Xavier Morel
76852a2fad
Demoted to library in Python 3, see if uses can be replaced by more specific construct, just import functools.reduce otherwise. Futurize fixers: * lib2to3.fixes.fix_reduce
147 lines
4.6 KiB
ReStructuredText
147 lines
4.6 KiB
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==================================
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Python 3 compatibility/conversions
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==================================
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Goal (notsure?): for v11 to provide an alpha/beta Python 3 compatibility, for
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v12 to provide official Python 3 support and drop Python 2 in either v12 or
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v13.
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Python 2 and Python 3 are somewhat different language, but following
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backports, forward ports and cross-compatibility library it is possible to
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use a subset of Python 2 and Python 3 in order to have a system compatible
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with both.
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Here are a few useful steps or reminders to make Python 2 code compatible
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with Python 3.
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References/useful documents:
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* `How do I port to Python 3? <https://eev.ee/blog/2016/07/31/python-faq-how-do-i-port-to-python-3/>`_
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* `Python-Future <http://python-future.org/index.html>`_
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* `Porting Python 2 code to Python 3 <https://docs.python.org/3/howto/pyporting.html>`_
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* `Porting to Python 3: A Guide <http://lucumr.pocoo.org/2010/2/11/porting-to-python-3-a-guide/>`_ (a bit outdated but useful for the extensive comments on strings and IO)
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Versions Support
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================
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A cross compatible Odoo would only support Python 2.7 and Python 3.5 and
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above: Python 2.7 backported some Python 3 features, and Python 2 features
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were reintroduced in various Python 3 in order to make conversion easier.
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Python 3.6 adds great features (f-strings, ...) and performance improvements
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(ordered compact dicts) but does not seem to reintroduce compatibility
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features whereas:
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* Python 3.5 reintroduced ``%`` for bytes/bytestrings (:pep:`461`)
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* Python 3.4 has no specific compatibility improvement but is the lowest P3
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version for PyLint
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* Python 3.3 reintroduced the "u" prefix for proper (unicode) strings
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* Python 3.2 made ``range`` views more list-like and reintroduced ``callable``
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.. warning::
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Python 3 adds plenty of great features (keyword-only parameters,
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generator delegation, pathlib, ...), do not use them until Python 2
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support is dropped
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Fixes
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=====
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Exception Handlers
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------------------
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.. important::
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All exception handlers must be converted to ``except ... as ..``. Valid
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forms are::
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except Exception:
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except (Exception1, ...):
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except Exception as name:
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except (Exception1, ...) as name:
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In Python 2, ``except`` statements are of the form::
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except Exception[, name]:
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or::
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except (Exception1, Exception2)[, name]:
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But because the name is optional, this gets confusing and people can stumble
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into the first form when trying for the second and write::
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except Exception1, Exception:
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which will *not* yield the expected result.
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Python 3 changes this syntax to::
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except Exception[ as name]:
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or::
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except (Exception1, Exception2)[ as name]:
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This form was implemented in Python 2.5 and is thus compatible across the
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board.
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Removed Operators
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-----------------
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.. important::
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* The backtick operator ``\`foo\``` must be converted to an explicit call
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to the ``repr()`` builtin
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* The ``<>`` operator must be replaced by ``!=``
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These two operators were long recommended against/deprecated in Python 2,
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Python 3 removed them from the language.
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Removed/renamed builtins
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------------------------
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``reduce``
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##########
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In Python 3, ``reduce`` has been demoted from builtin to ``functools.reduce``.
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However this is because many (if not most) uses of ``reduce`` can be replaced
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by ``sum``, ``all``, ``any`` or a list comprehension for a more readable and
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faster result.
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It is easy enough to just add ``from functools import reduce`` to the file
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and compatible with Python 2.6 and later, but consider whether you get better
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code by replacing it with some other method altogether.
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Removed/renamed methods
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-----------------------
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.. important::
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* the ``has_key`` method on dicts must be replaced by use of the ``in``
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operator e.g. ``foo.has_key(bar)`` becomes ``bar in foo``.
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``in`` for dicts was introduced in Python 2.3, leading to ``has_key`` being
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redundant, and removed in Python 3.
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Minor syntax changes
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--------------------
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* the ability to unpack a parameter (in the parameter declaration list) has
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been removed in Python 3 e.g.::
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def foo((bar, baz), qux):
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…
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is now invalid
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* octal literals must be prefixed by ``0o`` (or ``0O``). Following the C
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family, in Python 2 an octal literal simply has a leading 0, which can be
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confusing and easy to get wrong when e.g. padding for readability (e.g.
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``0013`` would be the decimal 11 rather than 13).
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In Python 3, leading zeroes followed by neither a 0 nor a period is an
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error, octal literals now follow the hexadecimal convention with a ``0o``
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prefix.
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