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python import module from a package

folder structure:

<current dir>
 main.py
 packages <dir>
 __init__.py
 mod.py

main py:

import packages
print packages.mod.hello()

mod.py:

def hello():
 return 'hello'

__init__.py:

from packages import mod

If I run main.py, I get no error. But if I edit __init__.py to 'from packages import *' , I get this error: AttributeError: 'module' object has no attribute 'mod'

I'm not asking how to make that 'print' command work. I can use other 'import' syntax in main.py to make it work. The question is: I'm curious about that 'from packages import mod' in the __init__.py. If i can do import mod then when I replace to import *, which means import everything, why do I get an error instead?

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

Anyone can help? Thanks

Answer*

## 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][1] 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.

 [1]: https://docs.python.org/3/tutorial/modules.html#importing-from-a-package

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i can do from package import * since there will be no error when compiled, only if i print then it said no module 'mod'. So that command is valid , only i'm confused about what the code is doing .

andio
– andio

2016年04月09日 10:13:55 +00:00
Commented Apr 9, 2016 at 10:13
"You can only import modules, not packages," suggests that packages and modules are mutually exclusive. However, per The Python Language Reference, §5.2 Packages: "[A]ll packages are modules, but not all modules are packages. ... [P]ackages are just a special kind of module." Further a package can be imported: "When a regular package is imported, this __init__.py file is implicitly executed, and the objects it defines are bound to names in the package’s namespace."

Jim Ratliff
– Jim Ratliff

2022年05月06日 16:23:43 +00:00
Commented May 6, 2022 at 16:23
See When you "import" a package you actually import the module __init__.py. in answer. This is essentially what you say. Not?

Mike Müller
– Mike Müller

2022年05月19日 07:30:24 +00:00
Commented May 19, 2022 at 7:30

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