全局变量变为局部--UnboundLocalError：分配前引用了局部变量

Question

I initialized rate as a global var:

import os, sys
rate=30

def foo():
    print('#########rate:', rate)
    if False:
        rate=int(sys.argv[2])


foo()

but when running the script, I get the following error:

Traceback (most recent call last):
  Line 10, in <module>
    foo()
  Line 5, in foo
    print('#########rate:', rate)
UnboundLocalError: local variable 'rate' referenced before assignment

although the if False: rate=int(sys.argv[2]) statement is not executed, it seems has some influence, is there some python rules explains this?

Answer 1

You should declare rate as global:

import os, sys
rate=30

def foo():
    global rate # <----
    print('#########rate:', rate)
    if False:
        rate=int(sys.argv[2])


foo()

If there is assignment to a varaint (without global declaration), it is treated as local variable.