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asyncio-dev.rst.txt	13398	0644	editdlrm
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asyncio.rst.txt	2272	0644	editdlrm
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doctest.rst.txt	71743	0644	editdlrm
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faulthandler.rst.txt	6136	0644	editdlrm
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fnmatch.rst.txt	3103	0644	editdlrm
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fpectl.rst.txt	4180	0644	editdlrm
fractions.rst.txt	6364	0644	editdlrm
frameworks.rst.txt	391	0644	editdlrm
ftplib.rst.txt	17552	0644	editdlrm
functional.rst.txt	365	0644	editdlrm
functions.rst.txt	71621	0644	editdlrm
functools.rst.txt	18503	0644	editdlrm
gc.rst.txt	9694	0644	editdlrm
getopt.rst.txt	6557	0644	editdlrm
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heapq.rst.txt	13402	0644	editdlrm
hmac.rst.txt	4020	0644	editdlrm
html.entities.rst.txt	1319	0644	editdlrm
html.parser.rst.txt	11275	0644	editdlrm
html.rst.txt	1304	0644	editdlrm
http.client.rst.txt	18870	0644	editdlrm
http.cookiejar.rst.txt	27832	0644	editdlrm
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http.rst.txt	7096	0644	editdlrm
http.server.rst.txt	17639	0644	editdlrm
i18n.rst.txt	408	0644	editdlrm
idle.rst.txt	25834	0644	editdlrm
imaplib.rst.txt	20367	0644	editdlrm
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ipc.rst.txt	649	0644	editdlrm
itertools.rst.txt	38459	0644	editdlrm
json.rst.txt	27876	0644	editdlrm
keyword.rst.txt	617	0644	editdlrm
language.rst.txt	522	0644	editdlrm
linecache.rst.txt	2408	0644	editdlrm
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logging.config.rst.txt	33886	0644	editdlrm
logging.handlers.rst.txt	42497	0644	editdlrm
logging.rst.txt	60479	0644	editdlrm
lzma.rst.txt	17361	0644	editdlrm
macpath.rst.txt	707	0644	editdlrm
mailbox.rst.txt	63055	0644	editdlrm
mailcap.rst.txt	3691	0644	editdlrm
markup.rst.txt	679	0644	editdlrm
marshal.rst.txt	4855	0644	editdlrm
math.rst.txt	14375	0644	editdlrm
mimetypes.rst.txt	9839	0644	editdlrm
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mm.rst.txt	431	0644	editdlrm
mmap.rst.txt	11232	0644	editdlrm
modulefinder.rst.txt	3240	0644	editdlrm
modules.rst.txt	355	0644	editdlrm
msilib.rst.txt	18612	0644	editdlrm
msvcrt.rst.txt	4392	0644	editdlrm
multiprocessing.rst.txt	104494	0644	editdlrm
netdata.rst.txt	339	0644	editdlrm
netrc.rst.txt	3007	0644	editdlrm
nis.rst.txt	1993	0644	editdlrm
nntplib.rst.txt	21821	0644	editdlrm
numbers.rst.txt	8089	0644	editdlrm
numeric.rst.txt	696	0644	editdlrm
operator.rst.txt	18989	0644	editdlrm
optparse.rst.txt	76996	0644	editdlrm
os.path.rst.txt	16419	0644	editdlrm
os.rst.txt	137534	0644	editdlrm
ossaudiodev.rst.txt	17845	0644	editdlrm
othergui.rst.txt	2822	0644	editdlrm
parser.rst.txt	15148	0644	editdlrm
pathlib.rst.txt	30045	0644	editdlrm
pdb.rst.txt	19454	0644	editdlrm
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pickle.rst.txt	37440	0644	editdlrm
pickletools.rst.txt	3729	0644	editdlrm
pipes.rst.txt	2557	0644	editdlrm
pkgutil.rst.txt	8730	0644	editdlrm
platform.rst.txt	9624	0644	editdlrm
plistlib.rst.txt	7398	0644	editdlrm
poplib.rst.txt	8197	0644	editdlrm
posix.rst.txt	3694	0644	editdlrm
pprint.rst.txt	14274	0644	editdlrm
profile.rst.txt	27969	0644	editdlrm
pty.rst.txt	3123	0644	editdlrm
pwd.rst.txt	2739	0644	editdlrm
pyclbr.rst.txt	3297	0644	editdlrm
pydoc.rst.txt	4688	0644	editdlrm
pyexpat.rst.txt	28620	0644	editdlrm
python.rst.txt	475	0644	editdlrm
py_compile.rst.txt	3851	0644	editdlrm
queue.rst.txt	7268	0644	editdlrm
quopri.rst.txt	2571	0644	editdlrm
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re.rst.txt	64936	0644	editdlrm
readline.rst.txt	11943	0644	editdlrm
reprlib.rst.txt	5162	0644	editdlrm
resource.rst.txt	12291	0644	editdlrm
rlcompleter.rst.txt	2294	0644	editdlrm
runpy.rst.txt	8285	0644	editdlrm
sched.rst.txt	4842	0644	editdlrm
secrets.rst.txt	5930	0644	editdlrm
select.rst.txt	28175	0644	editdlrm
selectors.rst.txt	8925	0644	editdlrm
shelve.rst.txt	8390	0644	editdlrm
shlex.rst.txt	16441	0644	editdlrm
shutil.rst.txt	26014	0644	editdlrm
signal.rst.txt	17218	0644	editdlrm
site.rst.txt	9628	0644	editdlrm
smtpd.rst.txt	10806	0644	editdlrm
smtplib.rst.txt	23439	0644	editdlrm
sndhdr.rst.txt	1994	0644	editdlrm
socket.rst.txt	68202	0644	editdlrm
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sqlite3.rst.txt	39215	0644	editdlrm
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stat.rst.txt	9805	0644	editdlrm
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stdtypes.rst.txt	179240	0644	editdlrm
string.rst.txt	35374	0644	editdlrm
stringprep.rst.txt	4279	0644	editdlrm
struct.rst.txt	19581	0644	editdlrm
subprocess.rst.txt	45793	0644	editdlrm
sunau.rst.txt	7390	0644	editdlrm
superseded.rst.txt	258	0644	editdlrm
symbol.rst.txt	975	0644	editdlrm
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sys.rst.txt	58273	0644	editdlrm
sysconfig.rst.txt	8749	0644	editdlrm
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tabnanny.rst.txt	2007	0644	editdlrm
tarfile.rst.txt	31749	0644	editdlrm
telnetlib.rst.txt	7899	0644	editdlrm
tempfile.rst.txt	13857	0644	editdlrm
termios.rst.txt	3751	0644	editdlrm
test.rst.txt	25764	0644	editdlrm
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unittest.rst.txt	93304	0644	editdlrm
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xml.rst.txt	6071	0644	editdlrm
xml.sax.handler.rst.txt	15399	0644	editdlrm
xml.sax.reader.rst.txt	12133	0644	editdlrm
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xml.sax.utils.rst.txt	3901	0644	editdlrm
xmlrpc.client.rst.txt	23071	0644	editdlrm
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zipapp.rst.txt	17392	0644	editdlrm
zipfile.rst.txt	24005	0644	editdlrm
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zlib.rst.txt	13912	0644	editdlrm
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__future__.rst.txt	5245	0644	editdlrm
__main__.rst.txt	904	0644	editdlrm

:mod:`csv` --- CSV File Reading and Writing =========================================== .. module:: csv :synopsis: Write and read tabular data to and from delimited files. .. sectionauthor:: Skip Montanaro **Source code:** :source:`Lib/csv.py` .. index:: single: csv pair: data; tabular -------------- 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 :rfc:`4180`. 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 :mod:`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 :mod:`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. .. seealso:: :pep:`305` - CSV File API The Python Enhancement Proposal which proposed this addition to Python. .. _csv-contents: Module Contents --------------- The :mod:`csv` module defines the following functions: .. index:: single: universal newlines; csv.reader function .. function:: 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 :term:`iterator` protocol and returns a string each time its :meth:`!__next__` method is called --- :term:`file objects ` and list objects are both suitable. If *csvfile* is a file object, it should be opened with ``newline=''``. [1]_ 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:`Dialect` class or one of the strings returned by the :func:`list_dialects` 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 :ref:`csv-fmt-params`. Each row read from the csv file is returned as a list of strings. No automatic data type conversion is performed unless the ``QUOTE_NONNUMERIC`` format option 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 Beans Spam, Lovely Spam, Wonderful Spam .. function:: 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 :func:`write` method. If *csvfile* is a file object, it should be opened with ``newline=''`` [1]_. 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:`Dialect` class or one of the strings returned by the :func:`list_dialects` 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 :ref:`csv-fmt-params`. To make it as easy as possible to interface with modules which implement the DB API, the value :const:`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 ``cursor.fetch*`` call. All other non-string data are stringified with :func:`str` before being written. A short usage example:: import csv with 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']) .. function:: register_dialect(name[, dialect[, **fmtparams]]) Associate *dialect* with *name*. *name* must be a string. The dialect can be specified either by passing a sub-class of :class:`Dialect`, or by *fmtparams* 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`. .. function:: unregister_dialect(name) Delete the dialect associated with *name* from the dialect registry. An :exc:`Error` is raised if *name* is not a registered dialect name. .. function:: get_dialect(name) Return the dialect associated with *name*. An :exc:`Error` is raised if *name* is not a registered dialect name. This function returns an immutable :class:`Dialect`. .. function:: list_dialects() Return the names of all registered dialects. .. function:: 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 :mod:`csv` module defines the following classes: .. class:: 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 an :mod:`OrderedDict ` whose keys are given by the optional *fieldnames* parameter. The *fieldnames* parameter is a :term:`sequence`. If *fieldnames* is omitted, the values in the first row of file *f* will be used as the fieldnames. Regardless of how the fieldnames are determined, the ordered 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 ``None``. All other optional or keyword arguments are passed to the underlying :class:`reader` instance. .. versionchanged:: 3.6 Returned rows are now of type :class:`OrderedDict`. A short usage example:: >>> import csv >>> with open('names.csv', newline='') as csvfile: ... reader = csv.DictReader(csvfile) ... for row in reader: ... print(row['first_name'], row['last_name']) ... Eric Idle John Cleese >>> print(row) OrderedDict([('first_name', 'John'), ('last_name', 'Cleese')]) .. class:: 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 :mod:`sequence ` of keys that identify the order in which values in the dictionary passed to the :meth:`writerow` 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 :meth:`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 :exc:`ValueError` 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 :class:`writer` instance. Note that unlike the :class:`DictReader` class, the *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 file *f*. A short usage example:: import csv with open('names.csv', 'w', newline='') 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:: 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. .. class:: excel() The :class:`excel` class defines the usual properties of an Excel-generated CSV file. It is registered with the dialect name ``'excel'``. .. class:: 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 ``'excel-tab'``. .. class:: unix_dialect() The :class:`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'``. .. versionadded:: 3.2 .. class:: Sniffer() The :class:`Sniffer` class is used to deduce the format of a CSV file. The :class:`Sniffer` class provides two methods: .. method:: sniff(sample, delimiters=None) Analyze the given *sample* and return a :class:`Dialect` subclass reflecting the parameters found. If the optional *delimiters* parameter is given, it is interpreted as a string containing possible valid delimiter characters. .. method:: has_header(sample) Analyze the sample text (presumed to be in CSV format) and return :const:`True` if the first row appears to be a series of column headers. An example for :class:`Sniffer` use:: with open('example.csv', newline='') as csvfile: dialect = csv.Sniffer().sniff(csvfile.read(1024)) csvfile.seek(0) reader = csv.reader(csvfile, dialect) # ... process CSV file contents here ... The :mod:`csv` module defines the following constants: .. data:: QUOTE_ALL Instructs :class:`writer` objects to quote all fields. .. data:: QUOTE_MINIMAL Instructs :class:`writer` objects to only quote those fields which contain special characters such as *delimiter*, *quotechar* or any of the characters in *lineterminator*. .. data:: QUOTE_NONNUMERIC Instructs :class:`writer` objects to quote all non-numeric fields. Instructs the reader to convert all non-quoted fields to type *float*. .. data:: QUOTE_NONE Instructs :class:`writer` objects to never quote fields. When the current *delimiter* occurs in output data it is preceded by the current *escapechar* character. If *escapechar* is not set, the writer will raise :exc:`Error` if any characters that require escaping are encountered. Instructs :class:`reader` to perform no special processing of quote characters. The :mod:`csv` module defines the following exception: .. exception:: Error Raised by any of the functions when an error is detected. .. _csv-fmt-params: 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:`Dialect` class having a set of specific methods and a single :meth:`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 *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: .. attribute:: Dialect.delimiter A one-character string used to separate fields. It defaults to ``','``. .. attribute:: Dialect.doublequote Controls how instances of *quotechar* appearing inside a field should themselves be quoted. When :const:`True`, the character is doubled. When :const:`False`, the *escapechar* is used as a prefix to the *quotechar*. It defaults to :const:`True`. On output, if *doublequote* is :const:`False` and no *escapechar* is set, :exc:`Error` is raised if a *quotechar* is found in a field. .. attribute:: Dialect.escapechar A one-character string used by the writer to escape the *delimiter* if *quoting* is set to :const:`QUOTE_NONE` and the *quotechar* if *doublequote* is :const:`False`. On reading, the *escapechar* removes any special meaning from the following character. It defaults to :const:`None`, which disables escaping. .. attribute:: Dialect.lineterminator The string used to terminate lines produced by the :class:`writer`. It defaults to ``'\r\n'``. .. note:: The :class:`reader` is hard-coded to recognise either ``'\r'`` or ``'\n'`` as end-of-line, and ignores *lineterminator*. This behavior may change in the future. .. attribute:: Dialect.quotechar A one-character string used to quote fields containing special characters, such as the *delimiter* or *quotechar*, or which contain new-line characters. It defaults to ``'"'``. .. attribute:: Dialect.quoting Controls when quotes should be generated by the writer and recognised by the reader. It can take on any of the :const:`QUOTE_\*` constants (see section :ref:`csv-contents`) and defaults to :const:`QUOTE_MINIMAL`. .. attribute:: Dialect.skipinitialspace When :const:`True`, whitespace immediately following the *delimiter* is ignored. The default is :const:`False`. .. attribute:: Dialect.strict When ``True``, raise exception :exc:`Error` on bad CSV input. The default is ``False``. Reader Objects -------------- Reader objects (:class:`DictReader` instances and objects returned by the :func:`reader` function) have the following public methods: .. method:: csvreader.__next__() Return the next row of the reader's iterable object as a list (if the object was returned from :func:`reader`) or a dict (if it is a :class:`DictReader` instance), parsed according to the current dialect. Usually you should call this as ``next(reader)``. Reader objects have the following public attributes: .. attribute:: csvreader.dialect A read-only description of the dialect in use by the parser. .. attribute:: 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: .. attribute:: csvreader.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 -------------- :class:`Writer` objects (:class:`DictWriter` instances and objects returned by the :func:`writer` function) have the following public methods. A *row* must be an iterable of strings or numbers for :class:`Writer` objects and a dictionary mapping fieldnames to strings or numbers (by passing them through :func:`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). .. method:: csvwriter.writerow(row) Write the *row* parameter to the writer's file object, formatted according to the current dialect. .. versionchanged:: 3.5 Added support of arbitrary iterables. .. method:: 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: .. attribute:: csvwriter.dialect A read-only description of the dialect in use by the writer. DictWriter objects have the following public method: .. method:: DictWriter.writeheader() Write a row with the field names (as specified in the constructor). .. versionadded:: 3.2 .. _csv-examples: Examples -------- The simplest example of reading a CSV file:: import csv with open('some.csv', newline='') as f: reader = csv.reader(f) for row in reader: print(row) Reading a file with an alternate format:: import csv with 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 csv with open('some.csv', 'w', newline='') as f: writer = csv.writer(f) writer.writerows(someiterable) Since :func:`open` is used to open a CSV file for reading, the file will by default be decoded into unicode using the system default encoding (see :func:`locale.getpreferredencoding`). To decode a file using a different encoding, use the ``encoding`` argument of open:: import csv with 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 default encoding: specify the encoding argument when opening the output file. Registering a new dialect:: import csv csv.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, sys filename = '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 be done:: import csv for row in csv.reader(['one,two,three']): print(row) .. rubric:: Footnotes .. [1] If ``newline=''`` is not specified, newlines embedded inside quoted fields will not be interpreted correctly, and on platforms that use ``\r\n`` linendings on write an extra ``\r`` will be added. It should always be safe to specify ``newline=''``, since the csv module does its own (:term:`universal `) newline handling.