Python : How to set a global variable in a function

x = 1 # make a global module variable

def f():
      print x # try to print the global
      ...
      for j in range(100):
           if q > 3:
              x=4

Any variable assigned in a function is local to that function, unless it is specifically declared global. Since a value is bound to x as the last statement of the function body, the compiler assumes that x is local. Consequently the “print x” statement attempts to print an uninitialized local variable and will trigger aUnboundLocalError (or in earlier Python versions, a NameError).

The solution is to insert an explicit global declaration at the start of the function:

def f():
      global x
      print x # try to print the global
      ...
      for j in range(100):
           if q > 3:
              x=4

In this case, all references to x are interpreted as references to the x from the module namespace.

Note that the global declarations must be placed at the beginning of the function, and that it affects all uses of the variable inside the function.