BUG: Breaking Python None Semantics

[smythi93]

In src/fandango/constraints/base.py line 179 contd is a None check, which breaks Python's semantics.
Also, this perfect fitness looks extremely sus...

[andreas-zeller]

I think this is there to accomodate constraints that simply call print() (or otherwise produce no value). Hence, ignoring None as a return value is not a bad thing.

[smythi93]

Then the printing should return evaluate to True for constraints, otherwise this would not be consistent with Python's semantic, e.g., things like
x = None
x and True

Would be completely skipped but should definitely evaluate to False.

[andreas-zeller]

My biggest concern would be functions like re.match() (which return None if there is no match), which could well be part of a constraint. To accommodate print() statements, we can suggest simply using print(x) or True.

I have commented ignoring None values out for now, so this is addressed.