I am learning Python and while studying modules two doubts came to my mind.
Doubt #1:
I know that module is just simply python file such as filename.py, but what is a sub-module in Python?
Doubt #2:
Consider the following three lines of code:
import modulename
from pkgname import modulename
from pkgname import *
Do all three statements use __init__.py (which is inside pkgname package) or only the third statement?
To make things a bit more clearer about your second question.
When you do -
import packagename.modulename
or
from packagename import modulename
Python internally first imports packagename , and when I say python imports packagename , I mean it imports the __init__.py of that package , and then after that it imports modulename . This is the reason why when you do any of the above it imports the __init__.py .
When you do -
from packagename import *
Please note, this does not import modulename from packagename by default , this would only import the __init__.py from packagename , and all modules that you have listed in __all__ list in __init__.py , if no modules are listed in that list, none would be imported. Example -
Lets say I have
shared
-- __init__.py
-- a.py
__init__.py looks like -
print("In Shared")
a.py look like -
print("In A")
Now when in the directory above shared ,and openning python, if you do -
from shared import *
It would print out -
In Shared
But if you change that code in __init__.py to -
print("In Shared")
__all__ = ['a']
And do the same import from same location, It would print out -
In Shared
In A
As you can see it only imports submodules that are defined in the __all__ list.
Finally , when you do -
import modulename
Lets say you do that directly from within packagename , by changing the directory to it and openning python interactive interpreter there. At that time, you are not asking Python to import packagename for you, so it does not need to import packagename , and hence it does not import __init__.py .
As Andrea Corbellini says in the comment:
__init__.py file.pkgname/__init__.py is used whenever you import pkgname or one of its submodules (that is, lines 2 and 3 of your example). Your first example doesn't import any package or module of a package therefore it doesn't need to use __init__.py (Which can be empty)If there are these files, and you are working on the same folder
./pkgname/__init__.py # Submodule of pkgname package used on example 3) to import all the submodules
./pkgname/Asdf.py # Submodule of pkgname package
./pkgname/modulename.py # Submodule of pkgname package imported in example 2) using __init__.py
./modulname.py # Module imported on example 1)
__init__.py to import ./modulname.py. Check it
pkgname/__init__.pyis used whenever you importpkgnameor one of its submodules (that is, lines 2 and 3 of your example).__init__.py) any module in that package can be considered a sub-module of that package/module.