If operators with different priorities are used, consider adding whitespace around the operators with the lowest priority(ies). Use your own judgment; however, never use more than one space, and always have the same amount of whitespace on both sides of a binary operator:
# Correct:
i = i + 1
submitted += 1
x = x*2 - 1
hypot2 = x*x + y*y
c = (a+b) * (a-b)
Note that SymPy does the right thing, so we know there is enough semantic information to do this correctly:
>>> from sympy import var
>>> var('x')
x
>>> ((3*x + 5) * (2*x - 4)).expand()
6*x**2 - 2*x - 20
Per PEP8:
Note that SymPy does the right thing, so we know there is enough semantic information to do this correctly: