Comment #3 on issue 3313 by laurence.gonsalves: Function class violates
Liskov substitution principle
http://code.google.com/p/sympy/issues/detail?id=3313
Thanks.
I actually came across this while trying to work around issue 3312. The
following code works around that issue:
f = sympy.Function(name)
f.is_negative = False
def lt(self, other):
if type(self) != type(other):
return type(self) < type(other)
return self.args < other.args
f.__lt__ = lt
I tried converting this code to subclass UndefinedFunction rather than
monkey-patching, but ran into complications. At least one of __lt__ or
is_negative needs to be on the function *application* rather than on the
function itself, and I haven't figured out how to make my subclass of
UndefinedFunction return my own subclass of AppliedUndef.
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