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. 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.