Question: How do you set a global variable in a function in Python?

x = 1 # make a global
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 attempts to print an uninitialized local variable and will trigger 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.