Fix Python – Should import statements always be at the top of a module?

Question

Asked By – Adam J. Forster

PEP 8 states:

Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants.

However if the class/method/function that I am importing is only used in rare cases, surely it is more efficient to do the import when it is needed?

Isn’t this:

class SomeClass(object):

    def not_often_called(self)
        from datetime import datetime
        self.datetime = datetime.now()

more efficient than this?

from datetime import datetime

class SomeClass(object):

    def not_often_called(self)
        self.datetime = datetime.now()

Now we will see solution for issue: Should import statements always be at the top of a module?


Answer

Module importing is quite fast, but not instant. This means that:

  • Putting the imports at the top of the module is fine, because it’s a trivial cost that’s only paid once.
  • Putting the imports within a function will cause calls to that function to take longer.

So if you care about efficiency, put the imports at the top. Only move them into a function if your profiling shows that would help (you did profile to see where best to improve performance, right??)


The best reasons I’ve seen to perform lazy imports are:

  • Optional library support. If your code has multiple paths that use different libraries, don’t break if an optional library is not installed.
  • In the __init__.py of a plugin, which might be imported but not actually used. Examples are Bazaar plugins, which use bzrlib‘s lazy-loading framework.

This question is answered By – John Millikin

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0