14.1. csv — CSV File Reading and Writing — Python 3.6.3 documentation (2024)

14.1.1. 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 astring 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 optionaldialect parameter can be given which is used to define a set of parametersspecific to a particular CSV dialect. It may be an instance of a subclass ofthe Dialect class or one of the strings returned by thelist_dialects() function. The other optional fmtparams keyword argumentscan be given to override individual formatting parameters in the currentdialect. For full details about the dialect and formatting parameters, seesection Dialects and Formatting Parameters.

Each row read from the csv file is returned as a list of strings. Noautomatic data type conversion is performed unless the QUOTE_NONNUMERIC formatoption 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 BeansSpam, Lovely Spam, Wonderful Spam

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

Return a writer object responsible for converting the user’s data into delimitedstrings on the given file-like object. csvfile can be any object with awrite() method. If csvfile is a file object, it should be opened withnewline='' [1]. An optional dialectparameter can be given which is used to define a set of parameters specific to aparticular CSV dialect. It may be an instance of a subclass of theDialect class or one of the strings returned by thelist_dialects() function. The other optional fmtparams keyword argumentscan be given to override individual formatting parameters in the currentdialect. For full details about the dialect and formatting parameters, seesection Dialects and Formatting Parameters. To make itas easy as possible to interface with modules which implement the DB API, thevalue None is written as the empty string. While this isn’t areversible transformation, it makes it easier to dump SQL NULL data values toCSV 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 csvwith 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. Thedialect can be specified either by passing a sub-class of Dialect, orby fmtparams keyword arguments, or both, with keyword arguments overridingparameters of the dialect. For full details about the dialect and formattingparameters, see section Dialects and Formatting Parameters.

csv.unregister_dialect(name)¶

Delete the dialect associated with name from the dialect registry. AnError is raised if name is not a registered dialect name.

csv.get_dialect(name)¶

Return the dialect associated with name. An Error is raised ifname is not a registered dialect name. This function returns an immutableDialect.

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 isgiven, 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 theinformation in each row to an OrderedDictwhose keys are given by the optional fieldnames parameter.

The fieldnames parameter is a sequence. If fieldnames isomitted, the values in the first row of file f will be used as thefieldnames. Regardless of how the fieldnames are determined, the ordereddictionary preserves their original ordering.

If a row has more fields than fieldnames, the remaining data is put in alist and stored with the fieldname specified by restkey (which defaultsto None). If a non-blank row has fewer fields than fieldnames, themissing values are filled-in with None.

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

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

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'])...Eric IdleJohn Cleese>>> print(row)OrderedDict([('first_name', 'John'), ('last_name', 'Cleese')])

See Also

Reading and Writing CSV Files in Python – Real PythonWorking with csv files in Python - GeeksforGeeksA Guide to the Python csv ModuleUsing the CSV module in Python - PythonForBeginners.com

class csv.DictWriter(f, fieldnames, restval='', extrasaction='raise', dialect='excel', *args, **kwds)¶

Create an object which operates like a regular writer but maps dictionariesonto output rows. The fieldnames parameter is a sequence of keys that identify the order in which values in thedictionary passed to the writerow() method are written to filef. The optional restval parameter specifies the value to bewritten if the dictionary is missing a key in fieldnames. If thedictionary passed to the writerow() method contains a key not found infieldnames, the optional extrasaction parameter indicates what action totake.If it is set to 'raise', the default value, a ValueErroris raised.If it is set to 'ignore', extra values in the dictionary are ignored.Any other optional or keyword arguments are passed to the underlyingwriter instance.

Note that unlike the DictReader class, the fieldnames parameterof the DictWriter is not optional. Since Python’s dictobjects are not ordered, there is not enough information available to deducethe order in which the row should be written to file f.

A short usage example:

import csvwith 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 itsattributes, which are used to define the parameters for a specificreader or writer instance.

class csv.excel¶

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

class csv.excel_tab¶

The excel_tab class defines the usual properties of an Excel-generatedTAB-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 filegenerated on UNIX systems, i.e. using '\n' as line terminator and quotingall 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 subclassreflecting the parameters found. If the optional delimiters parameteris given, it is interpreted as a string containing possible validdelimiter characters.

has_header(sample)¶

Analyze the sample text (presumed to be in CSV format) and returnTrue if the first row appears to be a series of column headers.

An example for Sniffer use:

with open('example.csv') 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 containspecial characters such as delimiter, quotechar or any of the characters inlineterminator.

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 currentdelimiter occurs in output data it is preceded by the current escapecharcharacter. If escapechar is not set, the writer will raise Error ifany 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.

14.1.2. Dialects and Formatting Parameters¶

To make it easier to specify the format of input and output records, specificformatting parameters are grouped together into dialects. A dialect is asubclass of the Dialect class having a set of specific methods and asingle validate() method. When creating reader orwriter objects, the programmer can specify a string or a subclass ofthe Dialect class as the dialect parameter. In addition to, or insteadof, the dialect parameter, the programmer can also specify individualformatting parameters, which have the same names as the attributes defined belowfor 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 shouldthemselves be quoted. When True, the character is doubled. WhenFalse, the escapechar is used as a prefix to the quotechar. Itdefaults 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 quotingis set to QUOTE_NONE and the quotechar if doublequote isFalse. On reading, the escapechar removes any special meaning fromthe following character. It defaults to None, which disables escaping.

Dialect.lineterminator¶

The string used to terminate lines produced by the writer. It defaultsto '\r\n'.

Note

The reader is hard-coded to recognise either '\r' or '\n' asend-of-line, and ignores lineterminator. This behavior may change in thefuture.

Dialect.quotechar¶

A one-character string used to quote fields containing special characters, suchas the delimiter or quotechar, or which contain new-line characters. Itdefaults to '"'.

Dialect.quoting¶

Controls when quotes should be generated by the writer and recognised by thereader. It can take on any of the QUOTE_* constants (see sectionModule 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.

14.1.3. Reader Objects¶

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

csvreader.__next__()¶

Return the next row of the reader’s iterable object as a list (if the objectwas returned from reader()) or a dict (if it is a DictReaderinstance), parsed according to the current dialect. Usually you should callthis 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 thenumber 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 isinitialized upon first access or when the first record is read from thefile.

14.1.4. Writer Objects¶

Writer objects (DictWriter instances and objects returned bythe writer() function) have the following public methods. A row must bean iterable of strings or numbers for Writer objects and a dictionarymapping fieldnames to strings or numbers (by passing them through str()first) for DictWriter objects. Note that complex numbers are writtenout surrounded by parens. This may cause some problems for other programs whichread CSV files (assuming they support complex numbers at all).

csvwriter.writerow(row)¶

Write the row parameter to the writer’s file object, formatted according tothe 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) tothe 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.

14.1.5. Examples¶

The simplest example of reading a CSV file:

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

Reading a file with an alternate format:

import csvwith 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 csvwith 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 filewill by default be decoded into unicode using the system defaultencoding (see locale.getpreferredencoding()). To decode a fileusing a different encoding, use the encoding argument of open:

import csvwith 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 defaultencoding: specify the encoding argument when opening the output file.

Registering a new dialect:

import csvcsv.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, sysfilename = '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 bedone:

import csvfor row in csv.reader(['one,two,three']): print(row)

Footnotes