Source code:
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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
. 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
and
objects read and write sequences. Programmers can also read and write data in dictionary form using the
and
classes.
See also
- CSV File APIThe 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
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=''.
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
class or one of the strings returned by the
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
format option is specified (in which case unquoted fields are transformed into floats).
A short usage example:
>>> importcsv>>> withopen('eggs.csv',newline='')ascsvfile:... spamreader=csv.reader(csvfile,delimiter=' ',quotechar='|')... forrowinspamreader:... print(', '.join(row))Spam, Spam, Spam, Spam, Spam, Baked BeansSpam, Lovely Spam, Wonderful Spamwhere 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
method. If csvfile is a file object, it should be opened with newline=''
. 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
class or one of the strings returned by the
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
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
before being written.
A short usage example:
importcsvwithopen('eggs.csv','w',newline='')ascsvfile: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
, 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
is raised if name is not a registered dialect name.
csv.get_dialect(name)
Return the dialect associated with name. An
is raised if name is not a registered dialect name. This function returns an immutable
.
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:
classcsv.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
whose keys are given by the optional fieldnames parameter.
The fieldnames parameter is a
. 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
instance.
If the argument passed to fieldnames is an iterator, it will be coerced to a
.
Changed in version 3.6: Returned rows are now of type OrderedDict.
Changed in version 3.8: Returned rows are now of type
.
A short usage example:
>>> importcsv>>> withopen('names.csv',newline='')ascsvfile:... reader=csv.DictReader(csvfile)... forrowinreader:... print(row['first_name'],row['last_name'])...Eric IdleJohn Cleese>>> print(row){'first_name': 'John', 'last_name': 'Cleese'}where names.csv contains:
first_name,last_name Eric,Idle John,Cleese classcsv.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
of keys that identify the order in which values in the dictionary passed to the
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
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
instance.
Note that unlike the
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
.
A short usage example:
importcsvwithopen('names.csv','w',newline='')ascsvfile: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 classcsv.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
and
instances behave.
All available Dialect names are returned by
, and they can be registered with specific
and
classes through their initializer (__init__) functions like this:
importcsvwithopen('students.csv','w',newline='')ascsvfile:writer=csv.writer(csvfile,dialect='unix')classcsv.excel
The excel class defines the usual properties of an Excel-generated CSV file. It is registered with the dialect name 'excel'.
classcsv.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'.
classcsv.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.
classcsv.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
subclass reflecting the parameters found. If the optional delimiters parameter is given, it is interpreted as a string containing possible valid delimiter characters.
If several delimiters fit the sample equally well — for example if both ',' and ';' split every row consistently — the delimiters listed in the
attribute are preferred, in that order, no matter how many times each of them occurs.
Analyze the sample text (presumed to be in CSV format) and return
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,
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
use:
withopen('example.csv',newline='')ascsvfile: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
objects to quote all fields.
csv.QUOTE_MINIMAL
Instructs
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
and escapechar is set, the quotechar is escaped instead of causing the field to be quoted.
csv.QUOTE_NONNUMERIC
Instructs
objects to quote all non-numeric fields.
Instructs
objects to convert all non-quoted fields to type
.
Note
Some numeric types, such as
,
, or
, have a string representation that cannot be converted to
. They cannot be read in the QUOTE_NONNUMERIC and
modes.
csv.QUOTE_NONE
Instructs
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
if any characters that require escaping are encountered. Set quotechar to None to prevent its escaping.
Instructs
objects to perform no special processing of quote characters.
csv.QUOTE_NOTNULL
Instructs
objects to quote all fields which are not None. This is similar to
, except that if a field value is None an empty (unquoted) string is written.
Instructs
objects to interpret an empty (unquoted) field as None and to otherwise behave as
.
Added in version 3.12.
csv.QUOTE_STRINGS
Instructs
objects to always place quotes around fields which are strings. This is similar to
, except that if a field value is None an empty (unquoted) string is written.
Instructs
objects to interpret an empty (unquoted) string as None and to otherwise behave as
.
Added in version 3.12.
The csv module defines the following exception:
exceptioncsv.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
class containing various attributes describing the format of the CSV file. When creating
or
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
, the character is doubled. When
, the escapechar is used as a prefix to the quotechar. It defaults to True.
On output, if doublequote is
and no escapechar is set,
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
;
the quotechar is escaped if doublequote is
;
the escapechar itself.
On reading, the escapechar removes any special meaning from the following character. It defaults to
, 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
. It defaults to '\r\n'.
Note
The
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
.
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
and defaults to
if quotechar is not None, and
otherwise.
Dialect.skipinitialspace
When
, spaces immediately following the delimiter are ignored. The default is
. When combining delimiter='' with skipinitialspace=True, unquoted empty fields are not allowed.
Dialect.strict
When True, raise exception
on bad CSV input. The default is False.
Reader Objects
Reader objects (
instances and objects returned by the
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
) or a dict (if it is a
instance), parsed according to the current
. 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
objects (
instances and objects returned by the
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
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
. 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:
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
call used internally.
Added in version 3.2.
Changed in version 3.8: writeheader() now also returns the value returned by the
method it uses internally.
Examples
The simplest example of reading a CSV file:
importcsvwithopen('some.csv',newline='')asf:reader=csv.reader(f)forrowinreader:print(row)Reading a file with an alternate format:
importcsvwithopen('passwd',newline='')asf:reader=csv.reader(f,delimiter=':',quoting=csv.QUOTE_NONE)forrowinreader:print(row)The corresponding simplest possible writing example is:
importcsvwithopen('some.csv','w',newline='')asf:writer=csv.writer(f)writer.writerows(someiterable)Since
is used to open a CSV file for reading, the file will by default be decoded into unicode using the system default encoding (see
). To decode a file using a different encoding, use the encoding argument of open:
importcsvwithopen('some.csv',newline='',encoding='utf-8')asf:reader=csv.reader(f)forrowinreader: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:
importcsvcsv.register_dialect('unixpwd',delimiter=':',quoting=csv.QUOTE_NONE)withopen('passwd',newline='')asf:reader=csv.reader(f,'unixpwd')A slightly more advanced use of the reader — catching and reporting errors:
importcsv,sysfilename='some.csv'withopen(filename,newline='')asf:reader=csv.reader(f)try:forrowinreader:print(row)exceptcsv.Errorase: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:
importcsvforrowincsv.reader(['one,two,three']):print(row)Footnotes