Module contents¶

The csv module defines the following functions:

csv.reader(csvfile, /, dialect='excel', **fmtparams)¶

Return a reader object that will process lines from the given csvfile. A csvfile must be an iterable of strings, each in the reader’s defined csv format. A csvfile is most commonly a file-like object or list. 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

where eggs.csv contains:

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 dialects and formatting parameters, see the Dialects and formatting parameters section. 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'])

which writes eggs.csv containing:

Spam Spam Spam Spam Spam |Baked Beans|
Spam |Lovely Spam| |Wonderful Spam|

csv.register_dialect(name, /, dialect='excel', **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 dialects 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()¶

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 a dict 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 and will be omitted from the results. If fieldnames is provided, they will be used and the first row will be included in the results. Regardless of how the fieldnames are determined, the 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 the value of restval (which defaults to None).

All other optional or keyword arguments are passed to the underlying reader instance.

If the argument passed to fieldnames is an iterator, it will be coerced to a list.

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

Changed in version 3.8: Returned rows are now of type dict.

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)
{'first_name': 'John', 'last_name': 'Cleese'}

where names.csv contains:

first_name,last_name
Eric,Idle
John,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 class is not optional.

If the argument passed to fieldnames is an iterator, it will be coerced to a list.

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'})

which writes names.csv containing:

first_name,last_name
Baked,Beans
Lovely,Spam
Wonderful,Spam

class csv.Dialect¶

The Dialect class is a container class whose attributes contain information for how to handle doublequotes, whitespace, delimiters, etc. Due to the lack of a strict CSV specification, different applications produce subtly different CSV data. Dialect instances define how reader and writer instances behave.

All available Dialect names are returned by list_dialects(), and they can be registered with specific reader and writer classes through their initializer (__init__) functions like this:

import csv

with open('students.csv', 'w', newline='') as csvfile:
    writer = csv.writer(csvfile, dialect='unix')

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'.

Added 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.

The dialect is deduced by parsing the sample with every plausible combination of parameters and choosing the combination which splits the sample into rows with the most consistent number of fields, so the returned dialect is consistent with how reader() will parse the sample. Raise Error if no combination fits the sample, in particular if it is a single column, so there is no delimiter to find.

If several combinations fit the sample equally well — for example if both ',' and ';' split every row consistently — the delimiters listed in the preferred attribute are preferred, in that order, no matter how many times each of them occurs.

The lineterminator parameter is deduced separately, by a majority vote among the line endings of the sample. A tie is broken in the order '\r\n', '\n', '\r', so a sample without a complete line gives '\r\n'.

Changed in version 3.16: The dialect is now deduced by trial parsing and the results may differ from those of earlier Python versions. The escapechar and lineterminator parameters can now be detected, and the requested delimiters are not restricted to ASCII.

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. Inspecting each column, one of two key criteria will be considered to estimate if the sample contains a header:

• the second through n-th rows contain numeric values

• the second through n-th rows contain strings where at least one value’s length differs from that of the putative header of that column.

Twenty-one rows after the header are sampled; if more than half of the columns + rows meet the criteria, True is returned.

Note

This method is a rough heuristic and may produce both false positives and negatives.

The Sniffer class has the following attribute:

preferred¶

The list of the delimiters preferred for breaking ties, in the order of preference. It can be modified. Its initial value is [',', '\t', ';', ' ', ':'].

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, '\r', '\n' or any of the characters in lineterminator. If doublequote is False and escapechar is set, the quotechar is escaped instead of causing the field to be quoted.

csv.QUOTE_NONNUMERIC¶

Instructs writer objects to quote all non-numeric fields.

Instructs reader objects to convert all non-quoted fields to type float.

Note

Some numeric types, such as bool, Fraction, or IntEnum, have a string representation that cannot be converted to float. They cannot be read in the QUOTE_NONNUMERIC and QUOTE_STRINGS modes.

csv.QUOTE_NONE¶

Instructs writer objects to never quote fields. When the current delimiter, quotechar, escapechar, '\r', '\n' or any of the characters in lineterminator 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. Set quotechar to None to prevent its escaping.

Instructs reader objects to perform no special processing of quote characters.

csv.QUOTE_NOTNULL¶

Instructs writer objects to quote all fields which are not None. This is similar to QUOTE_ALL, except that if a field value is None an empty (unquoted) string is written.

Instructs reader objects to interpret an empty (unquoted) field as None and to otherwise behave as QUOTE_ALL.

Added in version 3.12.

csv.QUOTE_STRINGS¶

Instructs writer objects to always place quotes around fields which are strings. This is similar to QUOTE_NONNUMERIC, except that if a field value is None an empty (unquoted) string is written.

Instructs reader objects to interpret an empty (unquoted) string as None and to otherwise behave as QUOTE_NONNUMERIC.

Added in version 3.12.

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 containing various attributes describing the format of the CSV file. 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 characters that require escaping:

• the delimiter, the quotechar, '\r', '\n' and any of the characters in lineterminator are escaped if quoting is set to QUOTE_NONE;

• the quotechar is escaped if doublequote is False;

• the escapechar itself.

On reading, the escapechar removes any special meaning from the following character. It defaults to None, which disables escaping.

Changed in version 3.10: Previously the escapechar itself was not escaped, which lost it on reading.

Changed in version 3.11: An empty escapechar is not allowed.

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 the quotechar, or which contain new-line characters ('\r', '\n' or any of the characters in lineterminator). It defaults to '"'. Can be set to None to prevent escaping '"' if quoting is set to QUOTE_NONE.

Changed in version 3.11: An empty quotechar is not allowed.

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 and defaults to QUOTE_MINIMAL if quotechar is not None, and QUOTE_NONE otherwise.

Dialect.skipinitialspace¶

When True, spaces immediately following the delimiter are ignored. The default is False. When combining delimiter=' ' with skipinitialspace=True, unquoted empty fields are not allowed.

Dialect.strict¶

When True, raise exception Error on bad CSV input. The default is False.

Dialects support copy.replace(), which returns a copy of the dialect with the specified formatting parameters replaced.

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:

DictReader.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. Return the return value of the call to the write method of the underlying file object.

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) to the writer’s file object, formatted according to the current dialect. Return the return value of the csvwriter.writerow() call used internally.

Added in version 3.2.

Changed in version 3.8: writeheader() now also returns the value returned by the csvwriter.writerow() method it uses internally.

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 UTF-8. To decode a file using a different encoding, use the encoding argument of open:

import csv
with open('some.csv', newline='', encoding='latin-1') as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

The same applies to writing in something other than the 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(f'file {filename}, line {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