Adam J. Forster Adam J. Forster - 4 days ago 5
Python Question

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

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.


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()

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.
Comments