How to modify meaning of builtin function "not" to "!"?

[grbgooglefan]

I am creating functions, the return result of which I am using to make
decisions in combined expressions.
In some expressions, I would like to inverse the return result of
function.

E.g. function contains(source,search) will return true if "search"
string is found in source string.
I want to make reverse of this by putting it as:
if ( ! contains(s1,s2) ):
     return 1

I found that "!" is not accepted by Python & compile fails with
"invalid syntax".
Corresponding to this Boolean Operator we've "not" in Python.

How can I make "not" as "!"?