import vs __import__( ) vs importlib.import_module( )?

Question

I noticed Flask was using Werkzeug to __import__ a module, and I was a little confused. I went and checked out the docs on it and saw that it seems to give you more control somehow in terms of where it looks for the module, but I'm not sure exactly how and I have zero idea how it's different from importlib.import_module.

The odd thing in the Werkzeug example is that it just says __import__(import_name), so I don't see how that's any different from just using the import statement, since it's ignoring the optional extra parameters.

Can anyone explain? I looked at other people having asked similar questions on SO previously but they weren't very clearly phrased questions and the answers didn't address this at all.

Answer 1

__import__ is a low-level hook function that's used to import modules; it can be used to import a module dynamically by giving the module name to import as a variable, something the import statement won't let you do.

importlib.import_module() is a wrapper around that hook^* to produce a nice API for the functionality; it is a very recent addition to Python 2, and has been more fleshed out in Python 3. Codebases that use __import__ generally do so because they want to remain compatible with older Python 2 releases, e.g. anything before Python 2.7.

One side-effect of using __import__ can be that it returns the imported module and doesn't add anything to the namespace; you can import with it without having then to delete the new name if you didn't want that new name; using import somename will add somename to your namespace, but __import__('somename') instead returns the imported module, which you can then ignore. Werkzeug uses the hook for that reason in one location.

All other uses are to do with dynamic imports. Werkzeug supports Python 2.6 still so cannot use importlib.

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^*importlib is a Pure-Python implementation, and import_module() will use that implementation, whist __import__ will use a C-optimised version. Both versions call back to importlib._bootstrap._find_and_load() so the difference is mostly academic.

Answer 2

__import__(import_name), so I don't see how that's any different from just using the import statement

Both __import__() and importlib.import_module() allow you to import a module when you have the module name as a string. You cannot write:

x = 're'
import x

or you'll get:

 File "1.py", line 3, in <module>
 import x ImportError: No module named x