gistfile1.py

import sys

def foo():
    if False:
        import sys
        sys.exit(1)
    print sys.argv[0]

foo()

# ➜  tmp  python foo.py
# Traceback (most recent call last):
#   File "foo.py", line 9, in <module>
#     foo()
#   File "foo.py", line 7, in foo
#     print sys.argv[0]
# UnboundLocalError: local variable 'sys' referenced before assignment
# ➜  tmp