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.. _stdlib-incompatibilities:
Standard library incompatibilities
==================================
Some standard library interfaces have changed in ways that require
different code than normal Py3 code in order to achieve Py2/3
compatibility.
Here we will attempt to document these, together with known workarounds:
.. csv-table:: Standard library incompatibilities
:header: "module", "object / feature", "section"
:widths: 10, 20, 15
``array``, ``array`` constructor, :ref:`stdlib-array-constructor`
``array``, ``array.read()`` method, :ref:`stdlib-array-read`
``base64``, ``decodebytes()`` function, :ref:`stdlib-base64-decodebytes`
``re``, ``ASCII`` mode, :ref:`stdlib-re-ASCII`
To contribute to this, please email the python-porting list or send a
pull request. See :ref:`contributing`.
.. _stdlib-array-constructor:
array.array()
-------------
The first argument to ``array.array(typecode[, initializer])`` must be a native
platform string: unicode string on Python 3, byte string on Python 2.
Python 2::
>>> array.array(b'b')
array.array(b'b')
>>> array.array(u'u')
TypeError: must be char, not unicode
Python 3::
>>> array.array(b'b')
TypeError: must be a unicode character, not bytes
>>> array.array(u'b')
array('b')
This means that the typecode cannot be specified portably across Python 3 and Python 2
with a single string literal when ``from __future__ import unicode_literals`` is in effect.
You can use the following code on both Python 3 and Python 2::
from __future__ import unicode_literals
from future.utils import bytes_to_native_str
import array
# ...
a = array.array(bytes_to_native_str(b'b'))
This was `fixed in Python 2.7.11
<https://hg.python.org/cpython/file/6d1b6a68f775/Misc/NEWS#l233>`_.
Since then, ``array.array()`` now also accepts unicode format typecode.
.. _stdlib-array-read:
array.array.read()
------------------
This method has been removed in Py3. This crops up in e.g. porting ``http.client``.
.. _stdlib-base64-decodebytes:
base64.decodebytes() and base64.encodebytes()
---------------------------------------------
The ``base64`` module on Py2 has no ``decodebytes`` or ``encodebytes`` functions.
.. _stdlib-re-ASCII:
re.ASCII
--------
Python 3 code using regular expressions sometimes looks like this (from
:mod:`urllib.request`)::
re.compile(r":\d+$", re.ASCII)
This enables 'ASCII mode' for regular expressions (see the docs `here
<http://docs.python.org/3/library/re.html#re.ASCII>`_). Python 2's
:mod:`re` module has no equivalent mode.
struct.pack()
-------------
Before Python version 2.7.7, the :func:`struct.pack` function
required a native string as its format argument. For example::
>>> from __future__ import unicode_literals
>>> from struct import pack
>>> pack('<4H2I', version, rec_type, build, year, file_hist_flags, ver_can_read)
raised ``TypeError: Struct() argument 1 must be string, not unicode``.
This was `fixed in Python 2.7.7
<https://hg.python.org/cpython/raw-file/f89216059edf/Misc/NEWS>`_.
Since then, ``struct.pack()`` now also accepts unicode format
strings.
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