462 lines
18 KiB
TeX
462 lines
18 KiB
TeX
\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 generally be printable
|
|
\ASCII{} to be safe. 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 more information about the dialect and formatting
|
|
parameters, see section~\ref{csv-fmt-params}, ``Dialects and Formatting
|
|
Parameters'' for details of these parameters.
|
|
|
|
All data read are returned as strings. No automatic data type
|
|
conversion is performed.
|
|
\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 more information
|
|
about the dialect and formatting parameters, see
|
|
section~\ref{csv-fmt-params}, ``Dialects and Formatting Parameters'' for
|
|
details of these 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 more information about the dialect and formatting parameters, see
|
|
section~\ref{csv-fmt-params}, ``Dialects and Formatting Parameters''
|
|
for details of these 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.
|
|
\end{classdesc}
|
|
|
|
\begin{classdesc}{excel_tab}{}
|
|
The \class{excel_tab} class defines the usual properties of an
|
|
Excel-generated TAB-delimited file.
|
|
\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 a single method:
|
|
|
|
\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.
|
|
\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}
|
|
|
|
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
|
|
print csv.reader(['one,two,three'])[0]
|
|
\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 classes that handle the encoding and decoding
|
|
for you as long as you avoid encodings like utf-16 that use NULs.
|
|
|
|
\begin{verbatim}
|
|
import csv
|
|
|
|
class UnicodeReader:
|
|
def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
|
|
self.reader = csv.reader(f, dialect=dialect, **kwds)
|
|
self.encoding = encoding
|
|
|
|
def next(self):
|
|
row = self.reader.next()
|
|
return [unicode(s, self.encoding) for s in row]
|
|
|
|
def __iter__(self):
|
|
return self
|
|
|
|
class UnicodeWriter:
|
|
def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
|
|
self.writer = csv.writer(f, dialect=dialect, **kwds)
|
|
self.encoding = encoding
|
|
|
|
def writerow(self, row):
|
|
self.writer.writerow([s.encode("utf-8") for s in row])
|
|
|
|
def writerows(self, rows):
|
|
for row in rows:
|
|
self.writerow(row)
|
|
\end{verbatim}
|
|
|
|
They should work just like the \class{csv.reader} and \class{csv.writer}
|
|
classes but add an \var{encoding} parameter.
|