14.1. "csv" — CSV File Reading and Writing
******************************************

**Source code:** Lib/csv.py

======================================================================

The so-called CSV (Comma Separated Values) format is the most common
import and export format for spreadsheets and databases.  CSV format
was used for many years prior to attempts to describe the format in a
standardized way in **RFC 4180**.  The lack of a well-defined standard
means that subtle differences often exist in the data produced and
consumed by different applications.  These differences can make it
annoying to process CSV files from multiple sources. Still, while the
delimiters and quoting characters vary, the overall format is similar
enough that it is possible to write a single module which can
efficiently manipulate such data, hiding the details of reading and
writing the data from the programmer.

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.

The "csv" module’s "reader" and "writer" objects read and write
sequences.  Programmers can also read and write data in dictionary
form using the "DictReader" and "DictWriter" classes.

See also:

  **PEP 305** - CSV File API
     The Python Enhancement Proposal which proposed this addition to
     Python.


14.1.1. Module Contents
=======================

The "csv" module defines the following functions:

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).

   A short usage example:

      >>> 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.

   A short usage example:

      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 an "OrderedDict" 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 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" instance.

   Changed in version 3.6: Returned rows are now of type
   "OrderedDict".

   A short usage example:

      >>> 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" 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" 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 file *f*.

   A short usage example:

      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'".

   New in version 3.2.

class csv.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.

An example for "Sniffer" use:

   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.


14.1.2. 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'".

   Note: 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 Module Contents) 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".


14.1.3. 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.


14.1.4. 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.

   Changed in version 3.5: 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).

   New in version 3.2.


14.1.5. Examples
================

The simplest example of reading a CSV file:

   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)

-[ Footnotes ]-

[1] 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.
