4. Execution model¶
4.1. Structure of a program¶
A Python program is constructed from code blocks.
A block is a piece of Python program text that is executed as a unit.
The following are blocks: a module, a function body, and a class definition.
Each command typed interactively is a block. A script file (a file given as
standard input to the interpreter or specified as a command line argument to the
interpreter) is a code block. A script command (a command specified on the
interpreter command line with the ‘-c‘ option) is a code block. The string
argument passed to the built-in functions eval() and exec() is a
code block.
A code block is executed in an execution frame. A frame contains some administrative information (used for debugging) and determines where and how execution continues after the code block’s execution has completed.
4.2. Naming and binding¶
4.2.1. Binding of names¶
Names refer to objects. Names are introduced by name binding operations.
The following constructs bind names: formal parameters to functions,
import statements, class and function definitions (these bind the
class or function name in the defining block), and targets that are identifiers
if occurring in an assignment, for loop header, or after
as in a with statement or except clause.
The import statement
of the form from ... import * binds all names defined in the imported
module, except those beginning with an underscore. This form may only be used
at the module level.
A target occurring in a del statement is also considered bound for
this purpose (though the actual semantics are to unbind the name).
Each assignment or import statement occurs within a block defined by a class or function definition or at the module level (the top-level code block).
If a name is bound in a block, it is a local variable of that block, unless
declared as nonlocal or global. If a name is bound at
the module level, it is a global variable. (The variables of the module code
block are local and global.) If a variable is used in a code block but not
defined there, it is a free variable.
Each occurrence of a name in the program text refers to the binding of that name established by the following name resolution rules.
4.2.2. Resolution of names¶
A scope defines the visibility of a name within a block. If a local variable is defined in a block, its scope includes that block. If the definition occurs in a function block, the scope extends to any blocks contained within the defining one, unless a contained block introduces a different binding for the name.
When a name is used in a code block, it is resolved using the nearest enclosing scope. The set of all such scopes visible to a code block is called the block’s environment.
When a name is not found at all, a NameError exception is raised.
If the current scope is a function scope, and the name refers to a local
variable that has not yet been bound to a value at the point where the name is
used, an UnboundLocalError exception is raised.
UnboundLocalError is a subclass of NameError.
If a name binding operation occurs anywhere within a code block, all uses of the name within the block are treated as references to the current block. This can lead to errors when a name is used within a block before it is bound. This rule is subtle. Python lacks declarations and allows name binding operations to occur anywhere within a code block. The local variables of a code block can be determined by scanning the entire text of the block for name binding operations.
If the global statement occurs within a block, all uses of the name
specified in the statement refer to the binding of that name in the top-level
namespace. Names are resolved in the top-level namespace by searching the
global namespace, i.e. the namespace of the module containing the code block,
and the builtins namespace, the namespace of the module builtins. The
global namespace is searched first. If the name is not found there, the
builtins namespace is searched. The global statement must precede
all uses of the name.
The global statement has the same scope as a name binding operation
in the same block. If the nearest enclosing scope for a free variable contains
a global statement, the free variable is treated as a global.
The nonlocal statement causes corresponding names to refer
to previously bound variables in the nearest enclosing function scope.
SyntaxError is raised at compile time if the given name does not
exist in any enclosing function scope.
The namespace for a module is automatically created the first time a module is
imported. The main module for a script is always called __main__.
Class definition blocks and arguments to exec() and eval() are
special in the context of name resolution.
A class definition is an executable statement that may use and define names.
These references follow the normal rules for name resolution with an exception
that unbound local variables are looked up in the global namespace.
The namespace of the class definition becomes the attribute dictionary of
the class. The scope of names defined in a class block is limited to the
class block; it does not extend to the code blocks of methods – this includes
comprehensions and generator expressions since they are implemented using a
function scope. This means that the following will fail:
class A:
a = 42
b = list(a + i for i in range(10))
4.2.3. Builtins and restricted execution¶
The builtins namespace associated with the execution of a code block is actually
found by looking up the name __builtins__ in its global namespace; this
should be a dictionary or a module (in the latter case the module’s dictionary
is used). By default, when in the __main__ module, __builtins__ is
the built-in module builtins; when in any other module,
__builtins__ is an alias for the dictionary of the builtins module
itself. __builtins__ can be set to a user-created dictionary to create a
weak form of restricted execution.
4.2.4. Interaction with dynamic features¶
Name resolution of free variables occurs at runtime, not at compile time. This means that the following code will print 42:
i = 10
def f():
print(i)
i = 42
f()
There are several cases where Python statements are illegal when used in conjunction with nested scopes that contain free variables.
If a variable is referenced in an enclosing scope, it is illegal to delete the name. An error will be reported at compile time.
The eval() and exec() functions do not have access to the full
environment for resolving names. Names may be resolved in the local and global
namespaces of the caller. Free variables are not resolved in the nearest
enclosing namespace, but in the global namespace. [1] The exec() and
eval() functions have optional arguments to override the global and local
namespace. If only one namespace is specified, it is used for both.
4.3. Exceptions¶
Exceptions are a means of breaking out of the normal flow of control of a code block in order to handle errors or other exceptional conditions. An exception is raised at the point where the error is detected; it may be handled by the surrounding code block or by any code block that directly or indirectly invoked the code block where the error occurred.
The Python interpreter raises an exception when it detects a run-time error
(such as division by zero). A Python program can also explicitly raise an
exception with the raise statement. Exception handlers are specified
with the try ... except statement. The finally
clause of such a statement can be used to specify cleanup code which does not
handle the exception, but is executed whether an exception occurred or not in
the preceding code.
Python uses the “termination” model of error handling: an exception handler can find out what happened and continue execution at an outer level, but it cannot repair the cause of the error and retry the failing operation (except by re-entering the offending piece of code from the top).
When an exception is not handled at all, the interpreter terminates execution of
the program, or returns to its interactive main loop. In either case, it prints
a stack backtrace, except when the exception is SystemExit.
Exceptions are identified by class instances. The except clause is
selected depending on the class of the instance: it must reference the class of
the instance or a base class thereof. The instance can be received by the
handler and can carry additional information about the exceptional condition.
Note
Exception messages are not part of the Python API. Their contents may change from one version of Python to the next without warning and should not be relied on by code which will run under multiple versions of the interpreter.
See also the description of the try statement in section The try statement
and raise statement in section The raise statement.
Footnotes
| [1] | This limitation occurs because the code that is executed by these operations is not available at the time the module is compiled. |