PEP 08 states:
Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants.<
Putting the import statement inside of a function can prevent circular dependencies. For example, if you have 2 modules, X.py and Y.py, and they both need to import each other, this will cause a circular dependency when you import one of the modules causing an infinite loop. If you move the import statement in one of the modules then it won't try to import the other module till the function is called, and that module will already be imported, so no infinite loop. Read here for more - effbot.org/zone/import-confusion.htm