Basic Usage¶

Recall from Engine Configuration that an Engine is created via the create_engine() call:

engine = create_engine('mysql://scott:tiger@localhost/test')

The typical usage of create_engine() is once per particular database URL, held globally for the lifetime of a single application process. A single Engine manages many individual DBAPI connections on behalf of the process and is intended to be called upon in a concurrent fashion. The Engine is not synonymous to the DBAPI connect function, which represents just one connection resource - the Engine is most efficient when created just once at the module level of an application, not per-object or per-function call.

For a multiple-process application that uses the os.fork system call, or for example the Python multiprocessing module, it’s usually required that a separate Engine be used for each child process. This is because the Engine maintains a reference to a connection pool that ultimately references DBAPI connections - these tend to not be portable across process boundaries. An Engine that is configured not to use pooling (which is achieved via the usage of NullPool) does not have this requirement.

The engine can be used directly to issue SQL to the database. The most generic way is first procure a connection resource, which you get via the Engine.connect() method:

connection = engine.connect()
result = connection.execute("select username from users")
for row in result:
    print "username:", row['username']
connection.close()

The connection is an instance of Connection, which is a proxy object for an actual DBAPI connection. The DBAPI connection is retrieved from the connection pool at the point at which Connection is created.

The returned result is an instance of ResultProxy, which references a DBAPI cursor and provides a largely compatible interface with that of the DBAPI cursor. The DBAPI cursor will be closed by the ResultProxy when all of its result rows (if any) are exhausted. A ResultProxy that returns no rows, such as that of an UPDATE statement (without any returned rows), releases cursor resources immediately upon construction.

When the close() method is called, the referenced DBAPI connection is released to the connection pool. From the perspective of the database itself, nothing is actually “closed”, assuming pooling is in use. The pooling mechanism issues a rollback() call on the DBAPI connection so that any transactional state or locks are removed, and the connection is ready for its next usage.

The above procedure can be performed in a shorthand way by using the execute() method of Engine itself:

result = engine.execute("select username from users")
for row in result:
    print "username:", row['username']

Where above, the execute() method acquires a new Connection on its own, executes the statement with that object, and returns the ResultProxy. In this case, the ResultProxy contains a special flag known as close_with_result, which indicates that when its underlying DBAPI cursor is closed, the Connection object itself is also closed, which again returns the DBAPI connection to the connection pool, releasing transactional resources.

If the ResultProxy potentially has rows remaining, it can be instructed to close out its resources explicitly:

result.close()

If the ResultProxy has pending rows remaining and is dereferenced by the application without being closed, Python garbage collection will ultimately close out the cursor as well as trigger a return of the pooled DBAPI connection resource to the pool (SQLAlchemy achieves this by the usage of weakref callbacks - never the __del__ method) - however it’s never a good idea to rely upon Python garbage collection to manage resources.

Our example above illustrated the execution of a textual SQL string. The execute() method can of course accommodate more than that, including the variety of SQL expression constructs described in SQL Expression Language Tutorial.

Using Transactions¶

The Connection object provides a begin() method which returns a Transaction object. This object is usually used within a try/except clause so that it is guaranteed to invoke Transaction.rollback() or Transaction.commit():

connection = engine.connect()
trans = connection.begin()
try:
    r1 = connection.execute(table1.select())
    connection.execute(table1.insert(), col1=7, col2='this is some data')
    trans.commit()
except:
    trans.rollback()
    raise

The above block can be created more succinctly using context managers, either given an Engine:

# runs a transaction
with engine.begin() as connection:
    r1 = connection.execute(table1.select())
    connection.execute(table1.insert(), col1=7, col2='this is some data')

Or from the Connection, in which case the Transaction object is available as well:

with connection.begin() as trans:
    r1 = connection.execute(table1.select())
    connection.execute(table1.insert(), col1=7, col2='this is some data')

# method_a starts a transaction and calls method_b
def method_a(connection):
    trans = connection.begin() # open a transaction
    try:
        method_b(connection)
        trans.commit()  # transaction is committed here
    except:
        trans.rollback() # this rolls back the transaction unconditionally
        raise

# method_b also starts a transaction
def method_b(connection):
    trans = connection.begin() # open a transaction - this runs in the context of method_a's transaction
    try:
        connection.execute("insert into mytable values ('bat', 'lala')")
        connection.execute(mytable.insert(), col1='bat', col2='lala')
        trans.commit()  # transaction is not committed yet
    except:
        trans.rollback() # this rolls back the transaction unconditionally
        raise

# open a Connection and call method_a
conn = engine.connect()
method_a(conn)
conn.close()

Understanding Autocommit¶

The previous transaction example illustrates how to use Transaction so that several executions can take part in the same transaction. What happens when we issue an INSERT, UPDATE or DELETE call without using Transaction? While some DBAPI implementations provide various special “non-transactional” modes, the core behavior of DBAPI per PEP-0249 is that a transaction is always in progress, providing only rollback() and commit() methods but no begin(). SQLAlchemy assumes this is the case for any given DBAPI.

Given this requirement, SQLAlchemy implements its own “autocommit” feature which works completely consistently across all backends. This is achieved by detecting statements which represent data-changing operations, i.e. INSERT, UPDATE, DELETE, as well as data definition language (DDL) statements such as CREATE TABLE, ALTER TABLE, and then issuing a COMMIT automatically if no transaction is in progress. The detection is based on the presence of the autocommit=True execution option on the statement. If the statement is a text-only statement and the flag is not set, a regular expression is used to detect INSERT, UPDATE, DELETE, as well as a variety of other commands for a particular backend:

conn = engine.connect()
conn.execute("INSERT INTO users VALUES (1, 'john')")  # autocommits

The “autocommit” feature is only in effect when no Transaction has otherwise been declared. This means the feature is not generally used with the ORM, as the Session object by default always maintains an ongoing Transaction.

Full control of the “autocommit” behavior is available using the generative Connection.execution_options() method provided on Connection, Engine, Executable, using the “autocommit” flag which will turn on or off the autocommit for the selected scope. For example, a text() construct representing a stored procedure that commits might use it so that a SELECT statement will issue a COMMIT:

engine.execute(text("SELECT my_mutating_procedure()").execution_options(autocommit=True))

Connectionless Execution, Implicit Execution¶

Recall from the first section we mentioned executing with and without explicit usage of Connection. “Connectionless” execution refers to the usage of the execute() method on an object which is not a Connection. This was illustrated using the execute() method of Engine:

result = engine.execute("select username from users")
for row in result:
    print "username:", row['username']

In addition to “connectionless” execution, it is also possible to use the execute() method of any Executable construct, which is a marker for SQL expression objects that support execution. The SQL expression object itself references an Engine or Connection known as the bind, which it uses in order to provide so-called “implicit” execution services.

Given a table as below:

from sqlalchemy import MetaData, Table, Column, Integer

meta = MetaData()
users_table = Table('users', meta,
    Column('id', Integer, primary_key=True),
    Column('name', String(50))
)

Explicit execution delivers the SQL text or constructed SQL expression to the execute() method of Connection:

engine = create_engine('sqlite:///file.db')
connection = engine.connect()
result = connection.execute(users_table.select())
for row in result:
    # ....
connection.close()

Explicit, connectionless execution delivers the expression to the execute() method of Engine:

engine = create_engine('sqlite:///file.db')
result = engine.execute(users_table.select())
for row in result:
    # ....
result.close()

Implicit execution is also connectionless, and makes usage of the execute() method on the expression itself. This method is provided as part of the Executable class, which refers to a SQL statement that is sufficient for being invoked against the database. The method makes usage of the assumption that either an Engine or Connection has been bound to the expression object. By “bound” we mean that the special attribute MetaData.bind has been used to associate a series of Table objects and all SQL constructs derived from them with a specific engine:

engine = create_engine('sqlite:///file.db')
meta.bind = engine
result = users_table.select().execute()
for row in result:
    # ....
result.close()

Above, we associate an Engine with a MetaData object using the special attribute MetaData.bind. The select() construct produced from the Table object has a method execute(), which will search for an Engine that’s “bound” to the Table.

Overall, the usage of “bound metadata” has three general effects:

• SQL statement objects gain an Executable.execute() method which automatically locates a “bind” with which to execute themselves.
• The ORM Session object supports using “bound metadata” in order to establish which Engine should be used to invoke SQL statements on behalf of a particular mapped class, though the Session also features its own explicit system of establishing complex Engine/ mapped class configurations.
• The MetaData.create_all(), MetaData.drop_all(), Table.create(), Table.drop(), and “autoload” features all make usage of the bound Engine automatically without the need to pass it explicitly.

In both “connectionless” examples, the Connection is created behind the scenes; the ResultProxy returned by the execute() call references the Connection used to issue the SQL statement. When the ResultProxy is closed, the underlying Connection is closed for us, resulting in the DBAPI connection being returned to the pool with transactional resources removed.

Engine Disposal¶

The Engine refers to a connection pool, which means under normal circumstances, there are open database connections present while the Engine object is still resident in memory. When an Engine is garbage collected, its connection pool is no longer referred to by that Engine, and assuming none of its connections are still checked out, the pool and its connections will also be garbage collected, which has the effect of closing out the actual database connections as well. But otherwise, the Engine will hold onto open database connections assuming it uses the normally default pool implementation of QueuePool.

The Engine is intended to normally be a permanent fixture established up-front and maintained throughout the lifespan of an application. It is not intended to be created and disposed on a per-connection basis; it is instead a registry that maintains both a pool of connections as well as configurational information about the database and DBAPI in use, as well as some degree of internal caching of per-database resources.

However, there are many cases where it is desirable that all connection resources referred to by the Engine be completely closed out. It’s generally not a good idea to rely on Python garbage collection for this to occur for these cases; instead, the Engine can be explicitly disposed using the Engine.dispose() method. This disposes of the engine’s underlying connection pool and replaces it with a new one that’s empty. Provided that the Engine is discarded at this point and no longer used, all checked-in connections which it refers to will also be fully closed.

Valid use cases for calling Engine.dispose() include:

• When a program wants to release any remaining checked-in connections held by the connection pool and expects to no longer be connected to that database at all for any future operations.
• When a program uses multiprocessing or fork(), and an Engine object is copied to the child process, Engine.dispose() should be called so that the engine creates brand new database connections local to that fork. Database connections generally do not travel across process boundaries.
• Within test suites or multitenancy scenarios where many ad-hoc, short-lived Engine objects may be created and disposed.

Connections that are checked out are not discarded when the engine is disposed or garbage collected, as these connections are still strongly referenced elsewhere by the application. However, after Engine.dispose() is called, those connections are no longer associated with that Engine; when they are closed, they will be returned to their now-orphaned connection pool which will ultimately be garbage collected, once all connections which refer to it are also no longer referenced anywhere. Since this process is not easy to control, it is strongly recommended that Engine.dispose() is called only after all checked out connections are checked in or otherwise de-associated from their pool.

An alternative for applications that are negatively impacted by the Engine object’s use of connection pooling is to disable pooling entirely. This typically incurs only a modest performance impact upon the use of new connections, and means that when a connection is checked in, it is entirely closed out and is not held in memory. See Switching Pool Implementations for guidelines on how to disable pooling.

Using the Threadlocal Execution Strategy¶

The “threadlocal” engine strategy is an optional feature which can be used by non-ORM applications to associate transactions with the current thread, such that all parts of the application can participate in that transaction implicitly without the need to explicitly reference a Connection.

Enabling threadlocal is achieved as follows:

db = create_engine('mysql://localhost/test', strategy='threadlocal')

The above Engine will now acquire a Connection using connection resources derived from a thread-local variable whenever Engine.execute() or Engine.contextual_connect() is called. This connection resource is maintained as long as it is referenced, which allows multiple points of an application to share a transaction while using connectionless execution:

def call_operation1():
    engine.execute("insert into users values (?, ?)", 1, "john")

def call_operation2():
    users.update(users.c.user_id==5).execute(name='ed')

db.begin()
try:
    call_operation1()
    call_operation2()
    db.commit()
except:
    db.rollback()

Explicit execution can be mixed with connectionless execution by using the Engine.connect() method to acquire a Connection that is not part of the threadlocal scope:

db.begin()
conn = db.connect()
try:
    conn.execute(log_table.insert(), message="Operation started")
    call_operation1()
    call_operation2()
    db.commit()
    conn.execute(log_table.insert(), message="Operation succeeded")
except:
    db.rollback()
    conn.execute(log_table.insert(), message="Operation failed")
finally:
    conn.close()

To access the Connection that is bound to the threadlocal scope, call Engine.contextual_connect():

conn = db.contextual_connect()
call_operation3(conn)
conn.close()

Calling close() on the “contextual” connection does not release its resources until all other usages of that resource are closed as well, including that any ongoing transactions are rolled back or committed.

Working with Raw DBAPI Connections¶

There are some cases where SQLAlchemy does not provide a genericized way at accessing some DBAPI functions, such as calling stored procedures as well as dealing with multiple result sets. In these cases, it’s just as expedient to deal with the raw DBAPI connection directly.

The most common way to access the raw DBAPI connection is to get it from an already present Connection object directly. It is present using the Connection.connection attribute:

connection = engine.connect()
dbapi_conn = connection.connection

The DBAPI connection here is actually a “proxied” in terms of the originating connection pool, however this is an implementation detail that in most cases can be ignored. As this DBAPI connection is still contained within the scope of an owning Connection object, it is best to make use of the Connection object for most features such as transaction control as well as calling the Connection.close() method; if these operations are performed on the DBAPI connection directly, the owning Connection will not be aware of these changes in state.

To overcome the limitations imposed by the DBAPI connection that is maintained by an owning Connection, a DBAPI connection is also available without the need to procure a Connection first, using the Engine.raw_connection() method of Engine:

dbapi_conn = engine.raw_connection()

This DBAPI connection is again a “proxied” form as was the case before. The purpose of this proxying is now apparent, as when we call the .close() method of this connection, the DBAPI connection is typically not actually closed, but instead released back to the engine’s connection pool:

dbapi_conn.close()

While SQLAlchemy may in the future add built-in patterns for more DBAPI use cases, there are diminishing returns as these cases tend to be rarely needed and they also vary highly dependent on the type of DBAPI in use, so in any case the direct DBAPI calling pattern is always there for those cases where it is needed.

Some recipes for DBAPI connection use follow.

connection = engine.raw_connection()
try:
    cursor = connection.cursor()
    cursor.callproc("my_procedure", ['x', 'y', 'z'])
    results = list(cursor.fetchall())
    cursor.close()
    connection.commit()
finally:
    connection.close()

connection = engine.raw_connection()
try:
    cursor = connection.cursor()
    cursor.execute("select * from table1; select * from table2")
    results_one = cursor.fetchall()
    cursor.nextset()
    results_two = cursor.fetchall()
    cursor.close()
finally:
    connection.close()

Registering New Dialects¶

The create_engine() function call locates the given dialect using setuptools entrypoints. These entry points can be established for third party dialects within the setup.py script. For example, to create a new dialect “foodialect://”, the steps are as follows:

1. Create a package called foodialect.

2. The package should have a module containing the dialect class, which is typically a subclass of sqlalchemy.engine.default.DefaultDialect. In this example let’s say it’s called FooDialect and its module is accessed via foodialect.dialect.

3. The entry point can be established in setup.py as follows:

   entry_points="""
   [sqlalchemy.dialects]
   foodialect = foodialect.dialect:FooDialect
   """

If the dialect is providing support for a particular DBAPI on top of an existing SQLAlchemy-supported database, the name can be given including a database-qualification. For example, if FooDialect were in fact a MySQL dialect, the entry point could be established like this:

entry_points="""
[sqlalchemy.dialects]
mysql.foodialect = foodialect.dialect:FooDialect
"""

The above entrypoint would then be accessed as create_engine("mysql+foodialect://").

from sqlalchemy.dialects import registry
registry.register("mysql.foodialect", "myapp.dialect", "MyMySQLDialect")

Connection / Engine API¶

class sqlalchemy.engine.Connection(engine, connection=None, close_with_result=False, _branch_from=None, _execution_options=None, _dispatch=None, _has_events=None)¶

Bases: sqlalchemy.engine.Connectable

Provides high-level functionality for a wrapped DB-API connection.

Provides execution support for string-based SQL statements as well as ClauseElement, Compiled and DefaultGenerator objects. Provides a begin() method to return Transaction objects.

The Connection object is not thread-safe. While a Connection can be shared among threads using properly synchronized access, it is still possible that the underlying DBAPI connection may not support shared access between threads. Check the DBAPI documentation for details.

The Connection object represents a single dbapi connection checked out from the connection pool. In this state, the connection pool has no affect upon the connection, including its expiration or timeout state. For the connection pool to properly manage connections, connections should be returned to the connection pool (i.e. connection.close()) whenever the connection is not in use.

__init__(engine, connection=None, close_with_result=False, _branch_from=None, _execution_options=None, _dispatch=None, _has_events=None)¶

Construct a new Connection.

The constructor here is not public and is only called only by an Engine. See Engine.connect() and Engine.contextual_connect() methods.

begin()¶

Begin a transaction and return a transaction handle.

The returned object is an instance of Transaction. This object represents the “scope” of the transaction, which completes when either the Transaction.rollback() or Transaction.commit() method is called.

Nested calls to begin() on the same Connection will return new Transaction objects that represent an emulated transaction within the scope of the enclosing transaction, that is:

trans = conn.begin()   # outermost transaction
trans2 = conn.begin()  # "nested"
trans2.commit()        # does nothing
trans.commit()         # actually commits

Calls to Transaction.commit() only have an effect when invoked via the outermost Transaction object, though the Transaction.rollback() method of any of the Transaction objects will roll back the transaction.

See also:

Connection.begin_nested() - use a SAVEPOINT

Connection.begin_twophase() - use a two phase /XID transaction

Engine.begin() - context manager available from Engine.

begin_nested()¶

Begin a nested transaction and return a transaction handle.

The returned object is an instance of NestedTransaction.

Nested transactions require SAVEPOINT support in the underlying database. Any transaction in the hierarchy may commit and rollback, however the outermost transaction still controls the overall commit or rollback of the transaction of a whole.

See also Connection.begin(), Connection.begin_twophase().

begin_twophase(xid=None)¶

Begin a two-phase or XA transaction and return a transaction handle.

The returned object is an instance of TwoPhaseTransaction, which in addition to the methods provided by Transaction, also provides a prepare() method.

Parameters:	xid¶ – the two phase transaction id. If not supplied, a random id will be generated.

See also Connection.begin(), Connection.begin_twophase().

close()¶

Close this Connection.

This results in a release of the underlying database resources, that is, the DBAPI connection referenced internally. The DBAPI connection is typically restored back to the connection-holding Pool referenced by the Engine that produced this Connection. Any transactional state present on the DBAPI connection is also unconditionally released via the DBAPI connection’s rollback() method, regardless of any Transaction object that may be outstanding with regards to this Connection.

After close() is called, the Connection is permanently in a closed state, and will allow no further operations.

closed¶

Return True if this connection is closed.

connect()¶

Returns a branched version of this Connection.

The Connection.close() method on the returned Connection can be called and this Connection will remain open.

This method provides usage symmetry with Engine.connect(), including for usage with context managers.

connection¶

The underlying DB-API connection managed by this Connection.

See also

Working with Raw DBAPI Connections

contextual_connect(**kwargs)¶

Returns a branched version of this Connection.

The Connection.close() method on the returned Connection can be called and this Connection will remain open.

This method provides usage symmetry with Engine.contextual_connect(), including for usage with context managers.

default_isolation_level¶

The default isolation level assigned to this Connection.

This is the isolation level setting that the Connection has when first procured via the Engine.connect() method. This level stays in place until the Connection.execution_options.isolation_level is used to change the setting on a per-Connection basis.

Unlike Connection.get_isolation_level(), this attribute is set ahead of time from the first connection procured by the dialect, so SQL query is not invoked when this accessor is called.

New in version 0.9.9.

See also

Connection.get_isolation_level() - view current level

create_engine.isolation_level - set per Engine isolation level

Connection.execution_options.isolation_level - set per Connection isolation level

detach()¶

Detach the underlying DB-API connection from its connection pool.

E.g.:

with engine.connect() as conn:
    conn.detach()
    conn.execute("SET search_path TO schema1, schema2")

    # work with connection

# connection is fully closed (since we used "with:", can
# also call .close())

This Connection instance will remain usable. When closed (or exited from a context manager context as above), the DB-API connection will be literally closed and not returned to its originating pool.

This method can be used to insulate the rest of an application from a modified state on a connection (such as a transaction isolation level or similar).

execute(object, *multiparams, **params)¶

Executes the a SQL statement construct and returns a ResultProxy.

Parameters:

object¶ – The statement to be executed. May be one of: a plain string any ClauseElement construct that is also a subclass of Executable, such as a select() construct a FunctionElement, such as that generated by func, will be automatically wrapped in a SELECT statement, which is then executed. a DDLElement object a DefaultGenerator object a Compiled object *multiparams/**params¶ – represent bound parameter values to be used in the execution. Typically, the format is either a collection of one or more dictionaries passed to *multiparams: conn.execute( table.insert(), {"id":1, "value":"v1"}, {"id":2, "value":"v2"} ) ...or individual key/values interpreted by **params: conn.execute( table.insert(), id=1, value="v1" ) In the case that a plain SQL string is passed, and the underlying DBAPI accepts positional bind parameters, a collection of tuples or individual values in *multiparams may be passed: conn.execute( "INSERT INTO table (id, value) VALUES (?, ?)", (1, "v1"), (2, "v2") ) conn.execute( "INSERT INTO table (id, value) VALUES (?, ?)", 1, "v1" ) Note above, the usage of a question mark ”?” or other symbol is contingent upon the “paramstyle” accepted by the DBAPI in use, which may be any of “qmark”, “named”, “pyformat”, “format”, “numeric”. See pep-249 for details on paramstyle. To execute a textual SQL statement which uses bound parameters in a DBAPI-agnostic way, use the text() construct.

execution_options(**opt)¶

Set non-SQL options for the connection which take effect during execution.

The method returns a copy of this Connection which references the same underlying DBAPI connection, but also defines the given execution options which will take effect for a call to execute(). As the new Connection references the same underlying resource, it’s usually a good idea to ensure that the copies will be discarded immediately, which is implicit if used as in:

result = connection.execution_options(stream_results=True).\
                    execute(stmt)

Note that any key/value can be passed to Connection.execution_options(), and it will be stored in the _execution_options dictionary of the Connection. It is suitable for usage by end-user schemes to communicate with event listeners, for example.

The keywords that are currently recognized by SQLAlchemy itself include all those listed under Executable.execution_options(), as well as others that are specific to Connection.

Parameters:

autocommit¶ – Available on: Connection, statement. When True, a COMMIT will be invoked after execution when executed in ‘autocommit’ mode, i.e. when an explicit transaction is not begun on the connection. Note that DBAPI connections by default are always in a transaction - SQLAlchemy uses rules applied to different kinds of statements to determine if COMMIT will be invoked in order to provide its “autocommit” feature. Typically, all INSERT/UPDATE/DELETE statements as well as CREATE/DROP statements have autocommit behavior enabled; SELECT constructs do not. Use this option when invoking a SELECT or other specific SQL construct where COMMIT is desired (typically when calling stored procedures and such), and an explicit transaction is not in progress. compiled_cache¶ – Available on: Connection. A dictionary where Compiled objects will be cached when the Connection compiles a clause expression into a Compiled object. It is the user’s responsibility to manage the size of this dictionary, which will have keys corresponding to the dialect, clause element, the column names within the VALUES or SET clause of an INSERT or UPDATE, as well as the “batch” mode for an INSERT or UPDATE statement. The format of this dictionary is not guaranteed to stay the same in future releases. Note that the ORM makes use of its own “compiled” caches for some operations, including flush operations. The caching used by the ORM internally supersedes a cache dictionary specified here. isolation_level¶ – Available on: Connection. Set the transaction isolation level for the lifespan of this Connection object (not the underyling DBAPI connection, for which the level is reset to its original setting upon termination of this Connection object). Valid values include those string values accepted by the create_engine.isolation_level parameter passed to create_engine(). These levels are semi-database specific; see individual dialect documentation for valid levels. Note that this option necessarily affects the underlying DBAPI connection for the lifespan of the originating Connection, and is not per-execution. This setting is not removed until the underlying DBAPI connection is returned to the connection pool, i.e. the Connection.close() method is called. Warning The isolation_level execution option should not be used when a transaction is already established, that is, the Connection.begin() method or similar has been called. A database cannot change the isolation level on a transaction in progress, and different DBAPIs and/or SQLAlchemy dialects may implicitly roll back or commit the transaction, or not affect the connection at all. Changed in version 0.9.9: A warning is emitted when the isolation_level execution option is used after a transaction has been started with Connection.begin() or similar. Note The isolation_level execution option is implicitly reset if the Connection is invalidated, e.g. via the Connection.invalidate() method, or if a disconnection error occurs. The new connection produced after the invalidation will not have the isolation level re-applied to it automatically. See also create_engine.isolation_level - set per Engine isolation level Connection.get_isolation_level() - view current level SQLite Transaction Isolation Postgresql Transaction Isolation MySQL Transaction Isolation Setting Transaction Isolation Levels - for the ORM no_parameters¶ – When True, if the final parameter list or dictionary is totally empty, will invoke the statement on the cursor as cursor.execute(statement), not passing the parameter collection at all. Some DBAPIs such as psycopg2 and mysql-python consider percent signs as significant only when parameters are present; this option allows code to generate SQL containing percent signs (and possibly other characters) that is neutral regarding whether it’s executed by the DBAPI or piped into a script that’s later invoked by command line tools. New in version 0.7.6. stream_results¶ – Available on: Connection, statement. Indicate to the dialect that results should be “streamed” and not pre-buffered, if possible. This is a limitation of many DBAPIs. The flag is currently understood only by the psycopg2 dialect.

get_isolation_level()¶

Return the current isolation level assigned to this Connection.

This will typically be the default isolation level as determined by the dialect, unless if the Connection.execution_options.isolation_level feature has been used to alter the isolation level on a per-Connection basis.

This attribute will typically perform a live SQL operation in order to procure the current isolation level, so the value returned is the actual level on the underlying DBAPI connection regardless of how this state was set. Compare to the Connection.default_isolation_level accessor which returns the dialect-level setting without performing a SQL query.

New in version 0.9.9.

See also

Connection.default_isolation_level - view default level

create_engine.isolation_level - set per Engine isolation level

Connection.execution_options.isolation_level - set per Connection isolation level

in_transaction()¶

Return True if a transaction is in progress.

info¶

Info dictionary associated with the underlying DBAPI connection referred to by this Connection, allowing user-defined data to be associated with the connection.

The data here will follow along with the DBAPI connection including after it is returned to the connection pool and used again in subsequent instances of Connection.

invalidate(exception=None)¶

Invalidate the underlying DBAPI connection associated with this Connection.

The underlying DBAPI connection is literally closed (if possible), and is discarded. Its source connection pool will typically lazily create a new connection to replace it.

Upon the next use (where “use” typically means using the Connection.execute() method or similar), this Connection will attempt to procure a new DBAPI connection using the services of the Pool as a source of connectivty (e.g. a “reconnection”).

If a transaction was in progress (e.g. the Connection.begin() method has been called) when Connection.invalidate() method is called, at the DBAPI level all state associated with this transaction is lost, as the DBAPI connection is closed. The Connection will not allow a reconnection to proceed until the Transaction object is ended, by calling the Transaction.rollback() method; until that point, any attempt at continuing to use the Connection will raise an InvalidRequestError. This is to prevent applications from accidentally continuing an ongoing transactional operations despite the fact that the transaction has been lost due to an invalidation.

The Connection.invalidate() method, just like auto-invalidation, will at the connection pool level invoke the PoolEvents.invalidate() event.

See also

More on Invalidation

invalidated¶

Return True if this connection was invalidated.

run_callable(callable_, *args, **kwargs)¶

Given a callable object or function, execute it, passing a Connection as the first argument.

The given *args and **kwargs are passed subsequent to the Connection argument.

This function, along with Engine.run_callable(), allows a function to be run with a Connection or Engine object without the need to know which one is being dealt with.

scalar(object, *multiparams, **params)¶

Executes and returns the first column of the first row.

The underlying result/cursor is closed after execution.

transaction(callable_, *args, **kwargs)¶

Execute the given function within a transaction boundary.

The function is passed this Connection as the first argument, followed by the given *args and **kwargs, e.g.:

def do_something(conn, x, y):
    conn.execute("some statement", {'x':x, 'y':y})

conn.transaction(do_something, 5, 10)

The operations inside the function are all invoked within the context of a single Transaction. Upon success, the transaction is committed. If an exception is raised, the transaction is rolled back before propagating the exception.

Note

The transaction() method is superseded by the usage of the Python with: statement, which can be used with Connection.begin():

with conn.begin():
    conn.execute("some statement", {'x':5, 'y':10})

As well as with Engine.begin():

with engine.begin() as conn:
    conn.execute("some statement", {'x':5, 'y':10})

See also:

Engine.begin() - engine-level transactional context

Engine.transaction() - engine-level version of Connection.transaction()

class sqlalchemy.engine.Connectable¶

Interface for an object which supports execution of SQL constructs.

The two implementations of Connectable are Connection and Engine.

Connectable must also implement the ‘dialect’ member which references a Dialect instance.

connect(**kwargs)¶

Return a Connection object.

Depending on context, this may be self if this object is already an instance of Connection, or a newly procured Connection if this object is an instance of Engine.

contextual_connect()¶

Return a Connection object which may be part of an ongoing context.

Depending on context, this may be self if this object is already an instance of Connection, or a newly procured Connection if this object is an instance of Engine.

create(entity, **kwargs)¶

Emit CREATE statements for the given schema entity.

Deprecated since version 0.7: Use the create() method on the given schema object directly, i.e. Table.create(), Index.create(), MetaData.create_all()

drop(entity, **kwargs)¶

Emit DROP statements for the given schema entity.

Deprecated since version 0.7: Use the drop() method on the given schema object directly, i.e. Table.drop(), Index.drop(), MetaData.drop_all()

execute(object, *multiparams, **params)¶

Executes the given construct and returns a ResultProxy.

scalar(object, *multiparams, **params)¶

Executes and returns the first column of the first row.

The underlying cursor is closed after execution.

class sqlalchemy.engine.Engine(pool, dialect, url, logging_name=None, echo=None, proxy=None, execution_options=None)¶

Bases: sqlalchemy.engine.Connectable, sqlalchemy.log.Identified

Connects a Pool and Dialect together to provide a source of database connectivity and behavior.

An Engine object is instantiated publicly using the create_engine() function.

See also:

Engine Configuration

Working with Engines and Connections

begin(close_with_result=False)¶

Return a context manager delivering a Connection with a Transaction established.

E.g.:

with engine.begin() as conn:
    conn.execute("insert into table (x, y, z) values (1, 2, 3)")
    conn.execute("my_special_procedure(5)")

Upon successful operation, the Transaction is committed. If an error is raised, the Transaction is rolled back.

The close_with_result flag is normally False, and indicates that the Connection will be closed when the operation is complete. When set to True, it indicates the Connection is in “single use” mode, where the ResultProxy returned by the first call to Connection.execute() will close the Connection when that ResultProxy has exhausted all result rows.

New in version 0.7.6.

See also:

Engine.connect() - procure a Connection from an Engine.

Connection.begin() - start a Transaction for a particular Connection.

connect(**kwargs)¶

Return a new Connection object.

The Connection object is a facade that uses a DBAPI connection internally in order to communicate with the database. This connection is procured from the connection-holding Pool referenced by this Engine. When the close() method of the Connection object is called, the underlying DBAPI connection is then returned to the connection pool, where it may be used again in a subsequent call to connect().

contextual_connect(close_with_result=False, **kwargs)¶

Return a Connection object which may be part of some ongoing context.

By default, this method does the same thing as Engine.connect(). Subclasses of Engine may override this method to provide contextual behavior.

Parameters:	close_with_result¶ – When True, the first ResultProxy created by the Connection will call the Connection.close() method of that connection as soon as any pending result rows are exhausted. This is used to supply the “connectionless execution” behavior provided by the Engine.execute() method.

dispose()¶

Dispose of the connection pool used by this Engine.

This has the effect of fully closing all currently checked in database connections. Connections that are still checked out will not be closed, however they will no longer be associated with this Engine, so when they are closed individually, eventually the Pool which they are associated with will be garbage collected and they will be closed out fully, if not already closed on checkin.

A new connection pool is created immediately after the old one has been disposed. This new pool, like all SQLAlchemy connection pools, does not make any actual connections to the database until one is first requested, so as long as the Engine isn’t used again, no new connections will be made.

See also

Engine Disposal

driver¶

Driver name of the Dialect in use by this Engine.

execute(statement, *multiparams, **params)¶

Executes the given construct and returns a ResultProxy.

The arguments are the same as those used by Connection.execute().

Here, a Connection is acquired using the contextual_connect() method, and the statement executed with that connection. The returned ResultProxy is flagged such that when the ResultProxy is exhausted and its underlying cursor is closed, the Connection created here will also be closed, which allows its associated DBAPI connection resource to be returned to the connection pool.

execution_options(**opt)¶

Return a new Engine that will provide Connection objects with the given execution options.

The returned Engine remains related to the original Engine in that it shares the same connection pool and other state:

• The Pool used by the new Engine is the same instance. The Engine.dispose() method will replace the connection pool instance for the parent engine as well as this one.
• Event listeners are “cascaded” - meaning, the new Engine inherits the events of the parent, and new events can be associated with the new Engine individually.
• The logging configuration and logging_name is copied from the parent Engine.

The intent of the Engine.execution_options() method is to implement “sharding” schemes where multiple Engine objects refer to the same connection pool, but are differentiated by options that would be consumed by a custom event:

primary_engine = create_engine("mysql://")
shard1 = primary_engine.execution_options(shard_id="shard1")
shard2 = primary_engine.execution_options(shard_id="shard2")

Above, the shard1 engine serves as a factory for Connection objects that will contain the execution option shard_id=shard1, and shard2 will produce Connection objects that contain the execution option shard_id=shard2.

An event handler can consume the above execution option to perform a schema switch or other operation, given a connection. Below we emit a MySQL use statement to switch databases, at the same time keeping track of which database we’ve established using the Connection.info dictionary, which gives us a persistent storage space that follows the DBAPI connection:

from sqlalchemy import event
from sqlalchemy.engine import Engine

shards = {"default": "base", shard_1: "db1", "shard_2": "db2"}

@event.listens_for(Engine, "before_cursor_execute")
def _switch_shard(conn, cursor, stmt,
        params, context, executemany):
    shard_id = conn._execution_options.get('shard_id', "default")
    current_shard = conn.info.get("current_shard", None)

    if current_shard != shard_id:
        cursor.execute("use %s" % shards[shard_id])
        conn.info["current_shard"] = shard_id

New in version 0.8.

See also

Connection.execution_options() - update execution options on a Connection object.

Engine.update_execution_options() - update the execution options for a given Engine in place.

has_table(table_name, schema=None)¶

Return True if the given backend has a table of the given name.

See also

Fine Grained Reflection with Inspector - detailed schema inspection using the Inspector interface.

quoted_name - used to pass quoting information along with a schema identifier.

name¶

String name of the Dialect in use by this Engine.

raw_connection(_connection=None)¶

Return a “raw” DBAPI connection from the connection pool.

The returned object is a proxied version of the DBAPI connection object used by the underlying driver in use. The object will have all the same behavior as the real DBAPI connection, except that its close() method will result in the connection being returned to the pool, rather than being closed for real.

This method provides direct DBAPI connection access for special situations when the API provided by Connection is not needed. When a Connection object is already present, the DBAPI connection is available using the Connection.connection accessor.

See also

Working with Raw DBAPI Connections

run_callable(callable_, *args, **kwargs)¶

Given a callable object or function, execute it, passing a Connection as the first argument.

The given *args and **kwargs are passed subsequent to the Connection argument.

This function, along with Connection.run_callable(), allows a function to be run with a Connection or Engine object without the need to know which one is being dealt with.

table_names(schema=None, connection=None)¶

Return a list of all table names available in the database.

Parameters:	schema¶ – Optional, retrieve names from a non-default schema. connection¶ – Optional, use a specified connection. Default is the contextual_connect for this Engine.

transaction(callable_, *args, **kwargs)¶

Execute the given function within a transaction boundary.

The function is passed a Connection newly procured from Engine.contextual_connect() as the first argument, followed by the given *args and **kwargs.

e.g.:

def do_something(conn, x, y):
    conn.execute("some statement", {'x':x, 'y':y})

engine.transaction(do_something, 5, 10)

The operations inside the function are all invoked within the context of a single Transaction. Upon success, the transaction is committed. If an exception is raised, the transaction is rolled back before propagating the exception.

Note

The transaction() method is superseded by the usage of the Python with: statement, which can be used with Engine.begin():

with engine.begin() as conn:
    conn.execute("some statement", {'x':5, 'y':10})

See also:

Engine.begin() - engine-level transactional context

Connection.transaction() - connection-level version of Engine.transaction()

update_execution_options(**opt)¶

Update the default execution_options dictionary of this Engine.

The given keys/values in **opt are added to the default execution options that will be used for all connections. The initial contents of this dictionary can be sent via the execution_options parameter to create_engine().

See also

Connection.execution_options()

Engine.execution_options()

class sqlalchemy.engine.ExceptionContext¶

Encapsulate information about an error condition in progress.

This object exists solely to be passed to the ConnectionEvents.handle_error() event, supporting an interface that can be extended without backwards-incompatibility.

New in version 0.9.7.

chained_exception = None¶

The exception that was returned by the previous handler in the exception chain, if any.

If present, this exception will be the one ultimately raised by SQLAlchemy unless a subsequent handler replaces it.

May be None.

connection = None¶

The Connection in use during the exception.

This member is present, except in the case of a failure when first connecting.

See also

ExceptionContext.engine

cursor = None¶

The DBAPI cursor object.

May be None.

engine = None¶

The Engine in use during the exception.

This member should always be present, even in the case of a failure when first connecting.

New in version 1.0.0.

execution_context = None¶

The ExecutionContext corresponding to the execution operation in progress.

This is present for statement execution operations, but not for operations such as transaction begin/end. It also is not present when the exception was raised before the ExecutionContext could be constructed.

Note that the ExceptionContext.statement and ExceptionContext.parameters members may represent a different value than that of the ExecutionContext, potentially in the case where a ConnectionEvents.before_cursor_execute() event or similar modified the statement/parameters to be sent.

May be None.

invalidate_pool_on_disconnect = True¶

Represent whether all connections in the pool should be invalidated when a “disconnect” condition is in effect.

Setting this flag to False within the scope of the ConnectionEvents.handle_error() event will have the effect such that the full collection of connections in the pool will not be invalidated during a disconnect; only the current connection that is the subject of the error will actually be invalidated.

The purpose of this flag is for custom disconnect-handling schemes where the invalidation of other connections in the pool is to be performed based on other conditions, or even on a per-connection basis.

New in version 1.0.3.

is_disconnect = None¶

Represent whether the exception as occurred represents a “disconnect” condition.

This flag will always be True or False within the scope of the ConnectionEvents.handle_error() handler.

SQLAlchemy will defer to this flag in order to determine whether or not the connection should be invalidated subsequently. That is, by assigning to this flag, a “disconnect” event which then results in a connection and pool invalidation can be invoked or prevented by changing this flag.

original_exception = None¶

The exception object which was caught.

This member is always present.

parameters = None¶

Parameter collection that was emitted directly to the DBAPI.

May be None.

sqlalchemy_exception = None¶

The sqlalchemy.exc.StatementError which wraps the original, and will be raised if exception handling is not circumvented by the event.

May be None, as not all exception types are wrapped by SQLAlchemy. For DBAPI-level exceptions that subclass the dbapi’s Error class, this field will always be present.

statement = None¶

String SQL statement that was emitted directly to the DBAPI.

May be None.

class sqlalchemy.engine.NestedTransaction(connection, parent)¶

Bases: sqlalchemy.engine.Transaction

Represent a ‘nested’, or SAVEPOINT transaction.

A new NestedTransaction object may be procured using the Connection.begin_nested() method.

The interface is the same as that of Transaction.

class sqlalchemy.engine.ResultProxy(context)¶

Wraps a DB-API cursor object to provide easier access to row columns.

Individual columns may be accessed by their integer position, case-insensitive column name, or by schema.Column object. e.g.:

row = fetchone()

col1 = row[0]    # access via integer position

col2 = row['col2']   # access via name

col3 = row[mytable.c.mycol] # access via Column object.

ResultProxy also handles post-processing of result column data using TypeEngine objects, which are referenced from the originating SQL statement that produced this result set.

_cursor_description()¶

May be overridden by subclasses.

_process_row¶

alias of RowProxy

_soft_close(_autoclose_connection=True)¶

Soft close this ResultProxy.

This releases all DBAPI cursor resources, but leaves the ResultProxy “open” from a semantic perspective, meaning the fetchXXX() methods will continue to return empty results.

This method is called automatically when:

• all result rows are exhausted using the fetchXXX() methods.
• cursor.description is None.

This method is not public, but is documented in order to clarify the “autoclose” process used.

New in version 1.0.0.

See also

ResultProxy.close()

close()¶

Close this ResultProxy.

This closes out the underlying DBAPI cursor corresonding to the statement execution, if one is stil present. Note that the DBAPI cursor is automatically released when the ResultProxy exhausts all available rows. ResultProxy.close() is generally an optional method except in the case when discarding a ResultProxy that still has additional rows pending for fetch.

In the case of a result that is the product of connectionless execution, the underyling Connection object is also closed, which releases DBAPI connection resources.

After this method is called, it is no longer valid to call upon the fetch methods, which will raise a ResourceClosedError on subsequent use.

Changed in version 1.0.0: - the ResultProxy.close() method has been separated out from the process that releases the underlying DBAPI cursor resource. The “auto close” feature of the Connection now performs a so-called “soft close”, which releases the underlying DBAPI cursor, but allows the ResultProxy to still behave as an open-but-exhausted result set; the actual ResultProxy.close() method is never called. It is still safe to discard a ResultProxy that has been fully exhausted without calling this method.

See also

Working with Engines and Connections

ResultProxy._soft_close()

fetchall()¶

Fetch all rows, just like DB-API cursor.fetchall().

After all rows have been exhausted, the underlying DBAPI cursor resource is released, and the object may be safely discarded.

Subsequent calls to ResultProxy.fetchall() will return an empty list. After the ResultProxy.close() method is called, the method will raise ResourceClosedError.

Changed in version 1.0.0: - Added “soft close” behavior which allows the result to be used in an “exhausted” state prior to calling the ResultProxy.close() method.

fetchmany(size=None)¶

Fetch many rows, just like DB-API cursor.fetchmany(size=cursor.arraysize).

After all rows have been exhausted, the underlying DBAPI cursor resource is released, and the object may be safely discarded.

Calls to ResultProxy.fetchmany() after all rows have been exhuasted will return an empty list. After the ResultProxy.close() method is called, the method will raise ResourceClosedError.

Changed in version 1.0.0: - Added “soft close” behavior which allows the result to be used in an “exhausted” state prior to calling the ResultProxy.close() method.

fetchone()¶

Fetch one row, just like DB-API cursor.fetchone().

After all rows have been exhausted, the underlying DBAPI cursor resource is released, and the object may be safely discarded.

Calls to ResultProxy.fetchone() after all rows have been exhausted will return None. After the ResultProxy.close() method is called, the method will raise ResourceClosedError.

Changed in version 1.0.0: - Added “soft close” behavior which allows the result to be used in an “exhausted” state prior to calling the ResultProxy.close() method.

first()¶

Fetch the first row and then close the result set unconditionally.

Returns None if no row is present.

After calling this method, the object is fully closed, e.g. the ResultProxy.close() method will have been called.

inserted_primary_key¶

Return the primary key for the row just inserted.

The return value is a list of scalar values corresponding to the list of primary key columns in the target table.

This only applies to single row insert() constructs which did not explicitly specify Insert.returning().

Note that primary key columns which specify a server_default clause, or otherwise do not qualify as “autoincrement” columns (see the notes at Column), and were generated using the database-side default, will appear in this list as None unless the backend supports “returning” and the insert statement executed with the “implicit returning” enabled.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() construct.

is_insert¶

True if this ResultProxy is the result of a executing an expression language compiled expression.insert() construct.

When True, this implies that the inserted_primary_key attribute is accessible, assuming the statement did not include a user defined “returning” construct.

keys()¶

Return the current set of string keys for rows.

last_inserted_params()¶

Return the collection of inserted parameters from this execution.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() construct.

last_updated_params()¶

Return the collection of updated parameters from this execution.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an update() construct.

lastrow_has_defaults()¶

Return lastrow_has_defaults() from the underlying ExecutionContext.

See ExecutionContext for details.

lastrowid¶

return the ‘lastrowid’ accessor on the DBAPI cursor.

This is a DBAPI specific method and is only functional for those backends which support it, for statements where it is appropriate. It’s behavior is not consistent across backends.

Usage of this method is normally unnecessary when using insert() expression constructs; the inserted_primary_key attribute provides a tuple of primary key values for a newly inserted row, regardless of database backend.

postfetch_cols()¶

Return postfetch_cols() from the underlying ExecutionContext.

See ExecutionContext for details.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() or update() construct.

prefetch_cols()¶

Return prefetch_cols() from the underlying ExecutionContext.

See ExecutionContext for details.

Raises InvalidRequestError if the executed statement is not a compiled expression construct or is not an insert() or update() construct.

returned_defaults¶

Return the values of default columns that were fetched using the ValuesBase.return_defaults() feature.

The value is an instance of RowProxy, or None if ValuesBase.return_defaults() was not used or if the backend does not support RETURNING.

New in version 0.9.0.

See also

ValuesBase.return_defaults()

returns_rows¶

True if this ResultProxy returns rows.

I.e. if it is legal to call the methods fetchone(), fetchmany() fetchall().

rowcount¶

Return the ‘rowcount’ for this result.

The ‘rowcount’ reports the number of rows matched by the WHERE criterion of an UPDATE or DELETE statement.

Note

Notes regarding ResultProxy.rowcount:

• This attribute returns the number of rows matched, which is not necessarily the same as the number of rows that were actually modified - an UPDATE statement, for example, may have no net change on a given row if the SET values given are the same as those present in the row already. Such a row would be matched but not modified. On backends that feature both styles, such as MySQL, rowcount is configured by default to return the match count in all cases.
• ResultProxy.rowcount is only useful in conjunction with an UPDATE or DELETE statement. Contrary to what the Python DBAPI says, it does not return the number of rows available from the results of a SELECT statement as DBAPIs cannot support this functionality when rows are unbuffered.
• ResultProxy.rowcount may not be fully implemented by all dialects. In particular, most DBAPIs do not support an aggregate rowcount result from an executemany call. The ResultProxy.supports_sane_rowcount() and ResultProxy.supports_sane_multi_rowcount() methods will report from the dialect if each usage is known to be supported.
• Statements that use RETURNING may not return a correct rowcount.

scalar()¶

Fetch the first column of the first row, and close the result set.

Returns None if no row is present.

After calling this method, the object is fully closed, e.g. the ResultProxy.close() method will have been called.

supports_sane_multi_rowcount()¶

Return supports_sane_multi_rowcount from the dialect.

See ResultProxy.rowcount for background.

supports_sane_rowcount()¶

Return supports_sane_rowcount from the dialect.

See ResultProxy.rowcount for background.

class sqlalchemy.engine.RowProxy(parent, row, processors, keymap)¶

Bases: sqlalchemy.engine.BaseRowProxy

Proxy values from a single cursor row.

Mostly follows “ordered dictionary” behavior, mapping result values to the string-based column name, the integer position of the result in the row, as well as Column instances which can be mapped to the original Columns that produced this result set (for results that correspond to constructed SQL expressions).

has_key(key)¶

Return True if this RowProxy contains the given key.

items()¶

Return a list of tuples, each tuple containing a key/value pair.

keys()¶

Return the list of keys as strings represented by this RowProxy.

class sqlalchemy.engine.Transaction(connection, parent)¶

Represent a database transaction in progress.

The Transaction object is procured by calling the begin() method of Connection:

from sqlalchemy import create_engine
engine = create_engine("postgresql://scott:tiger@localhost/test")
connection = engine.connect()
trans = connection.begin()
connection.execute("insert into x (a, b) values (1, 2)")
trans.commit()

The object provides rollback() and commit() methods in order to control transaction boundaries. It also implements a context manager interface so that the Python with statement can be used with the Connection.begin() method:

with connection.begin():
    connection.execute("insert into x (a, b) values (1, 2)")

The Transaction object is not threadsafe.

See also: Connection.begin(), Connection.begin_twophase(), Connection.begin_nested().

close()¶

Close this Transaction.

If this transaction is the base transaction in a begin/commit nesting, the transaction will rollback(). Otherwise, the method returns.

This is used to cancel a Transaction without affecting the scope of an enclosing transaction.

commit()¶

Commit this Transaction.

rollback()¶

Roll back this Transaction.

class sqlalchemy.engine.TwoPhaseTransaction(connection, xid)¶

Bases: sqlalchemy.engine.Transaction

Represent a two-phase transaction.

A new TwoPhaseTransaction object may be procured using the Connection.begin_twophase() method.

The interface is the same as that of Transaction with the addition of the prepare() method.

prepare()¶

Prepare this TwoPhaseTransaction.

After a PREPARE, the transaction can be committed.