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Developer’s tips for writing code for Python 2 and 3

As of matplotlib 1.4, the six library is used to support Python 2 and 3 from a single code base. The 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.

Welcome to the __future__

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 six gets used a lot.

Finding places to use six

The only way to make sure code works on both Python 2 and 3 is to make sure it is covered by unit tests.

However, the 2to3 commandline tool can also be used to locate places that require special handling with six.

(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.

The dreaded \u escapes

When 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 r"\usepackage{foo}". 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 slashes, e.g. "\\usepackage{foo}".

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'

Iteration

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 zip, range and 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 dict.items():

Python 2 Python 3 six
d.items() list(d.items()) list(six.iteritems(d))
d.iteritems() d.items() six.iteritems(d)

Numpy-specific things

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.