Module Contents¶

Thecsv module defines the following functions:

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

Return areader object that will processlines from the givencsvfile. A csvfile must be an iterable ofstrings, each in the reader’s defined csv format.A csvfile is most commonly a file-like object or list.Ifcsvfile is a file object,it should be opened withnewline=''.[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 oftheDialect class or one of the strings returned by thelist_dialects() function. The other optionalfmtparams keyword argumentscan be given to override individual formatting parameters in the currentdialect. For full details about the dialect and formatting parameters, seesectionDialects and Formatting Parameters.

Each row read from the csv file is returned as a list of strings. Noautomatic data type conversion is performed unless theQUOTE_NONNUMERIC formatoption 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 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. Ifcsvfile is a file object, it should be opened withnewline=''[1]. An optionaldialectparameter 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 optionalfmtparams keyword argumentscan be given to override individual formatting parameters in the currentdialect. For full details about dialects and formatting parameters, seetheDialects and Formatting Parameters section. To make itas easy as possible to interface with modules which implement the DB API, thevalueNone 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 acursor.fetch* call.All other non-string data are stringified withstr() 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'])

csv.register_dialect(name[,dialect[,**fmtparams]])¶

Associatedialect withname.name must be a string. Thedialect can be specified either by passing a sub-class ofDialect, orbyfmtparams keyword arguments, or both, with keyword arguments overridingparameters of the dialect. For full details about dialects and formattingparameters, see sectionDialects and Formatting Parameters.

csv.unregister_dialect(name)¶

Delete the dialect associated withname from the dialect registry. AnError is raised ifname is not a registered dialect name.

csv.get_dialect(name)¶

Return the dialect associated withname. AnError 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. Ifnew_limit isgiven, this becomes the new limit.

Thecsv 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 theinformation in each row to adict whose keys are given by theoptionalfieldnames parameter.

Thefieldnames parameter is asequence. Iffieldnames isomitted, the values in the first row of filef will be used as thefieldnames and will be omitted from the results. Iffieldnames is provided, they will be used and the first row will beincluded 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 alist and stored with the fieldname specified byrestkey (which defaultstoNone). If a non-blank row has fewer fields than fieldnames, themissing values are filled-in with the value ofrestval (which defaultstoNone).

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

If the argument passed tofieldnames is an iterator, it will be coerced to alist.

Changed in version 3.6:Returned rows are now of typeOrderedDict.

Changed in version 3.8:Returned rows are now of typedict.

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'}

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

Create an object which operates like a regular writer but maps dictionariesonto output rows. Thefieldnames parameter is asequence of keys that identify the order in which values in thedictionary passed to thewriterow() method are written to filef. The optionalrestval parameter specifies the value to bewritten if the dictionary is missing a key infieldnames. If thedictionary passed to thewriterow() method contains a key not found infieldnames, the optionalextrasaction parameter indicates what action totake.If it is set to'raise', the default value, aValueErroris 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 theDictReader class, thefieldnames parameterof theDictWriter class is not optional.

If the argument passed tofieldnames is an iterator, it will be coerced to alist.

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'})

classcsv.Dialect¶

TheDialect class is a container class whose attributes containinformation for how to handle doublequotes, whitespace, delimiters, etc.Due to the lack of a strict CSV specification, different applicationsproduce subtly different CSV data.Dialect instances define howreader andwriter instances behave.

All availableDialect names are returned bylist_dialects(),and they can be registered with specificreader andwriterclasses through their initializer (__init__) functions like this:

importcsvwithopen('students.csv','w',newline='')ascsvfile:writer=csv.writer(csvfile,dialect='unix')

classcsv.excel¶

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

classcsv.excel_tab¶

Theexcel_tab class defines the usual properties of an Excel-generatedTAB-delimited file. It is registered with the dialect name'excel-tab'.

classcsv.unix_dialect¶

Theunix_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'.

Added in version 3.2.

classcsv.Sniffer¶

TheSniffer class is used to deduce the format of a CSV file.

TheSniffer class provides two methods:

sniff(sample,delimiters=None)¶

Analyze the givensample and return aDialect subclassreflecting the parameters found. If the optionaldelimiters 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.Inspecting each column, one of two key criteria will be considered toestimate 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’slength differs from that of the putative header of that column.

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

Note

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

An example forSniffer 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 ...

Thecsv module defines the following constants:

csv.QUOTE_ALL¶

Instructswriter objects to quote all fields.

csv.QUOTE_MINIMAL¶

Instructswriter objects to only quote those fields which containspecial characters such asdelimiter,quotechar or any of the characters inlineterminator.

csv.QUOTE_NONNUMERIC¶

Instructswriter objects to quote all non-numeric fields.

Instructsreader objects to convert all non-quoted fields to typefloat.

csv.QUOTE_NONE¶

Instructswriter objects to never quote fields. When the currentdelimiter occurs in output data it is preceded by the currentescapecharcharacter. Ifescapechar is not set, the writer will raiseError ifany characters that require escaping are encountered.

Instructsreader objects to perform no special processing of quote characters.

csv.QUOTE_NOTNULL¶

Instructswriter objects to quote all fields which are notNone. This is similar toQUOTE_ALL, except that if afield value isNone an empty (unquoted) string is written.

Instructsreader objects to interpret an empty (unquoted) fieldasNone and to otherwise behave asQUOTE_ALL.

Added in version 3.12.

csv.QUOTE_STRINGS¶

Instructswriter objects to always place quotes around fieldswhich are strings. This is similar toQUOTE_NONNUMERIC, except that if afield value isNone an empty (unquoted) string is written.

Instructsreader objects to interpret an empty (unquoted) string asNone andto otherwise behave asQUOTE_NONNUMERIC.

Added in version 3.12.

Thecsv 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, specificformatting parameters are grouped together into dialects. A dialect is asubclass of theDialect class containing various attributesdescribing the format of the CSV file. When creatingreader orwriter objects, the programmer can specify a string or a subclass oftheDialect class as the dialect parameter. In addition to, or insteadof, thedialect parameter, the programmer can also specify individualformatting parameters, which have the same names as the attributes defined belowfor theDialect class.

Dialects support the following attributes:

Dialect.delimiter¶

A one-character string used to separate fields. It defaults to','.

Dialect.doublequote¶

Controls how instances ofquotechar appearing inside a field shouldthemselves be quoted. WhenTrue, the character is doubled. WhenFalse, theescapechar is used as a prefix to thequotechar. Itdefaults toTrue.

On output, ifdoublequote isFalse and noescapechar is set,Error is raised if aquotechar is found in a field.

Dialect.escapechar¶

A one-character string used by the writer to escape thedelimiter ifquotingis set toQUOTE_NONE and thequotechar ifdoublequote isFalse. On reading, theescapechar removes any special meaning fromthe following character. It defaults toNone, which disables escaping.

Changed in version 3.11:An emptyescapechar is not allowed.

Dialect.lineterminator¶

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

Note

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

Dialect.quotechar¶

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

Changed in version 3.11:An emptyquotechar is not allowed.

Dialect.quoting¶

Controls when quotes should be generated by the writer and recognised by thereader. It can take on any of theQUOTE_* constantsand defaults toQUOTE_MINIMAL.

Dialect.skipinitialspace¶

WhenTrue, spaces immediately following thedelimiter are ignored.The default isFalse.

Dialect.strict¶

WhenTrue, raise exceptionError on bad CSV input.The default isFalse.

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 fromreader()) or a dict (if it is aDictReaderinstance), parsed according to the currentDialect. Usually youshould call this asnext(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:

DictReader.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.

Writer Objects¶

writer objects (DictWriter instances and objects returned bythewriter() function) have the following public methods. Arow must bean iterable of strings or numbers forwriter objects and a dictionarymapping fieldnames to strings or numbers (by passing them throughstr()first) forDictWriter 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 therow parameter to the writer’s file object, formatted accordingto the currentDialect. Return the return value of the call to thewrite method of the underlying file object.

Changed in version 3.5:Added support of arbitrary iterables.

csvwriter.writerows(rows)¶

Write all elements inrows (an iterable ofrow objects as describedabove) to the writer’s file object, formatted according to the currentdialect.

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

Added in version 3.2.

Changed in version 3.8:writeheader() now also returns the value returned bythecsvwriter.writerow() 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)

Sinceopen() is used to open a CSV file for reading, the filewill by default be decoded into unicode using the system defaultencoding (seelocale.getencoding()). To decode a fileusing a different encoding, use theencoding 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 defaultencoding: 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('file{}, line{}:{}'.format(filename,reader.line_num,e))

And while the module doesn’t directly support parsing strings, it can easily bedone:

importcsvforrowincsv.reader(['one,two,three']):print(row)

Footnotes

Ifnewline='' is not specified, newlines embedded inside quoted fieldswill not be interpreted correctly, and on platforms that use\r\n linendingson write an extra\r will be added. It should always be safe to specifynewline='', since the csv module does its own(universal) newline handling.