Python Import – How to Import a Module in Nested Packages

importpackagepython

This is a python newbie question:

I have the following directory structure:

test
 -- test_file.py
a
 -- b
   -- module.py

where test, a and b are folders. Both test and a are on the same level.

module.py has a class called shape, and I want to instantiate an instance of it in test_file.py. How can I do so?

I have tried:

from a.b import module

but I got:

ImportError: No module named a.b

Best Answer

What you want is a relative import like:

from ..a.b import module

The problem with this is that it doesn't work if you are calling test_file.py as your main module. As stated here:

Note that both explicit and implicit relative imports are based on the name of the current module. Since the name of the main module is always "main", modules intended for use as the main module of a Python application should always use absolute imports.

So, if you want to call test_file.py as your main module, then you should consider changing the structure of your modules and using an absolute import, else just use the relative import from above.

EDIT:

Looks like from . import some_module will do what you're after.

Python – How to Import a Module from a Package

Short answer

So what does the from packages import * really mean inside that __init__.py?

The __init__.py imports itself.

Explanation

You can only import modules, not packages. Packages are just containers for modules or sub-packages. When you "import" a package you actually import the module __init__.py.

The __init__.py with this content:

from packages import mod

imports the module mod into __init__.py. Therefore, it will be available in your main.py via packages.mod (remember packages is represented by __init__.py).

When you change the content of __init__.py to:

from packages import *

You are importing the module __init__.py, the very same file you are in. This works (a second import just triggers a lookup in sys.modules) but won't give you the content of mod.

This means, you can use:

from module import *

but you cannot sensibly use this with an empty __init__.py:

from package import *

Because package is actually represented by the __init__.py and there is nothing in it yet. You can check this (interactively or in file):

>>> import packages
>>> print(packages)
<module 'packages' from '/.../packages/__init__.py'>

In __init__.py you can write:

from packages.mod import *

and then in main.py:

print packages.hello()

works. Because the function hello() is now in the global name space of the file __init__.py.

As mentioned in the answer by mozman, you can use __all__ in __init__.py to list the modules that should be imported if from packages import * is used. This is designed for this case.

The __init__.py has only this content:

__all__ = ['mod']

Now you can do this in main.py:

from packages import *

print mod.hello()

If you extend your __init__.py:

__all__ = ['mod']

from packages import *

You can do this in main.py:

import packages

print packages.mod.hello()

But if you remove the from packages import * from __init__.py:

__all__ = ['mod']

You will get an error:

AttributeError: 'module' object has no attribute 'mod'

because the __all__ is only used for the from packages import * case. Now we are back to the __init__.py imports itself.

Best Answer

Related Solutions

Python – Import Package in Module Inside Same Package

EDIT:

Python – How to Import a Module from a Package

Short answer

Explanation

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