Why UnboundLocalError occurs when importing inside function

Question

For some reason this code produces error:

import os

def main():
    print(os.path.isfile('/bin/cat'))
    import os

if __name__ == '__main__':
    main()

Result:

Traceback (most recent call last):
  File "test.py", line 10, in <module>
    main()
  File "test.py", line 5, in main
    print(os.path.isfile('/bin/cat'))
UnboundLocalError: local variable 'os' referenced before assignment

Why it happens? Note that at beginning of both cases there is import os. Somehow additional import in the end of the body of a function affects whole scope of this function.

If you remove import inside the function, everything is fine (which is not surprising).

import os

def main():
    print(os.path.isfile('/bin/cat'))
    # import os

if __name__ == '__main__':
    main()

Result:

True

About possible duplicates: There are some similar questions, but regarding global variables, not imports.

If you import os in the global scope, you're creating a global variable called `os`. If you import os in local scope, you're creating a local variable called `os`. And if you try and use a local variable in a function before it's created, you get that error. — khelwood, Nov 07 '19 at 14:09

score 1 · Accepted Answer · answered Nov 07 '19 at 14:10

1

If you import os in the global scope, you're creating a global variable called os. If you import os in local scope, you're creating a local variable called os. And if you try and use a local variable in a function before it's created, you get that error. Same as if you were explicitly assigning a variable.

The same solutions apply, if you want the import inside the function to create a global variable, you can use the global keyword:

def main():
    global os
    print(os.path.isfile('/bin/cat'))
    import os

Or you could change your local import to use a different variable name so that your use of os is unambiguous.

def main():
    print(os.path.isfile('/bin/cat'))
    import os as _os

Though obviously this is just an example for demonstration, and there's no reason in this case to reimport os inside your function when you've already imported it globally.

answered Nov 07 '19 at 14:10

khelwood

55,782
14
81
108

Note that I do `import os` at beginning in both examples. I just still do not understand why **additional** import in the line somewhere below causes error at the beginning of the function. – dankal444 Nov 08 '19 at 13:50
1

Because if you're creating a variable called `os` within your function (which you are, with `import os`), then the interpreter regards all references to the name `os` in the function as local variables rather than global variables. That's exactly the same as all the other questions about global variable use in a function. – khelwood Nov 08 '19 at 13:54
Ok, so defining variable inside a function shadows whole scope of the function, not just lines after this definition? If so, that was the thing I lacked – dankal444 Nov 08 '19 at 14:05
Since your question said `There are some similar questions, but regarding global variables, not imports` I thought you might have known that part, since that's exactly what all those dupes are about. – khelwood Nov 08 '19 at 14:24
Global variables are a bit different than imports. They share a lot, but, for example, you do not need to specify imports at the beginning of each function as you have to do with global variables. I still do not know why it is so, but I think it is material for another question. – dankal444 Nov 08 '19 at 14:32

Why UnboundLocalError occurs when importing inside function

1 Answers1