Simplify get_legend_handler method

[Kojoley]

I have simplified the get_legend_handler method. There is no need to create list from keys of legend_handler_map and no need in two dictionary lookups (if key in dict: return dict[key]).
Loop can be optimized even more, but will result in a code bloat, which does not worth of saving few cpu cycles.

[Kojoley]

It is not clear for me why there was no check for unhashble orig_handle in original method as if it is then it cannot be a key of a dictionary and maybe lines below was some unclear way to do such check?

        legend_handler_keys = list(six.iterkeys(legend_handler_map))
        if orig_handle in legend_handler_keys:

Should I add such check or close pull request?

[tacaswell]

It is side-stepping having to do a check by using a simpler data structure. The __contains__ check on a list is just a linear search (so hashability does not matter). If it is is the list, then it is a key (and hence must be hashable), if the key is not hashible, it can not be a key and hence will not be in the list.

I would revert the first set of changes, add a comment as to why it is that way, and leave the second half.

Adding a test to the test suite that exercises the pathological input would be good too.

[Kojoley]

Yes, I understand this, but here we have a linear search in a dictionary by creating temporary list of dictionary keys and this list lives only withing the function.
As we cannot use isinstance(orig_handle, collections.Hashable) (because some objects define itself as hashable, but can become unhashable in runtime e.g. tuple) here I think check below is more suitable than the linear search.

    def get_legend_handler(legend_handler_map, orig_handle):
        try:
            hash(orig_handle)
        except TypeError:
            return None
        ...

[tacaswell]

This is different behavior, If it is not hash-able, then we need to walk it's MRO to see if we have a handle for any of it's classes.

This is apparently not tested well enough.

[tacaswell]

in cbook.py we have a ishashable function which basically does this, but returns a boolean. I can see going either way on using it here.

[Kojoley]

I have updated PR as per your suggestion and squashed multiple commits