## 12.20.1 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. 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 more information about the dialect and formatting parameters, see section , ``Dialects and Formatting Parameters'' for details of these parameters.

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

`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 more information about the dialect and formatting parameters, see section , ``Dialects and Formatting Parameters'' for details of these 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)

  Associate dialect with name. dialect must be a subclass of `csv.Dialect`. name must be a string or Unicode object.

`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.

`list_dialects`( | )

  Return the names of all registered dialects.

The `csv` module defines the following classes:

class `DictReader`( | csvfile, fieldnames[, restkey=`None`[, restval=`None`[, dialect=`'excel'`[, fmtparam]]]])

  Create an object which operates like a regular reader but maps the information read into a dict whose keys are given by the fieldnames parameter. 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 optiona restval parameter. All other parameters are interpreted as for `reader` objects.

class `DictWriter`( | csvfile, fieldnames[, restval=""[, extrasaction=`'raise'`[, dialect=`'excel'`[, fmtparam]]]])

  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. All other parameters are interpreted as for `writer` objects.

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 `Sniffer`( | )

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

The `Sniffer` class provides a single method:

`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 the current delimiter or begin with the current quotechar.

`QUOTE_NONNUMERIC`
  Instructs `writer` objects to quote all non-numeric fields.

`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. When `QUOTE_NONE` is in effect, it is an error not to have a single-character escapechar defined, even if no data to be written contains the delimiter character.

The `csv` module defines the following exception:

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