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### 导航 - [索引](../genindex.xhtml "总目录") - [模块](../py-modindex.xhtml "Python 模块索引") | - [下一页](configparser.xhtml "configparser --- Configuration file parser") | - [上一页](fileformats.xhtml "文件格式") | - ![](https://box.kancloud.cn/a721fc7ec672275e257bbbfde49a4d4e_16x16.png) - [Python](https://www.python.org/) » - zh\_CN 3.7.3 [文档](../index.xhtml) » - [Python 标准库](index.xhtml) » - [文件格式](fileformats.xhtml) » - $('.inline-search').show(0); | # [`csv`](#module-csv "csv: Write and read tabular data to and from delimited files.") --- CSV 文件读写 **源代码:** [Lib/csv.py](https://github.com/python/cpython/tree/3.7/Lib/csv.py) \[https://github.com/python/cpython/tree/3.7/Lib/csv.py\] - - - - - - 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**](https://tools.ietf.org/html/rfc4180.html) \[https://tools.ietf.org/html/rfc4180.html\] 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-csv "csv: Write and read tabular data to and from delimited files.") 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`](#module-csv "csv: Write and read tabular data to and from delimited files.") 模块中的 [`reader`](#csv.reader "csv.reader") 类和 [`writer`](#csv.writer "csv.writer") 类可用于读写序列化的数据。也可使用 [`DictReader`](#csv.DictReader "csv.DictReader") 类和 [`DictWriter`](#csv.DictWriter "csv.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`](#module-csv "csv: Write and read tabular data to and from delimited files.") 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](../glossary.xhtml#term-iterator) protocol and returns a string each time its `__next__()` method is called --- [file objects](../glossary.xhtml#term-file-object) and list objects are both suitable. If *csvfile* is a file object, it should be opened with `newline=''`. [1](#id3) 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`](#csv.Dialect "csv.Dialect") class or one of the strings returned by the [`list_dialects()`](#csv.list_dialects "csv.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](#csv-fmt-params). 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](#id3). 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`](#csv.Dialect "csv.Dialect") class or one of the strings returned by the [`list_dialects()`](#csv.list_dialects "csv.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](#csv-fmt-params). To make it as easy as possible to interface with modules which implement the DB API, the value [`None`](constants.xhtml#None "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()`](stdtypes.xhtml#str "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`](#csv.Dialect "csv.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-fmt-params). `csv.``unregister_dialect`(*name*)Delete the dialect associated with *name* from the dialect registry. An [`Error`](#csv.Error "csv.Error") is raised if *name* is not a registered dialect name. `csv.``get_dialect`(*name*)Return the dialect associated with *name*. An [`Error`](#csv.Error "csv.Error") is raised if *name* is not a registered dialect name. This function returns an immutable [`Dialect`](#csv.Dialect "csv.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-csv "csv: Write and read tabular data to and from delimited files.") 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 an [`OrderedDict`](collections.xhtml#collections.OrderedDict "collections.OrderedDict")whose keys are given by the optional *fieldnames* parameter. The *fieldnames* parameter is a [sequence](../glossary.xhtml#term-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 ordered 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`](#csv.reader "csv.reader") instance. 在 3.6 版更改: Returned rows are now of type `OrderedDict`. 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) OrderedDict([('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`](collections.abc.xhtml#module-collections.abc "collections.abc: Abstract base classes for containers") 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`](exceptions.xhtml#ValueError "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`](#csv.writer "csv.writer") instance. Note that unlike the [`DictReader`](#csv.DictReader "csv.DictReader") class, the *fieldnames* parameter of the [`DictWriter`](#csv.DictWriter "csv.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`](#csv.Dialect "csv.Dialect") class is a container class relied on primarily for its attributes, which are used to define the parameters for a specific [`reader`](#csv.reader "csv.reader") or [`writer`](#csv.writer "csv.writer") instance. *class* `csv.``excel`The [`excel`](#csv.excel "csv.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`](#csv.excel_tab "csv.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`](#csv.unix_dialect "csv.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`](#csv.Sniffer "csv.Sniffer") class is used to deduce the format of a CSV file. [`Sniffer`](#csv.Sniffer "csv.Sniffer") 类提供了两个方法: `sniff`(*sample*, *delimiters=None*)Analyze the given *sample* and return a [`Dialect`](#csv.Dialect "csv.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`](constants.xhtml#True "True") if the first row appears to be a series of column headers. 使用 [`Sniffer`](#csv.Sniffer "csv.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-csv "csv: Write and read tabular data to and from delimited files.") module defines the following constants: `csv.``QUOTE_ALL`Instructs [`writer`](#csv.writer "csv.writer") objects to quote all fields. `csv.``QUOTE_MINIMAL`Instructs [`writer`](#csv.writer "csv.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`](#csv.writer "csv.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`](#csv.writer "csv.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`](#csv.Error "csv.Error") if any characters that require escaping are encountered. Instructs [`reader`](#csv.reader "csv.reader") to perform no special processing of quote characters. The [`csv`](#module-csv "csv: Write and read tabular data to and from delimited files.") 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`](#csv.Dialect "csv.Dialect") class having a set of specific methods and a single `validate()` method. When creating [`reader`](#csv.reader "csv.reader") or [`writer`](#csv.writer "csv.writer") objects, the programmer can specify a string or a subclass of the [`Dialect`](#csv.Dialect "csv.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`](#csv.Dialect "csv.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`](constants.xhtml#True "True"), the character is doubled. When [`False`](constants.xhtml#False "False"), the *escapechar* is used as a prefix to the *quotechar*. It defaults to [`True`](constants.xhtml#True "True"). On output, if *doublequote* is [`False`](constants.xhtml#False "False") and no *escapechar* is set, [`Error`](#csv.Error "csv.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`](#csv.QUOTE_NONE "csv.QUOTE_NONE") and the *quotechar* if *doublequote* is [`False`](constants.xhtml#False "False"). On reading, the *escapechar* removes any special meaning from the following character. It defaults to [`None`](constants.xhtml#None "None"), which disables escaping. `Dialect.``lineterminator`The string used to terminate lines produced by the [`writer`](#csv.writer "csv.writer"). It defaults to `'\r\n'`. 注解 The [`reader`](#csv.reader "csv.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](#csv-contents)) and defaults to [`QUOTE_MINIMAL`](#csv.QUOTE_MINIMAL "csv.QUOTE_MINIMAL"). `Dialect.``skipinitialspace`When [`True`](constants.xhtml#True "True"), whitespace immediately following the *delimiter* is ignored. The default is [`False`](constants.xhtml#False "False"). `Dialect.``strict`When `True`, raise exception [`Error`](#csv.Error "csv.Error") on bad CSV input. The default is `False`. ## Reader Objects Reader objects ([`DictReader`](#csv.DictReader "csv.DictReader") instances and objects returned by the [`reader()`](#csv.reader "csv.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()`](#csv.reader "csv.reader")) or a dict (if it is a [`DictReader`](#csv.DictReader "csv.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`](#csv.DictWriter "csv.DictWriter") instances and objects returned by the [`writer()`](#csv.writer "csv.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()`](stdtypes.xhtml#str "str")first) for [`DictWriter`](#csv.DictWriter "csv.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. 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). 3\.2 新版功能. ## 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()`](functions.xhtml#open "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()`](locale.xhtml#locale.getpreferredencoding "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](#id1),[2](#id2))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](../glossary.xhtml#term-universal-newlines)) newline handling. ### 导航 - [索引](../genindex.xhtml "总目录") - [模块](../py-modindex.xhtml "Python 模块索引") | - [下一页](configparser.xhtml "configparser --- Configuration file parser") | - [上一页](fileformats.xhtml "文件格式") | - ![](https://box.kancloud.cn/a721fc7ec672275e257bbbfde49a4d4e_16x16.png) - [Python](https://www.python.org/) » - zh\_CN 3.7.3 [文档](../index.xhtml) » - [Python 标准库](index.xhtml) » - [文件格式](fileformats.xhtml) » - $('.inline-search').show(0); | © [版权所有](../copyright.xhtml) 2001-2019, Python Software Foundation. Python 软件基金会是一个非盈利组织。 [请捐助。](https://www.python.org/psf/donations/) 最后更新于 5月 21, 2019. [发现了问题](../bugs.xhtml)? 使用[Sphinx](http://sphinx.pocoo.org/)1.8.4 创建。