As of matplotlib 1.4, the six
library is used to support Python 2 and 3 from a single code base.
2to3 tool is no longer used.
This document describes some of the issues with that approach and some recommended solutions. It is not a complete guide to Python 2 and 3 compatibility.
The top of every
py file should include the following:
from __future__ import (absolute_import, division, print_function, unicode_literals) import six
This will make the Python 2 interpreter behave as close to Python 3 as possible.
All matplotlib files should also import
six, whether they are using
it or not, just to make moving code between modules easier, as
gets used a lot.
The only way to make sure code works on both Python 2 and 3 is to make sure it is covered by unit tests.
2to3 commandline tool can also be used to locate places
that require special handling with
(The modernize tool may also be handy, though I’ve never used it personally).
The six documentation serves as a good reference for the sorts of things that need to be updated.
from __future__ import unicode_literals is used, all string
literals (not preceded with a
b) will become unicode literals.
Normally, one would use “raw” string literals to encode strings that
contain a lot of slashes that we don’t want Python to interpret as
special characters. A common example in matplotlib is when it deals
with TeX and has to represent things like
Unfortunately, on Python 2there is no way to represent
u in a raw
unicode string literal, since it will always be interpreted as the
start of a unicode character escape, such as
u20af. The only
solution is to use a regular (non-raw) string literal and repeat all
The following shows the problem on Python 2:
>>> ur'\u' File "<stdin>", line 1 SyntaxError: (unicode error) 'rawunicodeescape' codec can't decode bytes in position 0-1: truncated \uXXXX >>> ur'\\u' u'\\\\u' >>> u'\u' File "<stdin>", line 1 SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 0-1: truncated \uXXXX escape >>> u'\\u' u'\\u'
This bug has been fixed in Python 3, however, we can’t take advantage of that and still support Python 2:
>>> r'\u' '\\u' >>> r'\\u' '\\\\u' >>> '\u' File "<stdin>", line 1 SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 0-1: truncated \uXXXX escape >>> '\\u' '\\u'
The behavior of the methods for iterating over the items, values and
keys of a dictionary has changed in Python 3. Additionally, other
built-in functions such as
map have changed to
return iterators rather than temporary lists.
In many cases, the performance implications of iterating vs. creating
a temporary list won’t matter, so it’s tempting to use the form that
is simplest to read. However, that results in code that behaves
differently on Python 2 and 3, leading to subtle bugs that may not be
detected by the regression tests. Therefore, unless the loop in
question is provably simple and doesn’t call into other code, the
six versions that ensure the same behavior on both Python 2 and 3
should be used. The following table shows the mapping of equivalent
semantics between Python 2, 3 and six for
|Python 2||Python 3||six|
When specifying dtypes, all strings must be byte strings on Python 2
and unicode strings on Python 3. The best way to handle this is to
force cast them using
str(). The same is true of structure
specifiers in the
struct built-in module.