Local import statements in Python

by

The other answers evince a mild confusion as to how import really works.

This statement:

import foo

is roughly equivalent to this statement:

foo = __import__('foo', globals(), locals(), [], -1)

That is, it creates a variable in the current scope with the same name as the requested module, and assigns it the result of calling __import__() with that module name and a boatload of default arguments.

The __import__() function handles conceptually converts a string ('foo') into a module object. Modules are cached in sys.modules, and that’s the first place __import__() looks–if sys.modules has an entry for 'foo', that’s what __import__('foo') will return, whatever it is. It really doesn’t care about the type. You can see this in action yourself; try running the following code:

import sys
sys.modules['boop'] = (1, 2, 3)
import boop
print boop

Leaving aside stylistic concerns for the moment, having an import statement inside a function works how you’d want. If the module has never been imported before, it gets imported and cached in sys.modules. It then assigns the module to the local variable with that name. It does not not not modify any module-level state. It does possibly modify some global state (adding a new entry to sys.modules).

That said, I almost never use import inside a function. If importing the module creates a noticeable slowdown in your program—like it performs a long computation in its static initialization, or it’s simply a massive module—and your program rarely actually needs the module for anything, it’s perfectly fine to have the import only inside the functions in which it’s used. (If this was distasteful, Guido would jump in his time machine and change Python to prevent us from doing it.) But as a rule, I and the general Python community put all our import statements at the top of the module in module scope.

More Related Contents:

  1. How do I import a module given the full path?
  2. How can I import other Python files?
  3. Import a module from a relative path
  4. Python error “ImportError: No module named”
  5. ImportError: No module named requests
  6. Circular import dependency in Python
  7. What exactly does “import *” import?
  8. Why is Python running my module when I import it, and how do I stop it?
  9. Should wildcard import be avoided?
  10. How to mock an import
  11. Trying to import module with the same name as a built-in module causes an import error
  12. Callable modules
  13. List all the modules that are part of a python package?
  14. Importing modules: __main__ vs import as module
  15. ModuleNotFoundError: What does it mean __main__ is not a package?
  16. How can I check if a module has been imported?
  17. Dynamic loading of python modules
  18. Cyclic module dependencies and relative imports in Python
  19. Python: Why should ‘from import *’ be prohibited?
  20. What happens when using mutual or circular (cyclic) imports?
  21. How can I import a module dynamically given its name as string?
  22. django import error – No module named core.management
  23. Pillow installed, but getting “no module named pillow” when importing
  24. Reading a file using a relative path in a Python project
  25. Can you define aliases for imported modules in Python?
  26. how to “reimport” module to python then code be changed after import
  27. What are good rules of thumb for Python imports?
  28. Python import error: cannot import name ‘six’ from ‘sklearn.externals’
  29. How can I do relative imports in Python?
  30. How to add a Python import path using a .pth file

Leave a Comment