\section{\module{csv} --- CSV File Reading and Writing} \declaremodule{standard}{csv}\modulesynopsis{Write and read tabular data to and from delimited files.}\sectionauthor{Skip Montanaro}{skip@pobox.com} \versionadded{2.3}\index{csv}\indexii{data}{tabular} The so-called CSV (Comma Separated Values) format is the most common importand export format for spreadsheets and databases. There is no ``CSVstandard'', so the format is operationally defined by the many applicationswhich read and write it. The lack of a standard means that subtledifferences often exist in the data produced and consumed by differentapplications. These differences can make it annoying to process CSV filesfrom multiple sources. Still, while the delimiters and quoting charactersvary, the overall format is similar enough that it is possible to write asingle module which can efficiently manipulate such data, hiding the detailsof reading and writing the data from the programmer. The \module{csv} module implements classes to read and write tabular data inCSV format. It allows programmers to say, ``write this data in the formatpreferred by Excel,'' or ``read data from this file which was generated byExcel,'' without knowing the precise details of the CSV format used byExcel. Programmers can also describe the CSV formats understood by otherapplications or define their own special-purpose CSV formats. The \module{csv} module's \class{reader} and \class{writer} objects read andwrite sequences. Programmers can also read and write data in dictionaryform using the \class{DictReader} and \class{DictWriter} classes. \begin{notice} This version of the \module{csv} module doesn't support Unicode input. Also, there are currently some issues regarding \ASCII{} NUL characters. Accordingly, all input should be UTF-8 or printable \ASCII{} to be safe; see the examples in section~\ref{csv-examples}. These restrictions will be removed in the future.\end{notice} \begin{seealso}% \seemodule{array}{Arrays of uniformly types numeric values.} \seepep{305}{CSV File API} {The Python Enhancement Proposal which proposed this addition to Python.}\end{seealso} \subsection{Module Contents \label{csv-contents}} The \module{csv} module defines the following functions: \begin{funcdesc}{reader}{csvfile\optional{, dialect=\code{'excel'}}\optional{, fmtparam}}Return a reader object which will iterate over lines in the given{}\var{csvfile}. \var{csvfile} can be any object which supports theiterator protocol and returns a string each time its \method{next}method is called --- file objects and list objects are both suitable. If \var{csvfile} is a file object, it must be opened withthe 'b' flag on platforms where that makes a difference. An optional{}\var{dialect} parameter can be givenwhich is used to define a set of parameters specific to a particular CSVdialect. It may be an instance of a subclass of the \class{Dialect}class or one of the strings returned by the \function{list_dialects}function. The other optional {}\var{fmtparam} keyword arguments can begiven to override individual formatting parameters in the currentdialect. For full details about the dialect and formattingparameters, see section~\ref{csv-fmt-params}, ``Dialects and FormattingParameters''. All data read are returned as strings. No automatic data typeconversion is performed. \versionchanged[The parser is now stricter with respect to multi-line quotedfields. Previously, if a line ended within a quoted field without aterminating newline character, a newline would be inserted into thereturned field. This behavior caused problems when reading fileswhich contained carriage return characters within fields. Thebehavior was changed to return the field without inserting newlines. Asa consequence, if newlines embedded within fields are important, theinput should be split into lines in a manner which preserves the newlinecharacters]{2.5} \end{funcdesc} \begin{funcdesc}{writer}{csvfile\optional{, dialect=\code{'excel'}}\optional{, fmtparam}}Return a writer object responsible for converting the user's data intodelimited strings on the given file-like object. \var{csvfile} can be anyobject with a \function{write} method. If \var{csvfile} is a file object,it must be opened with the 'b' flag on platforms where that makes adifference. An optional{}\var{dialect} parameter can be given which is used to define a set ofparameters specific to a particular CSV dialect. It may be an instanceof a subclass of the \class{Dialect} class or one of the stringsreturned by the \function{list_dialects} function. The other optional{}\var{fmtparam} keyword arguments can be given to override individualformatting parameters in the current dialect. For full detailsabout the dialect and formatting parameters, seesection~\ref{csv-fmt-params}, ``Dialects and Formatting Parameters''.To make it as easy as possible tointerface with modules which implement the DB API, the value\constant{None} is written as the empty string. While this isn't areversible transformation, it makes it easier to dump SQL NULL data valuesto CSV files without preprocessing the data returned from a\code{cursor.fetch*()} call. All other non-string data are stringifiedwith \function{str()} before being written.\end{funcdesc} \begin{funcdesc}{register_dialect}{name\optional{, dialect}\optional{, fmtparam}}Associate \var{dialect} with \var{name}. \var{name} must be a stringor Unicode object. The dialect can be specified either by passing asub-class of \class{Dialect}, or by \var{fmtparam} keyword arguments,or both, with keyword arguments overriding parameters of the dialect.For full details about the dialect and formatting parameters, seesection~\ref{csv-fmt-params}, ``Dialects and Formatting Parameters''.\end{funcdesc} \begin{funcdesc}{unregister_dialect}{name}Delete the dialect associated with \var{name} from the dialect registry. An\exception{Error} is raised if \var{name} is not a registered dialectname.\end{funcdesc} \begin{funcdesc}{get_dialect}{name}Return the dialect associated with \var{name}. An \exception{Error} israised if \var{name} is not a registered dialect name.\end{funcdesc} \begin{funcdesc}{list_dialects}{}Return the names of all registered dialects.\end{funcdesc} \begin{funcdesc}{field_size_limit}{\optional{new_limit}} Returns the current maximum field size allowed by the parser. If \var{new_limit} is given, this becomes the new limit. \versionadded{2.5}\end{funcdesc} The \module{csv} module defines the following classes: \begin{classdesc}{DictReader}{csvfile\optional{, fieldnames=\constant{None},\optional{, restkey=\constant{None}\optional{, restval=\constant{None}\optional{, dialect=\code{'excel'}\optional{, *args, **kwds}}}}}}Create an object which operates like a regular reader but maps theinformation read into a dict whose keys are given by the optional{} \var{fieldnames}parameter. If the \var{fieldnames} parameter is omitted, the values inthe first row of the \var{csvfile} will be used as the fieldnames.If the row read has fewer fields than the fieldnames sequence,the value of \var{restval} will be used as the default value. If the rowread has more fields than the fieldnames sequence, the remaining data isadded as a sequence keyed by the value of \var{restkey}. If the row readhas fewer fields than the fieldnames sequence, the remaining keys take thevalue of the optional \var{restval} parameter. Any other optional orkeyword arguments are passed to the underlying \class{reader} instance.\end{classdesc} \begin{classdesc}{DictWriter}{csvfile, fieldnames\optional{, restval=""\optional{, extrasaction=\code{'raise'}\optional{, dialect=\code{'excel'}\optional{, *args, **kwds}}}}}Create an object which operates like a regular writer but maps dictionariesonto output rows. The \var{fieldnames} parameter identifies the order inwhich values in the dictionary passed to the \method{writerow()} method arewritten to the \var{csvfile}. The optional \var{restval} parameterspecifies the value to be written if the dictionary is missing a key in\var{fieldnames}. If the dictionary passed to the \method{writerow()}method contains a key not found in \var{fieldnames}, the optional\var{extrasaction} parameter indicates what action to take. If it is setto \code{'raise'} a \exception{ValueError} is raised. If it is set to\code{'ignore'}, extra values in the dictionary are ignored. Any otheroptional or keyword arguments are passed to the underlying \class{writer}instance. Note that unlike the \class{DictReader} class, the \var{fieldnames}parameter of the \class{DictWriter} is not optional. Since Python's\class{dict} objects are not ordered, there is not enough informationavailable to deduce the order in which the row should be written to the\var{csvfile}. \end{classdesc} \begin{classdesc*}{Dialect}{}The \class{Dialect} class is a container class relied on primarily for itsattributes, which are used to define the parameters for a specific\class{reader} or \class{writer} instance.\end{classdesc*} \begin{classdesc}{excel}{}The \class{excel} class defines the usual properties of an Excel-generatedCSV file. It is registered with the dialect name \code{'excel'}.\end{classdesc} \begin{classdesc}{excel_tab}{}The \class{excel_tab} class defines the usual properties of anExcel-generated TAB-delimited file. It is registered with the dialect name\code{'excel-tab'}.\end{classdesc} \begin{classdesc}{Sniffer}{}The \class{Sniffer} class is used to deduce the format of a CSV file.\end{classdesc} The \class{Sniffer} class provides two methods: \begin{methoddesc}{sniff}{sample\optional{,delimiters=None}}Analyze the given \var{sample} and return a \class{Dialect} subclassreflecting the parameters found. If the optional \var{delimiters} parameteris given, it is interpreted as a string containing possible valid delimitercharacters.\end{methoddesc} \begin{methoddesc}{has_header}{sample}Analyze the sample text (presumed to be in CSV format) and return\constant{True} if the first row appears to be a series of columnheaders.\end{methoddesc} The \module{csv} module defines the following constants: \begin{datadesc}{QUOTE_ALL}Instructs \class{writer} objects to quote all fields.\end{datadesc} \begin{datadesc}{QUOTE_MINIMAL}Instructs \class{writer} objects to only quote those fields which containspecial characters such as \var{delimiter}, \var{quotechar} or any of thecharacters in \var{lineterminator}.\end{datadesc} \begin{datadesc}{QUOTE_NONNUMERIC}Instructs \class{writer} objects to quote all non-numericfields. Instructs the reader to convert all non-quoted fields to type \var{float}.\end{datadesc} \begin{datadesc}{QUOTE_NONE}Instructs \class{writer} objects to never quote fields. When the current\var{delimiter} occurs in output data it is preceded by the current\var{escapechar} character. If \var{escapechar} is not set, the writerwill raise \exception{Error} if any characters that require escapingare encountered. Instructs \class{reader} to perform no special processing of quote characters.\end{datadesc} The \module{csv} module defines the following exception: \begin{excdesc}{Error}Raised by any of the functions when an error is detected.\end{excdesc} \subsection{Dialects and Formatting Parameters\label{csv-fmt-params}} To make it easier to specify the format of input and output records,specific formatting parameters are grouped together into dialects. Adialect is a subclass of the \class{Dialect} class having a set of specificmethods and a single \method{validate()} method. When creating \class{reader}or \class{writer} objects, the programmer can specify a string or a subclassof the \class{Dialect} class as the dialect parameter. In addition to, orinstead of, the \var{dialect} parameter, the programmer can also specifyindividual formatting parameters, which have the same names as theattributes defined below for the \class{Dialect} class. Dialects support the following attributes: \begin{memberdesc}[Dialect]{delimiter}A one-character string used to separate fields. It defaults to \code{','}.\end{memberdesc} \begin{memberdesc}[Dialect]{doublequote}Controls how instances of \var{quotechar} appearing inside a field shouldbe themselves be quoted. When \constant{True}, the character is doubled.When \constant{False}, the \var{escapechar} is used as a prefix to the\var{quotechar}. It defaults to \constant{True}. On output, if \var{doublequote} is \constant{False} and no\var{escapechar} is set, \exception{Error} is raised if a \var{quotechar}is found in a field.\end{memberdesc} \begin{memberdesc}[Dialect]{escapechar}A one-character string used by the writer to escape the \var{delimiter} if\var{quoting} is set to \constant{QUOTE_NONE} and the \var{quotechar}if \var{doublequote} is \constant{False}. On reading, the \var{escapechar}removes any special meaning from the following character. It defaultsto \constant{None}, which disables escaping.\end{memberdesc} \begin{memberdesc}[Dialect]{lineterminator}The string used to terminate lines produced by the \class{writer}.It defaults to \code{'\e r\e n'}. \note{The \class{reader} is hard-coded to recognise either \code{'\e r'}or \code{'\e n'} as end-of-line, and ignores \var{lineterminator}. Thisbehavior may change in the future.}\end{memberdesc} \begin{memberdesc}[Dialect]{quotechar}A one-character string used to quote fields containing special characters,such as the \var{delimiter} or \var{quotechar}, or which contain new-linecharacters. It defaults to \code{'"'}.\end{memberdesc} \begin{memberdesc}[Dialect]{quoting}Controls when quotes should be generated by the writer and recognisedby the reader. It can take on any of the \constant{QUOTE_*} constants(see section~\ref{csv-contents}) and defaults to \constant{QUOTE_MINIMAL}.\end{memberdesc} \begin{memberdesc}[Dialect]{skipinitialspace}When \constant{True}, whitespace immediately following the \var{delimiter}is ignored. The default is \constant{False}.\end{memberdesc} \subsection{Reader Objects} Reader objects (\class{DictReader} instances and objects returned bythe \function{reader()} function) have the following public methods: \begin{methoddesc}[csv reader]{next}{}Return the next row of the reader's iterable object as a list, parsedaccording to the current dialect.\end{methoddesc} Reader objects have the following public attributes: \begin{memberdesc}[csv reader]{dialect}A read-only description of the dialect in use by the parser.\end{memberdesc} \begin{memberdesc}[csv reader]{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. \versionadded{2.5}\end{memberdesc} \subsection{Writer Objects} \class{Writer} objects (\class{DictWriter} instances and objects returned bythe \function{writer()} function) have the following public methods. A{}\var{row} must be a sequence of strings or numbers for \class{Writer}objects and a dictionary mapping fieldnames to strings or numbers (bypassing them through \function{str()} first) for {}\class{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). \begin{methoddesc}[csv writer]{writerow}{row}Write the \var{row} parameter to the writer's file object, formattedaccording to the current dialect.\end{methoddesc} \begin{methoddesc}[csv writer]{writerows}{rows}Write all the \var{rows} parameters (a list of \var{row} objects asdescribed above) to the writer's file object, formattedaccording to the current dialect.\end{methoddesc} Writer objects have the following public attribute: \begin{memberdesc}[csv writer]{dialect}A read-only description of the dialect in use by the writer.\end{memberdesc} \subsection{Examples\label{csv-examples}} The simplest example of reading a CSV file: \begin{verbatim}import csvreader = csv.reader(open("some.csv", "rb"))for row in reader: print row\end{verbatim} Reading a file with an alternate format: \begin{verbatim}import csvreader = csv.reader(open("passwd", "rb"), delimiter=':', quoting=csv.QUOTE_NONE)for row in reader: print row\end{verbatim} The corresponding simplest possible writing example is: \begin{verbatim}import csvwriter = csv.writer(open("some.csv", "wb"))writer.writerows(someiterable)\end{verbatim} Registering a new dialect: \begin{verbatim}import csv csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE) reader = csv.reader(open("passwd", "rb"), 'unixpwd')\end{verbatim} A slightly more advanced use of the reader --- catching and reporting errors: \begin{verbatim}import csv, sysfilename = "some.csv"reader = csv.reader(open(filename, "rb"))try: for row in reader: print rowexcept csv.Error, e: sys.exit('file %s, line %d: %s' % (filename, reader.line_num, e))\end{verbatim} And while the module doesn't directly support parsing strings, it caneasily be done: \begin{verbatim}import csvfor row in csv.reader(['one,two,three']): print row\end{verbatim} The \module{csv} module doesn't directly support reading and writingUnicode, but it is 8-bit-clean save for some problems with \ASCII{} NULcharacters. So you can write functions or classes that handle theencoding and decoding for you as long as you avoid encodings likeUTF-16 that use NULs. UTF-8 is recommended. \function{unicode_csv_reader} below is a generator that wraps\class{csv.reader} to handle Unicode CSV data (a list of Unicodestrings). \function{utf_8_encoder} is a generator that encodes theUnicode strings as UTF-8, one string (or row) at a time. The encodedstrings are parsed by the CSV reader, and\function{unicode_csv_reader} decodes the UTF-8-encoded cells backinto Unicode: \begin{verbatim}import csv def unicode_csv_reader(unicode_csv_data, dialect=csv.excel, **kwargs): # csv.py doesn't do Unicode; encode temporarily as UTF-8: csv_reader = csv.reader(utf_8_encoder(unicode_csv_data), dialect=dialect, **kwargs) for row in csv_reader: # decode UTF-8 back to Unicode, cell by cell: yield [unicode(cell, 'utf-8') for cell in row] def utf_8_encoder(unicode_csv_data): for line in unicode_csv_data: yield line.encode('utf-8')\end{verbatim} For all other encodings the following \class{UnicodeReader} and\class{UnicodeWriter} classes can be used. They take an additional\var{encoding} parameter in their constructor and make sure that the datapasses the real reader or writer encoded as UTF-8: \begin{verbatim}import csv, codecs, cStringIO class UTF8Recoder: """ Iterator that reads an encoded stream and reencodes the input to UTF-8 """ def __init__(self, f, encoding): self.reader = codecs.getreader(encoding)(f) def __iter__(self): return self def next(self): return self.reader.next().encode("utf-8") class UnicodeReader: """ A CSV reader which will iterate over lines in the CSV file "f", which is encoded in the given encoding. """ def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds): f = UTF8Recoder(f, encoding)Lines 1–500 of 538. Load more
\section{\module{csv} --- CSV File Reading and Writing} \declaremodule{standard}{csv} \modulesynopsis{Write and read tabular data to and from delimited files.} \sectionauthor{Skip Montanaro}{skip@pobox.com} \versionadded{2.3} \index{csv} \indexii{data}{tabular} The so-called CSV (Comma Separated Values) format is the most common import and export format for spreadsheets and databases. There is no ``CSV standard'', so the format is operationally defined by the many applications which read and write it. The lack of a 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 \module{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 \module{csv} module's \class{reader} and \class{writer} objects read and write sequences. Programmers can also read and write data in dictionary form using the \class{DictReader} and \class{DictWriter} classes. \begin{notice} This version of the \module{csv} module doesn't support Unicode input. Also, there are currently some issues regarding \ASCII{} NUL characters. Accordingly, all input should be UTF-8 or printable \ASCII{} to be safe; see the examples in section~\ref{csv-examples}. These restrictions will be removed in the future. \end{notice} \begin{seealso} % \seemodule{array}{Arrays of uniformly types numeric values.} \seepep{305}{CSV File API} {The Python Enhancement Proposal which proposed this addition to Python.} \end{seealso} \subsection{Module Contents \label{csv-contents}} The \module{csv} module defines the following functions: \begin{funcdesc}{reader}{csvfile\optional{, dialect=\code{'excel'}}\optional{, fmtparam}} Return a reader object which will iterate over lines in the given {}\var{csvfile}. \var{csvfile} can be any object which supports the iterator protocol and returns a string each time its \method{next} method is called --- file objects and list objects are both suitable. If \var{csvfile} is a file object, it must be opened with the 'b' flag on platforms where that makes a difference. An optional {}\var{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{Dialect} class or one of the strings returned by the \function{list_dialects} function. The other optional {}\var{fmtparam} 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~\ref{csv-fmt-params}, ``Dialects and Formatting Parameters''. All data read are returned as strings. No automatic data type conversion is performed. \versionchanged[ The parser is now stricter with respect to multi-line quoted fields. Previously, if a line ended within a quoted field without a terminating newline character, a newline would be inserted into the returned field. This behavior caused problems when reading files which contained carriage return characters within fields. The behavior was changed to return the field without inserting newlines. As a consequence, if newlines embedded within fields are important, the input should be split into lines in a manner which preserves the newline characters]{2.5} \end{funcdesc} \begin{funcdesc}{writer}{csvfile\optional{, dialect=\code{'excel'}}\optional{, fmtparam}} Return a writer object responsible for converting the user's data into delimited strings on the given file-like object. \var{csvfile} can be any object with a \function{write} method. If \var{csvfile} is a file object, it must be opened with the 'b' flag on platforms where that makes a difference. An optional {}\var{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{Dialect} class or one of the strings returned by the \function{list_dialects} function. The other optional {}\var{fmtparam} 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~\ref{csv-fmt-params}, ``Dialects and Formatting Parameters''. To make it as easy as possible to interface with modules which implement the DB API, the value \constant{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 \code{cursor.fetch*()} call. All other non-string data are stringified with \function{str()} before being written. \end{funcdesc} \begin{funcdesc}{register_dialect}{name\optional{, dialect}\optional{, fmtparam}} Associate \var{dialect} with \var{name}. \var{name} must be a string or Unicode object. The dialect can be specified either by passing a sub-class of \class{Dialect}, or by \var{fmtparam} keyword arguments, or both, with keyword arguments overriding parameters of the dialect. For full details about the dialect and formatting parameters, see section~\ref{csv-fmt-params}, ``Dialects and Formatting Parameters''. \end{funcdesc} \begin{funcdesc}{unregister_dialect}{name} Delete the dialect associated with \var{name} from the dialect registry. An \exception{Error} is raised if \var{name} is not a registered dialect name. \end{funcdesc} \begin{funcdesc}{get_dialect}{name} Return the dialect associated with \var{name}. An \exception{Error} is raised if \var{name} is not a registered dialect name. \end{funcdesc} \begin{funcdesc}{list_dialects}{} Return the names of all registered dialects. \end{funcdesc} \begin{funcdesc}{field_size_limit}{\optional{new_limit}} Returns the current maximum field size allowed by the parser. If \var{new_limit} is given, this becomes the new limit. \versionadded{2.5} \end{funcdesc} The \module{csv} module defines the following classes: \begin{classdesc}{DictReader}{csvfile\optional{, fieldnames=\constant{None},\optional{, restkey=\constant{None}\optional{, restval=\constant{None}\optional{, dialect=\code{'excel'}\optional{, *args, **kwds}}}}}} Create an object which operates like a regular reader but maps the information read into a dict whose keys are given by the optional {} \var{fieldnames} parameter. If the \var{fieldnames} parameter is omitted, the values in the first row of the \var{csvfile} will be used as the fieldnames. If the row read has fewer fields than the fieldnames sequence, the value of \var{restval} will be used as the default value. If the row read has more fields than the fieldnames sequence, the remaining data is added as a sequence keyed by the value of \var{restkey}. If the row read has fewer fields than the fieldnames sequence, the remaining keys take the value of the optional \var{restval} parameter. Any other optional or keyword arguments are passed to the underlying \class{reader} instance. \end{classdesc} \begin{classdesc}{DictWriter}{csvfile, fieldnames\optional{, restval=""\optional{, extrasaction=\code{'raise'}\optional{, dialect=\code{'excel'}\optional{, *args, **kwds}}}}} Create an object which operates like a regular writer but maps dictionaries onto output rows. The \var{fieldnames} parameter identifies the order in which values in the dictionary passed to the \method{writerow()} method are written to the \var{csvfile}. The optional \var{restval} parameter specifies the value to be written if the dictionary is missing a key in \var{fieldnames}. If the dictionary passed to the \method{writerow()} method contains a key not found in \var{fieldnames}, the optional \var{extrasaction} parameter indicates what action to take. If it is set to \code{'raise'} a \exception{ValueError} is raised. If it is set to \code{'ignore'}, extra values in the dictionary are ignored. Any other optional or keyword arguments are passed to the underlying \class{writer} instance. Note that unlike the \class{DictReader} class, the \var{fieldnames} parameter of the \class{DictWriter} is not optional. Since Python's \class{dict} objects are not ordered, there is not enough information available to deduce the order in which the row should be written to the \var{csvfile}. \end{classdesc} \begin{classdesc*}{Dialect}{} The \class{Dialect} class is a container class relied on primarily for its attributes, which are used to define the parameters for a specific \class{reader} or \class{writer} instance. \end{classdesc*} \begin{classdesc}{excel}{} The \class{excel} class defines the usual properties of an Excel-generated CSV file. It is registered with the dialect name \code{'excel'}. \end{classdesc} \begin{classdesc}{excel_tab}{} The \class{excel_tab} class defines the usual properties of an Excel-generated TAB-delimited file. It is registered with the dialect name \code{'excel-tab'}. \end{classdesc} \begin{classdesc}{Sniffer}{} The \class{Sniffer} class is used to deduce the format of a CSV file. \end{classdesc} The \class{Sniffer} class provides two methods: \begin{methoddesc}{sniff}{sample\optional{,delimiters=None}} Analyze the given \var{sample} and return a \class{Dialect} subclass reflecting the parameters found. If the optional \var{delimiters} parameter is given, it is interpreted as a string containing possible valid delimiter characters. \end{methoddesc} \begin{methoddesc}{has_header}{sample} Analyze the sample text (presumed to be in CSV format) and return \constant{True} if the first row appears to be a series of column headers. \end{methoddesc} The \module{csv} module defines the following constants: \begin{datadesc}{QUOTE_ALL} Instructs \class{writer} objects to quote all fields. \end{datadesc} \begin{datadesc}{QUOTE_MINIMAL} Instructs \class{writer} objects to only quote those fields which contain special characters such as \var{delimiter}, \var{quotechar} or any of the characters in \var{lineterminator}. \end{datadesc} \begin{datadesc}{QUOTE_NONNUMERIC} Instructs \class{writer} objects to quote all non-numeric fields. Instructs the reader to convert all non-quoted fields to type \var{float}. \end{datadesc} \begin{datadesc}{QUOTE_NONE} Instructs \class{writer} objects to never quote fields. When the current \var{delimiter} occurs in output data it is preceded by the current \var{escapechar} character. If \var{escapechar} is not set, the writer will raise \exception{Error} if any characters that require escaping are encountered. Instructs \class{reader} to perform no special processing of quote characters. \end{datadesc} The \module{csv} module defines the following exception: \begin{excdesc}{Error} Raised by any of the functions when an error is detected. \end{excdesc} \subsection{Dialects and Formatting Parameters\label{csv-fmt-params}} 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{Dialect} class having a set of specific methods and a single \method{validate()} method. When creating \class{reader} or \class{writer} objects, the programmer can specify a string or a subclass of the \class{Dialect} class as the dialect parameter. In addition to, or instead of, the \var{dialect} parameter, the programmer can also specify individual formatting parameters, which have the same names as the attributes defined below for the \class{Dialect} class. Dialects support the following attributes: \begin{memberdesc}[Dialect]{delimiter} A one-character string used to separate fields. It defaults to \code{','}. \end{memberdesc} \begin{memberdesc}[Dialect]{doublequote} Controls how instances of \var{quotechar} appearing inside a field should be themselves be quoted. When \constant{True}, the character is doubled. When \constant{False}, the \var{escapechar} is used as a prefix to the \var{quotechar}. It defaults to \constant{True}. On output, if \var{doublequote} is \constant{False} and no \var{escapechar} is set, \exception{Error} is raised if a \var{quotechar} is found in a field. \end{memberdesc} \begin{memberdesc}[Dialect]{escapechar} A one-character string used by the writer to escape the \var{delimiter} if \var{quoting} is set to \constant{QUOTE_NONE} and the \var{quotechar} if \var{doublequote} is \constant{False}. On reading, the \var{escapechar} removes any special meaning from the following character. It defaults to \constant{None}, which disables escaping. \end{memberdesc} \begin{memberdesc}[Dialect]{lineterminator} The string used to terminate lines produced by the \class{writer}. It defaults to \code{'\e r\e n'}. \note{The \class{reader} is hard-coded to recognise either \code{'\e r'} or \code{'\e n'} as end-of-line, and ignores \var{lineterminator}. This behavior may change in the future.} \end{memberdesc} \begin{memberdesc}[Dialect]{quotechar} A one-character string used to quote fields containing special characters, such as the \var{delimiter} or \var{quotechar}, or which contain new-line characters. It defaults to \code{'"'}. \end{memberdesc} \begin{memberdesc}[Dialect]{quoting} Controls when quotes should be generated by the writer and recognised by the reader. It can take on any of the \constant{QUOTE_*} constants (see section~\ref{csv-contents}) and defaults to \constant{QUOTE_MINIMAL}. \end{memberdesc} \begin{memberdesc}[Dialect]{skipinitialspace} When \constant{True}, whitespace immediately following the \var{delimiter} is ignored. The default is \constant{False}. \end{memberdesc} \subsection{Reader Objects} Reader objects (\class{DictReader} instances and objects returned by the \function{reader()} function) have the following public methods: \begin{methoddesc}[csv reader]{next}{} Return the next row of the reader's iterable object as a list, parsed according to the current dialect. \end{methoddesc} Reader objects have the following public attributes: \begin{memberdesc}[csv reader]{dialect} A read-only description of the dialect in use by the parser. \end{memberdesc} \begin{memberdesc}[csv reader]{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. \versionadded{2.5} \end{memberdesc} \subsection{Writer Objects} \class{Writer} objects (\class{DictWriter} instances and objects returned by the \function{writer()} function) have the following public methods. A {}\var{row} must be a sequence of strings or numbers for \class{Writer} objects and a dictionary mapping fieldnames to strings or numbers (by passing them through \function{str()} first) for {}\class{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). \begin{methoddesc}[csv writer]{writerow}{row} Write the \var{row} parameter to the writer's file object, formatted according to the current dialect. \end{methoddesc} \begin{methoddesc}[csv writer]{writerows}{rows} Write all the \var{rows} parameters (a list of \var{row} objects as described above) to the writer's file object, formatted according to the current dialect. \end{methoddesc} Writer objects have the following public attribute: \begin{memberdesc}[csv writer]{dialect} A read-only description of the dialect in use by the writer. \end{memberdesc} \subsection{Examples\label{csv-examples}} The simplest example of reading a CSV file: \begin{verbatim} import csv reader = csv.reader(open("some.csv", "rb")) for row in reader: print row \end{verbatim} Reading a file with an alternate format: \begin{verbatim} import csv reader = csv.reader(open("passwd", "rb"), delimiter=':', quoting=csv.QUOTE_NONE) for row in reader: print row \end{verbatim} The corresponding simplest possible writing example is: \begin{verbatim} import csv writer = csv.writer(open("some.csv", "wb")) writer.writerows(someiterable) \end{verbatim} Registering a new dialect: \begin{verbatim} import csv csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE) reader = csv.reader(open("passwd", "rb"), 'unixpwd') \end{verbatim} A slightly more advanced use of the reader --- catching and reporting errors: \begin{verbatim} import csv, sys filename = "some.csv" reader = csv.reader(open(filename, "rb")) try: for row in reader: print row except csv.Error, e: sys.exit('file %s, line %d: %s' % (filename, reader.line_num, e)) \end{verbatim} And while the module doesn't directly support parsing strings, it can easily be done: \begin{verbatim} import csv for row in csv.reader(['one,two,three']): print row \end{verbatim} The \module{csv} module doesn't directly support reading and writing Unicode, but it is 8-bit-clean save for some problems with \ASCII{} NUL characters. So you can write functions or classes that handle the encoding and decoding for you as long as you avoid encodings like UTF-16 that use NULs. UTF-8 is recommended. \function{unicode_csv_reader} below is a generator that wraps \class{csv.reader} to handle Unicode CSV data (a list of Unicode strings). \function{utf_8_encoder} is a generator that encodes the Unicode strings as UTF-8, one string (or row) at a time. The encoded strings are parsed by the CSV reader, and \function{unicode_csv_reader} decodes the UTF-8-encoded cells back into Unicode: \begin{verbatim} import csv def unicode_csv_reader(unicode_csv_data, dialect=csv.excel, **kwargs): # csv.py doesn't do Unicode; encode temporarily as UTF-8: csv_reader = csv.reader(utf_8_encoder(unicode_csv_data), dialect=dialect, **kwargs) for row in csv_reader: # decode UTF-8 back to Unicode, cell by cell: yield [unicode(cell, 'utf-8') for cell in row] def utf_8_encoder(unicode_csv_data): for line in unicode_csv_data: yield line.encode('utf-8') \end{verbatim} For all other encodings the following \class{UnicodeReader} and \class{UnicodeWriter} classes can be used. They take an additional \var{encoding} parameter in their constructor and make sure that the data passes the real reader or writer encoded as UTF-8: \begin{verbatim} import csv, codecs, cStringIO class UTF8Recoder: """ Iterator that reads an encoded stream and reencodes the input to UTF-8 """ def __init__(self, f, encoding): self.reader = codecs.getreader(encoding)(f) def __iter__(self): return self def next(self): return self.reader.next().encode("utf-8") class UnicodeReader: """ A CSV reader which will iterate over lines in the CSV file "f", which is encoded in the given encoding. """ def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds): f = UTF8Recoder(f, encoding)