Update documentation for csv module to reflect changes in code,

specifically that it now expects unicode input (and therefore the
paragraph about it not handling unicode is deleted, as well as
the 'how to handle unicode' examples) and that files should
be opened with "newline=''" instead of binary mode.  Also removed
unused BytesIO import from test_csv.

This addresses issue 4847.

1 files changed, 30 insertions, 120 deletions

diff --git a/Doc/library/csv.rst b/Doc/library/csv.rst
index 46ef246..fd56e46 100644
--- a/Doc/library/csv.rst
+++ b/Doc/library/csv.rst
@@ -33,14 +33,6 @@ 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.
 
-.. note::
-
-   This version of the :mod:`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.
-
-
 .. seealso::
 
    :pep:`305` - CSV File API
@@ -60,8 +52,8 @@ The :mod:`csv` module defines the following functions:
    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 --- file objects and list
-   objects are both suitable.   If *csvfile* is a file object, it must be opened
-   with the 'b' flag on platforms where that makes a difference.  An optional
+   objects are both suitable.   If *csvfile* is a file object, it should be opened
+   with ``newline=''``. [#]_  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
@@ -71,20 +63,13 @@ The :mod:`csv` module defines the following functions:
    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.
-
-   The parser is quite strict 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.
+   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
-      >>> spamReader = csv.reader(open('eggs.csv'), delimiter=' ', quotechar='|')
+      >>> spamReader = csv.reader(open('eggs.csv', newline=''), delimiter=' ', quotechar='|')
       >>> for row in spamReader:
       ...     print(', '.join(row))
       Spam, Spam, Spam, Spam, Spam, Baked Beans
@@ -95,8 +80,7 @@ The :mod:`csv` module defines the following functions:
 
    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 must be opened with the
-   'b' flag on platforms where that makes a difference.  An optional *dialect*
+   :func:`write` method.  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
@@ -270,7 +254,6 @@ The :mod:`csv` module defines the following exception:
 
    Raised by any of the functions when an error is detected.
 
-
 .. _csv-fmt-params:
 
 Dialects and Formatting Parameters
@@ -419,41 +402,52 @@ Examples
 The simplest example of reading a CSV file::
 
    import csv
-   reader = csv.reader(open("some.csv", "rb"))
+   reader = csv.reader(open("some.csv", newline=''))
    for row in reader:
        print(row)
 
 Reading a file with an alternate format::
 
    import csv
-   reader = csv.reader(open("passwd", "rb"), delimiter=':', quoting=csv.QUOTE_NONE)
+   reader = csv.reader(open("passwd"), delimiter=':', quoting=csv.QUOTE_NONE)
    for row in reader:
        print(row)
 
 The corresponding simplest possible writing example is::
 
    import csv
-   writer = csv.writer(open("some.csv", "wb"))
+   writer = csv.writer(open("some.csv", "w"))
    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
+    reader = csv.reader(open("some.csv", newline='', encoding='utf-8'))
+    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)
-
-   reader = csv.reader(open("passwd", "rb"), 'unixpwd')
+   reader = csv.reader(open("passwd"), 'unixpwd')
 
 A slightly more advanced use of the reader --- catching and reporting errors::
 
    import csv, sys
    filename = "some.csv"
-   reader = csv.reader(open(filename, "rb"))
+   reader = csv.reader(open(filename, newline=''))
    try:
        for row in reader:
            print(row)
    except csv.Error as e:
-       sys.exit('file %s, line %d: %s' % (filename, reader.line_num, 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::
@@ -462,94 +456,10 @@ done::
    for row in csv.reader(['one,two,three']):
        print(row)
 
-The :mod:`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.
-
-:func:`unicode_csv_reader` below is a :term:`generator` that wraps :class:`csv.reader`
-to handle Unicode CSV data (a list of Unicode strings).  :func:`utf_8_encoder`
-is a :term:`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
-:func:`unicode_csv_reader` decodes the UTF-8-encoded cells back into Unicode::
-
-   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')
-
-For all other encodings the following :class:`UnicodeReader` and
-:class:`UnicodeWriter` classes can be used. They take an additional *encoding*
-parameter in their constructor and make sure that the data passes the real
-reader or writer encoded as UTF-8::
-
-   import csv, codecs, io
-
-   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 next(self.reader).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)
-           self.reader = csv.reader(f, dialect=dialect, **kwds)
-
-       def __next__(self):
-           row = next(self.reader)
-           return [unicode(s, "utf-8") for s in row]
-
-       def __iter__(self):
-           return self
-
-   class UnicodeWriter:
-       """
-       A CSV writer which will write rows to CSV file "f",
-       which is encoded in the given encoding.
-       """
-
-       def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
-           # Redirect output to a queue
-           self.queue = io.StringIO()
-           self.writer = csv.writer(self.queue, dialect=dialect, **kwds)
-           self.stream = f
-           self.encoder = codecs.getincrementalencoder(encoding)()
-
-       def writerow(self, row):
-           self.writer.writerow([s.encode("utf-8") for s in row])
-           # Fetch UTF-8 output from the queue ...
-           data = self.queue.getvalue()
-           data = data.decode("utf-8")
-           # ... and reencode it into the target encoding
-           data = self.encoder.encode(data)
-           # write to the target stream
-           self.stream.write(data)
-           # empty queue
-           self.queue.truncate(0)
-
-       def writerows(self, rows):
-           for row in rows:
-               self.writerow(row)
+.. rubric:: Footnotes
 
+.. [#] If ``newline=''`` is not specified, newlines embedded inside quoted fields
+   will not be interpreted correctly.  It should always be safe to specify
+   ``newline=''``, since the csv module does its own universal newline handling
+   on input.