9.1.1 Module Contents

The csv module defines the following functions:

reader( csvfile[, dialect='excel'][, fmtparam])

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 must be opened with the 'b' flag on platforms where that makes a difference. 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 fmtparam 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 9.1.2, ``Dialects and Formatting Parameters''.

All data read are returned as strings. No automatic data type conversion is performed.

Changed in version 2.5: The parser is now stricter with respect to multi-line quoted fields. Previously, if a line ended within a quoted field without a terminating newline character, a newline would be inserted into the returned field. This behavior caused problems when reading files which contained carriage return characters within fields. The behavior was changed to return the field without inserting newlines. As a consequence, if newlines embedded within fields are important, the input should be split into lines in a manner which preserves the newline characters.

writer( csvfile[, dialect='excel'][, fmtparam]): 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 must be opened with the 'b' flag on platforms where that makes a difference. 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 fmtparam 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 9.1.2, ``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.

register_dialect( name[, dialect][, fmtparam]): Associate dialect with name. name must be a string or Unicode object. The dialect can be specified either by passing a sub-class of Dialect, or by fmtparam keyword arguments, or both, with keyword arguments overriding parameters of the dialect. For full details about the dialect and formatting parameters, see section 9.1.2, ``Dialects and Formatting Parameters''.

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.

get_dialect( name): Return the dialect associated with name. An Error is raised if name is not a registered dialect name.
Changed in version 2.5: This function now returns an immutable Dialect. Previously an instance of the requested dialect was returned. Users could modify the underlying class, changing the behavior of active readers and writers..

list_dialects( ): Return the names of all registered dialects.

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. New in version 2.5.

The csv module defines the following classes:

class DictReader( csvfile[, fieldnames=None,[, restkey=None[, restval=None[, dialect='excel'[, *args, **kwds]]]]]): Create an object which operates like a regular reader but maps the information read into a dict whose keys are given by the optional fieldnames parameter. If the fieldnames parameter is omitted, the values in the first row of the csvfile will be used as the fieldnames. If the row read has fewer fields than the fieldnames sequence, the value of restval will be used as the default value. If the row read has more fields than the fieldnames sequence, the remaining data is added as a sequence keyed by the value of restkey. If the row read has fewer fields than the fieldnames sequence, the remaining keys take the value of the optional restval parameter. Any other optional or keyword arguments are passed to the underlying reader instance.

class DictWriter( csvfile, 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 identifies the order in which values in the dictionary passed to the writerow() method are written to the csvfile. 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' 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 is not optional. Since Python's dict objects are not ordered, there is not enough information available to deduce the order in which the row should be written to the csvfile.

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

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

The Sniffer class provides two methods:

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.

The csv module defines the following constants:

QUOTE_ALL: Instructs writer objects to quote all fields.

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.

QUOTE_NONNUMERIC: Instructs writer objects to quote all non-numeric fields.
Instructs the reader to convert all non-quoted fields to type float.

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

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