Comment #6 on issue 2433 by [email protected]: Polys module function poly() needs gens argument
http://code.google.com/p/sympy/issues/detail?id=2433
Because Poly is a constructor and it already does a lot. It can construct a polynomial from a Poly instance, an expression, a list, tuple or other iterable and a dictionary. If it gets an expression, it uses expand() first and then tries to parse the expanded form. As we know, expand() is very slow, even if there is nothing to do. On the other hand, poly() applies it self to every subexpression and then uses polynomial arithmetics to obtain a polynomial (it uses Poly when a subexpression is "atomic"). poly() doesn't call expand() at all.
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