How to substitute multiple symbols in an expression in sympy?
The command x = Symbol('x') stores Sympy's Symbol('x') into Python's variable x. The Sympy expression f that you create afterwards does contain Symbol('x'), not the Python variable x.
When you reassign x = 0, the Python variable x is set to zero, and is no longer related to Symbol('x'). This has no effect on the Sympy expression, which still contains Symbol('x').
This is best explained in this page of the Sympy documentation:http://docs.sympy.org/latest/gotchas.html#variables
What you want to do is f.subs(x,0), as said in other answers.
Actually sympy is designed not to substitute values until you really want to substitute them with subs (see http://docs.sympy.org/latest/tutorial/basic_operations.html)
Try
f.subs({x:0})f.subs(x, 0) # as alternativeinstead of
x = 0