Question

# coding: utf-8

def func():
    print 'x is', x
    #x = 2   #if I add this line, there will be an error, why?
    print 'Changed local x to', x
x = 50
func()
print 'Value of x is', x 

1. I don’t add the global x in func function, but it can still find x is 50, why?
2. When I add the x=2 line in the func function, there will be an error (UnboundLocalError: local variable 'x' referenced before assignment), why?

Answer 1

The trick here is that local names are detected statically:

• As long as the name x is not assigned in the function, references to x resolve to the the global scope
• If the name x is assigned anywhere in the function, Python assumes that x is thus a local name everywhere in the function. As a consequence, the first line becomes an error because local name x is used before being assigned.

In other words: assigned name is treated as local everywhere in the function, not just after the point of assignment.