First off, class f(Function) is a strange way of doing things. Why aren't you
just using f = Function('f')?
Second, calling a bare sorted() on SymPy objects is a bad idea. SymPy objects
are not intended to be sorted. In Python 3, for example, this kind of sorting
among "uncomparable types" will not be allowed. The problem is that sorted()
calls < or > on the objects (i.e., __le__ or __ge__). But SymPy overrides
these to create Le() objects. These should work nicely with bool(), but if you
create a strange object like class f(Function), I guess it might not. (by the
way, how would others feel about making bool(Le) raise an error so that this
kind of thing doesn't even pretend to work?)
If you want to have a canonical order of sympy objects, you should pass some
kind of sorting key to sorted(). A good choice is hash, because it is quick
(i.e., sorted([Symbol('x'), Real(10)], key=hash)). hash has the disadvantage
of being platform dependent. In the future (i.e., the next release), we will
have a Basic.sorted_key() function that should sort things platform
independently (Mateusz, am I right about this?).
Aaron Meurer
On Feb 13, 2011, at 10:46 AM, Jeff wrote:
> Hi,
>
> I had a strange sympy error -- the last line in the code below leads
> to an infinite recursion and never ends.
>
> import sympy
> from sympy import Function, Symbol
>
> class f(Function):
> pass
>
> sorted([sympy.Real(10), Symbol('x')]) # ok
> f(10) # ok
> sorted([f(10), Symbol('x')]) # error never ends
>
> It is tested on sympy 0.6.7
>
> I appreciate any suggestions.
>
> Jeff
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