python - order of import for modules

问题:

For example there is some folder:

.
├── file_a.py
├── file_b
│   └── __init__.py
└── file_b.py

where file_a.py has something like:

from file_b import some_function

I know this is definitely not good practice, but what is the order of resolution behind?

i.e. How python decides which module to be imported for an "absolute import"?

回答1:

I'm not sure where (or whether) this information is in the documentation - a quick check in the import system docs didn't turn it up - but PEP 420 says the following:

While looking for a module or package named "foo", for each directory in the parent path:

• If <directory>/foo/__init__.py is found, a regular package is imported and returned.
• If not, but <directory>/foo.{py,pyc,so,pyd} is found, a module is imported and returned. The exact list of extension varies by platform and whether the -O flag is specified. The list here is representative.
• If not, but <directory>/foo is found and is a directory, it is recorded and the scan continues with the next directory in the parent path.
• Otherwise the scan continues with the next directory in the parent path.

If the scan completes without returning a module or package, and at least one directory was recorded, then a namespace package is created.

So according to PEP 420, if a package and a non-package module are found in the same directory with the same name, the package wins.

回答2:

Based on @JonKiparsky 's suggestion, I dug through importlib a bit, and finally found importlib._bootstrap.FileFinder.find_spec, which does indeed explicitly check for a directory before looking for valid files that fit the package name. Glad to see @user2357112 's answer, though, that this is supposed to be defined behavior, and isn't just happenstance.

回答3:

I did a little experiment, made somelib.py and somelib/__init__.py Then tried the following

>>> from somelib import foo
in somelib/__init__.py
>>>

Evidently python prefers a directory-based module to a file-based one

As @scnerd points out, this could be accidental behavior, but it's what I'm seeing in the default implementation.

If you want to know how this happens, you would probably review importlib for the gruesome details