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

New in version 2.3.

The so-called CSV (Comma Separated Values) format is the most common
import and export format for spreadsheets and databases.  There is no
“CSV standard”, so the format is operationally defined by the many
applications which read and write it.  The lack of a 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.

Note: This version of the "csv" module doesn’t support Unicode
  input. Also, there are currently some issues regarding ASCII NUL
  characters.  Accordingly, all input should be UTF-8 or printable
  ASCII to be safe; see the examples in section Examples.

See also:

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


13.1.1. 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 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 *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.

   A short usage example:

      >>> import csv
      >>> with open('eggs.csv', 'rb') 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

   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.

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 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 *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. Floats
   are stringified with "repr()" before being written. All other non-
   string data are stringified with "str()" before being written.

   A short usage example:

      import csv
      with open('eggs.csv', 'wb') 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 or
   Unicode object. 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.

   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.

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.

   New in version 2.5.

The "csv" module defines the following classes:

class csv.DictReader(f, 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.  The *fieldnames* parameter is a sequence
   whose elements are associated with the fields of the input data in
   order. These elements become the keys of the resulting dictionary.
   If the *fieldnames* parameter is omitted, the values in the first
   row of the file *f* will be used as the fieldnames.  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.

   A short usage example:

      >>> import csv
      >>> with open('names.csv') as csvfile:
      ...     reader = csv.DictReader(csvfile)
      ...     for row in reader:
      ...         print(row['first_name'], row['last_name'])
      ...
      Baked Beans
      Lovely Spam
      Wonderful Spam

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 the
   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'" 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 file
   *f*.

   A short usage example:

      import csv

      with open('names.csv', 'w') 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.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', 'rb') 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.


13.1.2. 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".


13.1.3. 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,
   parsed according to the current dialect.

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.

   New in version 2.5.

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.

   Changed in version 2.6.


13.1.4. Writer Objects
======================

"Writer" objects ("DictWriter" instances and objects returned by the
"writer()" function) have the following public methods.  A *row* must
be a sequence 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.

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 2.7.


13.1.5. Examples
================

The simplest example of reading a CSV file:

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

Reading a file with an alternate format:

   import csv
   with open('passwd', 'rb') 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', 'wb') as f:
       writer = csv.writer(f)
       writer.writerows(someiterable)

Registering a new dialect:

   import csv
   csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE)
   with open('passwd', 'rb') 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, 'rb') as f:
       reader = csv.reader(f)
       try:
           for row in reader:
               print row
       except csv.Error as e:
           sys.exit('file %s, line %d: %s' % (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

The "csv" module doesn’t directly support reading and writing Unicode,
but it is 8-bit-clean save for some problems with ASCII NUL
characters.  So you can write functions or classes that handle the
encoding and decoding for you as long as you avoid encodings like
UTF-16 that use NULs.  UTF-8 is recommended.

"unicode_csv_reader()" below is a *generator* that wraps "csv.reader"
to handle Unicode CSV data (a list of Unicode strings).
"utf_8_encoder()" is a *generator* that encodes the Unicode strings as
UTF-8, one string (or row) at a time.  The encoded strings are parsed
by the CSV reader, and "unicode_csv_reader()" decodes the
UTF-8-encoded cells back into Unicode:

   import csv

   def unicode_csv_reader(unicode_csv_data, dialect=csv.excel, **kwargs):
       # csv.py doesn't do Unicode; encode temporarily as UTF-8:
       csv_reader = csv.reader(utf_8_encoder(unicode_csv_data),
                               dialect=dialect, **kwargs)
       for row in csv_reader:
           # decode UTF-8 back to Unicode, cell by cell:
           yield [unicode(cell, 'utf-8') for cell in row]

   def utf_8_encoder(unicode_csv_data):
       for line in unicode_csv_data:
           yield line.encode('utf-8')

For all other encodings the following "UnicodeReader" and
"UnicodeWriter" classes can be used. They take an additional
*encoding* parameter in their constructor and make sure that the data
passes the real reader or writer encoded as UTF-8:

   import csv, codecs, cStringIO

   class UTF8Recoder:
       """
       Iterator that reads an encoded stream and reencodes the input to UTF-8
       """
       def __init__(self, f, encoding):
           self.reader = codecs.getreader(encoding)(f)

       def __iter__(self):
           return self

       def next(self):
           return self.reader.next().encode("utf-8")

   class UnicodeReader:
       """
       A CSV reader which will iterate over lines in the CSV file "f",
       which is encoded in the given encoding.
       """

       def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
           f = UTF8Recoder(f, encoding)
           self.reader = csv.reader(f, dialect=dialect, **kwds)

       def next(self):
           row = self.reader.next()
           return [unicode(s, "utf-8") for s in row]

       def __iter__(self):
           return self

   class UnicodeWriter:
       """
       A CSV writer which will write rows to CSV file "f",
       which is encoded in the given encoding.
       """

       def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
           # Redirect output to a queue
           self.queue = cStringIO.StringIO()
           self.writer = csv.writer(self.queue, dialect=dialect, **kwds)
           self.stream = f
           self.encoder = codecs.getincrementalencoder(encoding)()

       def writerow(self, row):
           self.writer.writerow([s.encode("utf-8") for s in row])
           # Fetch UTF-8 output from the queue ...
           data = self.queue.getvalue()
           data = data.decode("utf-8")
           # ... and reencode it into the target encoding
           data = self.encoder.encode(data)
           # write to the target stream
           self.stream.write(data)
           # empty queue
           self.queue.truncate(0)

       def writerows(self, rows):
           for row in rows:
               self.writerow(row)
