rcParams.keys() is not Python 3 compatible

[mdboom]

When I run rcParams.keys() in Python 3 I get the following exception:

AttributeError: 'dict_keys' object has no attribute 'sort'

This is due to the fact that the keys method does not return a list but a dict_keys object, which has no sort method.

To retain the behavior from Python 2 I suggest the following patch:

--- a/lib/matplotlib/__init__.py
+++ b/lib/matplotlib/__init__.py
@@ -733,7 +733,7 @@ See rcParams.keys() for a list of valid parameters.' % (key,))
         """
         Return sorted list of keys.
         """
-        k = dict.keys(self)
+        k = list(dict.keys(self))
         k.sort()
         return k

This works fine in Python 2 and 3.

For completeness, I used Python 3.2.3 and matplotlib 1.2.0 from Debian unstable/experimental.

Thanks for the great work.

[mdboom]

The transformation you quote should already have been performed by 2to3. Are you perhaps importing matplotlib from the source tree, or from a 2.x install?

[tukss]

I agree that 2to3 should do this but it doesn't. You can test that easily by running 2to3 on __init__.py.
The reason is probably that we directly call the method of the dict class instead of the keys method of a dict instance. In the latter case 2to3 would add list() as expected.

So this seems to be a deficiency in 2to3 rather than matplotlib. I can report this to the Python bug tracker but I am not sure there will be a fix for that.

For the time being I guess we need a manual fix. What is your opinion?

[dmcdougall]

There are no Travis failures indicating errors of this kind, so I am curious as to why you get this error. How does the error arise? Do you get it right after import matplotlib?

[dmcdougall]

Addendum: AFAIK, Travis runs 2to3 on the whole source tree.

[tukss]

Generally, matplotlib in Python 3 works fine. I got this error while trying to modify rcParams from ipython (If you just type rcParams to see all parameters, you get the error). It's clearly a minor issue that can easily go unnoticed. I guess none of the tests try the keys() method of rcParams.

[dmcdougall]

Has your ipython been built with python 3?

[tukss]

Yes, it has. This issue does not have anything to do with ipython, I just happened to notice it there first.

Steps to reproduce this with plain python3:

>>> import matplotlib
>>> matplotlib.rcParams.keys()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/matplotlib/__init__.py", line 726, in keys
    k.sort()
AttributeError: 'dict_keys' object has no attribute 'sort'

[mdboom]

Interesting -- I see this now -- 2to3 is not covering this as I expected.

2to3 does do this:

 x = {}
-for k in x.keys():
+for k in list(x.keys()):
     pass

But leaves this alone (perhaps because there is an argument to keys here).

class Foo(dict):
    def keys(self):
        k = dict.keys(self)
        k.sort()
        return k

I'll go ahead and attach a commit with the change you recommended.