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Diffstat (limited to 'Doc/library/csv.rst')
| -rw-r--r-- | Doc/library/csv.rst | 59 |
1 files changed, 35 insertions, 24 deletions
diff --git a/Doc/library/csv.rst b/Doc/library/csv.rst index 7fb4fc8256..52a8a310ec 100644 --- a/Doc/library/csv.rst +++ b/Doc/library/csv.rst @@ -146,21 +146,28 @@ The :mod:`csv` module defines the following functions: The :mod:`csv` module defines the following classes: -.. class:: DictReader(csvfile, fieldnames=None, restkey=None, restval=None, \ +.. class:: DictReader(f, fieldnames=None, restkey=None, restval=None, \ dialect='excel', *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 - *fieldnames* parameter. The *fieldnames* parameter is a :mod:`sequence - <collections.abc>` whose elements are associated with the fields of the - input data in order. These elements become the keys of the resulting - dictionary. If the *fieldnames* parameter is omitted, the values in the - first row of the *csvfile* will be used as the fieldnames. If the row read - has more fields than the fieldnames sequence, the remaining data is added as - a sequence keyed by the value of *restkey*. If the row read has fewer - fields than the fieldnames sequence, the remaining keys take the value of - the optional *restval* parameter. Any other optional or keyword arguments - are passed to the underlying :class:`reader` instance. + Create an object that operates like a regular reader but maps the + information in each row to an :mod:`OrderedDict <collections.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:: @@ -170,31 +177,35 @@ The :mod:`csv` module defines the following classes: ... for row in reader: ... print(row['first_name'], row['last_name']) ... - Baked Beans - Lovely Spam - Wonderful Spam + Eric Idle + John Cleese + + >>> print(row) + OrderedDict([('first_name', 'John'), ('last_name', 'Cleese')]) -.. class:: DictWriter(csvfile, fieldnames, restval='', extrasaction='raise', \ +.. 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 <collections.abc>` of keys that identify the order in which values in the - dictionary passed to the :meth:`writerow` method are written to the - *csvfile*. The optional *restval* parameter specifies the value to be + 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'`` 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. + 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 the *csvfile*. + the order in which the row should be written to file *f*. A short usage example:: |