Demonstration of 'import stomping' behavior of imports inside functions

import_stomp.py

import os as foo
import os as bar

print()
print(f"Initial {'rmdir' in dir(foo)=}")
print(f"Initial {'rmdir' in dir(bar)=}")
print()

def no_global():
    import sys as foo

    print(f"Inside no_global {'rmdir' in dir(foo)=}")
    print(f"Inside no_global {'rmdir' in dir(bar)=}")

no_global()
print()

print(f"After no_global() {'rmdir' in dir(foo)=}")
print(f"After no_global() {'rmdir' in dir(bar)=}")
print()

def with_global():
    global foo
    import sys as foo

    print(f"Inside with_global {'rmdir' in dir(foo)=}")
    print(f"Inside with_global {'rmdir' in dir(bar)=}")

with_global()
print()

print(f"After with_global() {'rmdir' in dir(foo)=}")
print(f"After with_global() {'rmdir' in dir(bar)=}")
print()

We print 'rmdir' in dir(thing) as a probe for whether thing is the os module or not. If the test yields True, then it's os.

Output on Windows Python 3.12.8:

>python import_stomp.py

Initial 'rmdir' in dir(foo)=True
Initial 'rmdir' in dir(bar)=True

Inside no_global 'rmdir' in dir(foo)=False
Inside no_global 'rmdir' in dir(bar)=True

After no_global() 'rmdir' in dir(foo)=True
After no_global() 'rmdir' in dir(bar)=True

Inside with_global 'rmdir' in dir(foo)=False
Inside with_global 'rmdir' in dir(bar)=True

After with_global() 'rmdir' in dir(foo)=False
After with_global() 'rmdir' in dir(bar)=True

foo and bar are both os at first.

Inside no_global, the import overwrites foo to be sys, but once the function exits and the overwritten foo goes out of scope, foo in the module scope is os again.

Inside with_global, though, the import overwrites the module-scope foo. Module-scope foo is then checked inside with_global and found to be sys, and then checked again outside with_global and found to be sys there also.