Writing code for Python 2 and 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.
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.
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.
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:
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:`~IPython.utils.traitlets.HasTraits` and :class:`~IPython.qt.util.SuperQObject`.