| Python 3.6.5 Documentation > "csv" — CSV File Reading and Writing
"csv" — CSV File Reading and Writing
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**Source code:** Lib/csv.py
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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.
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.
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".
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.
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 the *rows* parameters (a list 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.
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.
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