Comment 3 for bug 210177

The new diff looks much better. Thanks Hermann! I only have three more
comments :)

* I think we need a few more tests, especially for the edge cases. For
instance, what happens when you do:

   msg1 = _('Message 1 and $msg2', mapping={})
   msg1.mapping['msg2'] = _('Message 2 and $msg1', mapping={'msg1':
msg1})

I know, it's a bit of an edge case, but it should be dealt with.

* Copying the dictionary shouldn't be done with copy.copy. It can
easily be done with mapping.copy(). In other words, dictionaries have
a copy() method, you should use it.

* Style: Wrapping a long if-line is more nicely wrapped using
parantheses, rather than backspace. I suppose it's a matter of taste,
but PEP8 suggests using parantheses ("The preferred way of wrapping
long lines is by using Python's implied line continuation inside
parentheses, brackets and braces.").