._apply_module_morphism() in combinat/free_module.py doesn't handle zero element properly

[zabrocki]

a potential defect in ._apply_module_morphism function calls list( self.basis().keys() ) when self.is_zero() is True. This works fine if self.basis() is a finite set, but causes a problem if self.basis() is an infinite set with an iteration function. In this case, the function hangs trying to list the infinite set.

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Apply only: trac_13119_apply_module_morphism_bugfix-mz.patch​

[zabrocki]

I added a patch to this ticket. Discussion is at ​https://groups.google.com/d/topic/sage-combinat-devel/yPjfNINtVbk/discussion

I changed one thing in addition since I had originally reported this bug. That is, if the codomain is not known and it looks for the parent of an element of the image to determine the codomain then raise a value error if the element returned by the function doesn't have a parent.

[zabrocki]

Removed a bit of whitespace within the function

Apply: trac_13119_apply_module_morphism_bugfix-mz.2.patch

[tscrim]

Hey Mike,

Two minor things:

• Could you use the auto-linking :func:`on_basis` in the docstring?
• Since at some point, there is a shift to Python 3, I think we should start using that syntax:

  raise ValueError('Codomain could not be determined')
  

Looks good to me otherwise.

Thanks,
Travis

For patchbot:

Apply only: trac_13119_apply_module_morphism_bugfix-mz.patch

[zabrocki]

Hi Travis, Thanks for picking this up. I made your changes and caught one or two more fixes in the docstring. -Mike

[tscrim]

Looks good. Thanks Mike.