csv --- CSV 文件读写

源代码： Lib/csv.py

CSV (Comma Separated Vaules) 格式是电子表格和数据库中最常见的输入、输出文件格式。在 RFC 4180 规范推出的很多年前，CSV 格式就已经被开始使用了，由于当时并没有合理的标准，不同应用程序读写的数据会存在细微的差别。这种差别让处理多个来源的 CSV 文件变得困难。但尽管分隔符会变化，此类文件的大致格式是相似的，所以编写一个单独的模块以高效处理此类数据，将程序员从读写数据的繁琐细节中解放出来是有可能的。

The csv module implements classes to read and write tabular data in CSV format. It allows programmers to say, "write this data in the format preferred by Excel," or "read data from this file which was generated by Excel," without knowing the precise details of the CSV format used by Excel. Programmers can also describe the CSV formats understood by other applications or define their own special-purpose CSV formats.

csv 模块中的 reader 类和 writer 类可用于读写序列化的数据。也可使用 DictReader 类和 DictWriter 类以字典的形式读写数据。

参见

该实现在“Python 增强提议” - PEP 305 (CSV 文件 API) 中被提出

The Python Enhancement Proposal which proposed this addition to Python.

模块内容

csv 模块定义了以下函数：

csv.reader(csvfile, dialect='excel', **fmtparams)

Return a reader object which will iterate over lines in the given csvfile. csvfile can be any object which supports the iterator protocol and returns a string each time its __next__() method is called --- file objects and list objects are both suitable. If csvfile is a file object, it should be opened with newline=''. 1 An optional dialect parameter can be given which is used to define a set of parameters specific to a particular CSV dialect. It may be an instance of a subclass of the Dialect class or one of the strings returned by the list_dialects() function. The other optional fmtparams keyword arguments can be given to override individual formatting parameters in the current dialect. For full details about the dialect and formatting parameters, see section Dialects and Formatting Parameters.

Each row read from the csv file is returned as a list of strings. No automatic data type conversion is performed unless the QUOTE_NONNUMERIC format option is specified (in which case unquoted fields are transformed into floats).

一个简短的用法示例:

>>> import csv
>>> with open('eggs.csv', newline='') as csvfile:
...     spamreader = csv.reader(csvfile, delimiter=' ', quotechar='|')
...     for row in spamreader:
...         print(', '.join(row))
Spam, Spam, Spam, Spam, Spam, Baked Beans
Spam, Lovely Spam, Wonderful Spam
csv.writer(csvfile, dialect='excel', **fmtparams)

Return a writer object responsible for converting the user's data into delimited strings on the given file-like object. csvfile can be any object with a write() method. If csvfile is a file object, it should be opened with newline='' 1. An optional dialect parameter can be given which is used to define a set of parameters specific to a particular CSV dialect. It may be an instance of a subclass of the Dialect class or one of the strings returned by the list_dialects() function. The other optional fmtparams keyword arguments can be given to override individual formatting parameters in the current dialect. For full details about the dialect and formatting parameters, see section Dialects and Formatting Parameters. To make it as easy as possible to interface with modules which implement the DB API, the value None is written as the empty string. While this isn't a reversible transformation, it makes it easier to dump SQL NULL data values to CSV files without preprocessing the data returned from a cursor.fetch* call. All other non-string data are stringified with str() before being written.

一个简短的用法示例:

import csv
with open('eggs.csv', 'w', newline='') as csvfile:
    spamwriter = csv.writer(csvfile, delimiter=' ',
                            quotechar='|', quoting=csv.QUOTE_MINIMAL)
    spamwriter.writerow(['Spam'] * 5 + ['Baked Beans'])
    spamwriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])
csv.register_dialect(name[, dialect[, **fmtparams]])

Associate dialect with name. name must be a string. The dialect can be specified either by passing a sub-class of Dialect, or by fmtparams keyword arguments, or both, with keyword arguments overriding parameters of the dialect. For full details about the dialect and formatting parameters, see section Dialects and Formatting Parameters.

csv.unregister_dialect(name)

Delete the dialect associated with name from the dialect registry. An Error is raised if name is not a registered dialect name.

csv.get_dialect(name)

Return the dialect associated with name. An Error is raised if name is not a registered dialect name. This function returns an immutable Dialect.

csv.list_dialects()

Return the names of all registered dialects.

csv.field_size_limit([new_limit])

Returns the current maximum field size allowed by the parser. If new_limit is given, this becomes the new limit.

The csv module defines the following classes:

class csv.DictReader(f, fieldnames=None, restkey=None, restval=None, dialect='excel', *args, **kwds)

Create an object that operates like a regular reader but maps the information in each row to a dict whose keys are given by the optional fieldnames parameter.

The fieldnames parameter is a sequence. If fieldnames is omitted, the values in the first row of file f will be used as the fieldnames. Regardless of how the fieldnames are determined, the dictionary preserves their original ordering.

If a row has more fields than fieldnames, the remaining data is put in a list and stored with the fieldname specified by restkey (which defaults to None). If a non-blank row has fewer fields than fieldnames, the missing values are filled-in with None.

All other optional or keyword arguments are passed to the underlying reader instance.

在 3.8 版更改: Returned rows are now of type dict.

一个简短的用法示例:

>>> import csv
>>> with open('names.csv', newline='') as csvfile:
...     reader = csv.DictReader(csvfile)
...     for row in reader:
...         print(row['first_name'], row['last_name'])
...
Eric Idle
John Cleese

>>> print(row)
{'first_name': 'John', 'last_name': 'Cleese'}
class csv.DictWriter(f, fieldnames, restval='', extrasaction='raise', dialect='excel', *args, **kwds)

Create an object which operates like a regular writer but maps dictionaries onto output rows. The fieldnames parameter is a sequence of keys that identify the order in which values in the dictionary passed to the writerow() method are written to file f. The optional restval parameter specifies the value to be written if the dictionary is missing a key in fieldnames. If the dictionary passed to the writerow() method contains a key not found in fieldnames, the optional extrasaction parameter indicates what action to take. If it is set to 'raise', the default value, a ValueError is raised. If it is set to 'ignore', extra values in the dictionary are ignored. Any other optional or keyword arguments are passed to the underlying writer instance.

Note that unlike the DictReader class, the fieldnames parameter of the DictWriter class is not optional.

一个简短的用法示例:

import csv

with open('names.csv', 'w', newline='') as csvfile:
    fieldnames = ['first_name', 'last_name']
    writer = csv.DictWriter(csvfile, fieldnames=fieldnames)

    writer.writeheader()
    writer.writerow({'first_name': 'Baked', 'last_name': 'Beans'})
    writer.writerow({'first_name': 'Lovely', 'last_name': 'Spam'})
    writer.writerow({'first_name': 'Wonderful', 'last_name': 'Spam'})
class csv.Dialect

The Dialect class is a container class relied on primarily for its attributes, which are used to define the parameters for a specific reader or writer instance.

class csv.excel

The excel class defines the usual properties of an Excel-generated CSV file. It is registered with the dialect name 'excel'.

class csv.excel_tab

The excel_tab class defines the usual properties of an Excel-generated TAB-delimited file. It is registered with the dialect name 'excel-tab'.

class csv.unix_dialect

The unix_dialect class defines the usual properties of a CSV file generated on UNIX systems, i.e. using '\n' as line terminator and quoting all fields. It is registered with the dialect name 'unix'.

3.2 新版功能.

class csv.Sniffer

The Sniffer class is used to deduce the format of a CSV file.

Sniffer 类提供了两个方法：

sniff(sample, delimiters=None)

Analyze the given sample and return a Dialect subclass reflecting the parameters found. If the optional delimiters parameter is given, it is interpreted as a string containing possible valid delimiter characters.

has_header(sample)

Analyze the sample text (presumed to be in CSV format) and return True if the first row appears to be a series of column headers.

使用 Sniffer 的示例：

with open('example.csv', newline='') as csvfile:
    dialect = csv.Sniffer().sniff(csvfile.read(1024))
    csvfile.seek(0)
    reader = csv.reader(csvfile, dialect)
    # ... process CSV file contents here ...

The csv module defines the following constants:

csv.QUOTE_ALL

Instructs writer objects to quote all fields.

csv.QUOTE_MINIMAL

Instructs writer objects to only quote those fields which contain special characters such as delimiter, quotechar or any of the characters in lineterminator.

csv.QUOTE_NONNUMERIC

Instructs writer objects to quote all non-numeric fields.

Instructs the reader to convert all non-quoted fields to type float.

csv.QUOTE_NONE

Instructs writer objects to never quote fields. When the current delimiter occurs in output data it is preceded by the current escapechar character. If escapechar is not set, the writer will raise Error if any characters that require escaping are encountered.

Instructs reader to perform no special processing of quote characters.

The csv module defines the following exception:

exception csv.Error

Raised by any of the functions when an error is detected.

Dialects and Formatting Parameters

To make it easier to specify the format of input and output records, specific formatting parameters are grouped together into dialects. A dialect is a subclass of the Dialect class having a set of specific methods and a single validate() method. When creating reader or writer objects, the programmer can specify a string or a subclass of the Dialect class as the dialect parameter. In addition to, or instead of, the dialect parameter, the programmer can also specify individual formatting parameters, which have the same names as the attributes defined below for the Dialect class.

Dialects support the following attributes:

Dialect.delimiter

A one-character string used to separate fields. It defaults to ','.

Dialect.doublequote

Controls how instances of quotechar appearing inside a field should themselves be quoted. When True, the character is doubled. When False, the escapechar is used as a prefix to the quotechar. It defaults to True.

On output, if doublequote is False and no escapechar is set, Error is raised if a quotechar is found in a field.

Dialect.escapechar

A one-character string used by the writer to escape the delimiter if quoting is set to QUOTE_NONE and the quotechar if doublequote is False. On reading, the escapechar removes any special meaning from the following character. It defaults to None, which disables escaping.

Dialect.lineterminator

The string used to terminate lines produced by the writer. It defaults to '\r\n'.

注解

The reader is hard-coded to recognise either '\r' or '\n' as end-of-line, and ignores lineterminator. This behavior may change in the future.

Dialect.quotechar

A one-character string used to quote fields containing special characters, such as the delimiter or quotechar, or which contain new-line characters. It defaults to '"'.

Dialect.quoting

Controls when quotes should be generated by the writer and recognised by the reader. It can take on any of the QUOTE_* constants (see section 模块内容) and defaults to QUOTE_MINIMAL.

Dialect.skipinitialspace

When True, whitespace immediately following the delimiter is ignored. The default is False.

Dialect.strict

When True, raise exception Error on bad CSV input. The default is False.

Reader Objects

Reader objects (DictReader instances and objects returned by the reader() function) have the following public methods:

csvreader.__next__()

Return the next row of the reader's iterable object as a list (if the object was returned from reader()) or a dict (if it is a DictReader instance), parsed according to the current dialect. Usually you should call this as next(reader).

Reader objects have the following public attributes:

csvreader.dialect

A read-only description of the dialect in use by the parser.

csvreader.line_num

The number of lines read from the source iterator. This is not the same as the number of records returned, as records can span multiple lines.

DictReader objects have the following public attribute:

csvreader.fieldnames

If not passed as a parameter when creating the object, this attribute is initialized upon first access or when the first record is read from the file.

Writer Objects

Writer objects (DictWriter instances and objects returned by the writer() function) have the following public methods. A row must be an iterable of strings or numbers for Writer objects and a dictionary mapping fieldnames to strings or numbers (by passing them through str() first) for DictWriter objects. Note that complex numbers are written out surrounded by parens. This may cause some problems for other programs which read CSV files (assuming they support complex numbers at all).

csvwriter.writerow(row)

Write the row parameter to the writer's file object, formatted according to the current dialect. Return the return value of the call to the write method of the underlying file object.

在 3.5 版更改: Added support of arbitrary iterables.

csvwriter.writerows(rows)

Write all elements in rows (an iterable of row objects as described above) to the writer's file object, formatted according to the current dialect.

Writer objects have the following public attribute:

csvwriter.dialect

A read-only description of the dialect in use by the writer.

DictWriter objects have the following public method:

DictWriter.writeheader()

Write a row with the field names (as specified in the constructor) to the writer's file object, formatted according to the current dialect. Return the return value of the csvwriter.writerow() call used internally.

3.2 新版功能.

在 3.8 版更改: writeheader() now also returns the value returned by the csvwriter.writerow() method it uses internally.

示例

读取 CSV 文件最简单的一个例子:

import csv
with open('some.csv', newline='') as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

Reading a file with an alternate format:

import csv
with open('passwd', newline='') as f:
    reader = csv.reader(f, delimiter=':', quoting=csv.QUOTE_NONE)
    for row in reader:
        print(row)

The corresponding simplest possible writing example is:

import csv
with open('some.csv', 'w', newline='') as f:
    writer = csv.writer(f)
    writer.writerows(someiterable)

Since open() is used to open a CSV file for reading, the file will by default be decoded into unicode using the system default encoding (see locale.getpreferredencoding()). To decode a file using a different encoding, use the encoding argument of open:

import csv
with open('some.csv', newline='', encoding='utf-8') as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

The same applies to writing in something other than the system default encoding: specify the encoding argument when opening the output file.

Registering a new dialect:

import csv
csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE)
with open('passwd', newline='') as f:
    reader = csv.reader(f, 'unixpwd')

A slightly more advanced use of the reader --- catching and reporting errors:

import csv, sys
filename = 'some.csv'
with open(filename, newline='') as f:
    reader = csv.reader(f)
    try:
        for row in reader:
            print(row)
    except csv.Error as e:
        sys.exit('file {}, line {}: {}'.format(filename, reader.line_num, e))

And while the module doesn't directly support parsing strings, it can easily be done:

import csv
for row in csv.reader(['one,two,three']):
    print(row)

脚注

1(1,2)

If newline='' is not specified, newlines embedded inside quoted fields will not be interpreted correctly, and on platforms that use \r\n linendings on write an extra \r will be added. It should always be safe to specify newline='', since the csv module does its own (universal) newline handling.