UnboundLocalError on nested module reimport

Question

I get an UnboundLocalError when I reimport an already imported module in python 2.7. A minimal example is

#!/usr/bin/python

import sys

def foo():
    print sys
    import sys

foo()

Traceback (most recent call last):
  File "./ptest.py", line 9, in <module>
    foo()
  File "./ptest.py", line 6, in foo
    print sys
UnboundLocalError: local variable 'sys' referenced before assignment

Howver, when the nested import is placed as the first statement in the function definition then everything works:

#!/usr/bin/python

import sys

def foo():
    import sys
    print sys

foo()

<module 'sys' (built-in)>

Can someone please explain why the first script fails? Thanks.

Answer 1

This is the same as referencing global variable. It is well explained in Python FAQ

This is because when you make an assignment to a variable in a scope, that variable becomes local to that scope and shadows any similarly named variable in the outer scope. Since the last statement in foo assigns a new value to x, the compiler recognizes it as a local variable. Consequently when the earlier print(x) attempts to print the uninitialized local variable and an error results.