# features/pep-420 / Doc / library / csv.rst

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# :mod:csv --- CSV File Reading and Writing

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 :mod: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 :mod:csv module's :class:reader and :class:writer objects read and write sequences. Programmers can also read and write data in dictionary form using the :class:DictReader and :class:DictWriter classes.

Note

This version of the :mod: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 :ref:csv-examples. These restrictions will be removed in the future.

## Module Contents

The :mod:csv module defines the following functions:

The :mod:csv module defines the following classes:

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 :class:reader instance.

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 :meth: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 :meth: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 :exc: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 :class:writer instance.

Note that unlike the :class:DictReader class, the fieldnames parameter of the :class:DictWriter is not optional. Since Python's :class: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.

The :class:Dialect class is a container class relied on primarily for its attributes, which are used to define the parameters for a specific :class:reader or :class:writer instance.

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

The :class:excel_tab class defines the usual properties of an Excel-generated TAB-delimited file. It is registered with the dialect name 'excel-tab'.

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

The :class:Sniffer class provides two methods:

An example for :class:Sniffer use:

csvfile = open("example.csv")
csvfile.seek(0)
# ... process CSV file contents here ...


The :mod:csv module defines the following constants:

The :mod:csv module defines the following exception:

## 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 :class:Dialect class having a set of specific methods and a single :meth:validate method. When creating :class:reader or :class:writer objects, the programmer can specify a string or a subclass of the :class: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 :class:Dialect class.

Dialects support the following attributes:

Reader objects (:class:DictReader instances and objects returned by the :func:reader function) have the following public methods:

Reader objects have the following public attributes:

DictReader objects have the following public attribute:

## Writer Objects

:class:Writer objects (:class:DictWriter instances and objects returned by the :func:writer function) have the following public methods. A row must be a sequence of strings or numbers for :class:Writer objects and a dictionary mapping fieldnames to strings or numbers (by passing them through :func:str first) for :class: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).

Writer objects have the following public attribute:

## Examples

The simplest example of reading a CSV file:

import csv
print(row)


Reading a file with an alternate format:

import csv
print(row)


The corresponding simplest possible writing example is:

import csv
writer = csv.writer(open("some.csv", "wb"))
writer.writerows(someiterable)


Registering a new dialect:

import csv

csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE)



A slightly more advanced use of the reader --- catching and reporting errors:

import csv, sys
filename = "some.csv"
try:
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
print(row)


The :mod: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.

:func:unicode_csv_reader below is a :term:generator that wraps :class:csv.reader to handle Unicode CSV data (a list of Unicode strings). :func:utf_8_encoder is a :term: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 :func:unicode_csv_reader decodes the UTF-8-encoded cells back into Unicode:

import csv

# csv.py doesn't do Unicode; encode temporarily as UTF-8:
dialect=dialect, **kwargs)
# 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 :class:UnicodeReader and :class: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, io

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

def __iter__(self):
return self

def __next__(self):

"""
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)

def __next__(self):
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 = io.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)

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