diff --git a/docs/source/development/index.rst b/docs/source/development/index.rst index 389fdea..a75cafd 100644 --- a/docs/source/development/index.rst +++ b/docs/source/development/index.rst @@ -17,6 +17,5 @@ Developer's guide for third party tools and libraries wrapperkernels execution lexer - pycompat config inputhook_app diff --git a/docs/source/development/pycompat.rst b/docs/source/development/pycompat.rst index 22aef63..74ddd9b 100644 --- a/docs/source/development/pycompat.rst +++ b/docs/source/development/pycompat.rst @@ -1,233 +1,31 @@ +:orphan: + Writing code for Python 2 and 3 =============================== .. module:: IPython.utils.py3compat :synopsis: Python 2 & 3 compatibility helpers -.. data:: PY3 - - Boolean indicating whether we're currently in Python 3. - -Iterators ---------- - -Many built in functions and methods in Python 2 come in pairs, one -returning a list, and one returning an iterator (e.g. :func:`range` and -:func:`python:xrange`). In Python 3, there is usually only the iterator form, -but it has the name which gives a list in Python 2 (e.g. :func:`range`). - -The way to write compatible code depends on what you need: - -* A list, e.g. for serialisation, or to test if something is in it. -* Iteration, but it will never be used for very many items, so efficiency - isn't especially important. -* Iteration over many items, where efficiency is important. - -================ ================= ======================= -list iteration (small) iteration(large) -================ ================= ======================= -list(range(n)) range(n) py3compat.xrange(n) -list(map(f, it)) map(f, it) -- -list(zip(a, b)) zip(a, b) -- -list(d.items()) d.items() py3compat.iteritems(d) -list(d.values()) d.values() py3compat.itervalues(d) -================ ================= ======================= - -Iterating over a dictionary yields its keys, so there is rarely a need -to use :meth:`dict.keys` or :meth:`dict.iterkeys`. - -Avoid using :func:`map` to cause function side effects. This is more -clearly written with a simple for loop. - -.. data:: xrange - - A reference to ``range`` on Python 3, and :func:`python:xrange` on Python 2. - -.. function:: iteritems(d) - itervalues(d) - - Iterate over (key, value) pairs of a dictionary, or just over values. - ``iterkeys`` is not defined: iterating over the dictionary yields its keys. - -Changed standard library locations ----------------------------------- - -Several parts of the standard library have been renamed and moved. This -is a short list of things that we're using. A couple of them have names -in :mod:`IPython.utils.py3compat`, so you don't need both -imports in each module that uses them. - -================== ============ =========== -Python 2 Python 3 py3compat -================== ============ =========== -:func:`raw_input` input input -:mod:`__builtin__` builtins builtin_mod -:mod:`StringIO` io -:mod:`Queue` queue -:mod:`cPickle` pickle -:mod:`thread` _thread -:mod:`copy_reg` copyreg -:mod:`urlparse` urllib.parse -:mod:`repr` reprlib -:mod:`Tkinter` tkinter -:mod:`Cookie` http.cookie -:mod:`_winreg` winreg -================== ============ =========== - -Be careful with StringIO: :class:`io.StringIO` is available in Python 2.7, -but it behaves differently from :class:`StringIO.StringIO`, and much of -our code assumes the use of the latter on Python 2. So a try/except on -the import may cause problems. - -.. function:: input - - Behaves like :func:`python:raw_input` on Python 2. - -.. data:: builtin_mod - builtin_mod_name - - A reference to the module containing builtins, and its name as a string. - -Unicode -------- - -Always be explicit about what is text (unicode) and what is bytes. -*Encoding* goes from unicode to bytes, and *decoding* goes from bytes -to unicode. - -To open files for reading or writing text, use :func:`io.open`, which is -the Python 3 builtin ``open`` function, available on Python 2 as well. -We almost always need to specify the encoding parameter, because the -default is platform dependent. - -We have several helper functions for converting between string types. They all -use the encoding from :func:`IPython.utils.encoding.getdefaultencoding` by default, -and the ``errors='replace'`` option to do best-effort conversions for the user's -system. - -.. function:: unicode_to_str(u, encoding=None) - str_to_unicode(s, encoding=None) - - Convert between unicode and the native str type. No-ops on Python 3. - -.. function:: str_to_bytes(s, encoding=None) - bytes_to_str(u, encoding=None) - - Convert between bytes and the native str type. No-ops on Python 2. - -.. function:: cast_unicode(s, encoding=None) - cast_bytes(s, encoding=None) - - Convert strings to unicode/bytes when they may be of either type. - -.. function:: cast_unicode_py2(s, encoding=None) - cast_bytes_py2(s, encoding=None) - - Convert strings to unicode/bytes when they may be of either type on Python 2, - but return them unaltered on Python 3 (where string types are more - predictable). - -.. data:: unicode_type - - A reference to ``str`` on Python 3, and to ``unicode`` on Python 2. - -.. data:: string_types - - A tuple for isinstance checks: ``(str,)`` on Python 3, ``(str, unicode)`` on - Python 2. - -Relative imports ----------------- - -:: - - # This makes Python 2 behave like Python 3: - from __future__ import absolute_import - - import io # Imports the standard library io module - from . import io # Import the io module from the package - # containing the current module - from .io import foo # foo from the io module next to this module - from IPython.utils import io # This still works - -Print function --------------- - -:: - - # Support the print function on Python 2: - from __future__ import print_function - - print(a, b) - print(foo, file=sys.stderr) - print(bar, baz, sep='\t', end='') - -Metaclasses ------------ - -The syntax for declaring a class with a metaclass is different in -Python 2 and 3. A helper function works for most cases: - -.. function:: with_metaclass - - Create a base class with a metaclass. Copied from the six library. - - Used like this:: - - class FormatterABC(with_metaclass(abc.ABCMeta, object)): - ... - -Combining inheritance between Qt and the traitlets system, however, does -not work with this. Instead, we do this:: - - class QtKernelClientMixin(MetaQObjectHasTraits('NewBase', (HasTraits, SuperQObject), {})): - ... - -This gives the new class a metaclass of :class:`~IPython.qt.util.MetaQObjectHasTraits`, -and the parent classes :class:`~traitlets.HasTraits` and -:class:`~IPython.qt.util.SuperQObject`. - - -Doctests --------- - -.. function:: doctest_refactor_print(func_or_str) - - Refactors print statements in doctests in Python 3 only. Accepts a string - or a function, so it can be used as a decorator. - -.. function:: u_format(func_or_str) - - Handle doctests written with ``{u}'abcþ'``, replacing the ``{u}`` with ``u`` - for Python 2, and removing it for Python 3. - - Accepts a string or a function, so it can be used as a decorator. - -Execfile --------- - -.. function:: execfile(fname, glob, loc=None) - - Equivalent to the Python 2 :func:`python:execfile` builtin. We redefine it in - Python 2 to better handle non-ascii filenames. - -Miscellaneous -------------- - -.. autofunction:: safe_unicode -.. function:: isidentifier(s, dotted=False) +IPython 6 requires Python 3, so our compatibility module +``IPython.utils.py3compat`` is deprecated. In most cases, we recommend you use +the `six module `__ to support compatible code. +This is widely used by other projects, so it is familiar to many developers and +thoroughly battle-tested. - Checks whether the string s is a valid identifier in this version of Python. - In Python 3, non-ascii characters are allowed. If ``dotted`` is True, it - allows dots (i.e. attribute access) in the string. +Our ``py3compat`` module provided some more specific unicode conversions than +those offered by ``six``. If you want to use these, copy them into your own code +from IPython 5.x. Do not rely on importing them from IPython, as the module may +be removed in the future. -.. function:: getcwd() +.. seealso:: - Return the current working directory as unicode, like :func:`os.getcwdu` on - Python 2. + `Porting Python 2 code to Python 3 `_ + Official information in the Python docs. -.. function:: MethodType + `Python-Modernize `_ + A tool which helps make code compatible with Python 3. - Constructor for :class:`types.MethodType` that takes two arguments, like - the real constructor on Python 3. + `Python-Future `_ + Another compatibility tool, which focuses on writing code for Python 3 and + making it work on Python 2.