======
import
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When you use ``import``, python first checks ``sys.modules`` to see if the *module/pkg* has been imported already. 

If **not**, it will try to import based on the directories of ``sys.path``.

* If ``import`` line includes **pkg**, it will run the ``__init__.py``.
* If ``import`` line includes **module**, it will run the whole file.

if **yes**, it won’t re-run the file no matter it is pkg(``__init__.py``) or module file.

Let’s say you have the following directory structure:

.. code-block:: none
 
  └── project
      ├── package1
      │   ├── module1.py
      │   └── module2.py
      └── package2
          ├── __init__.py
          ├── module3.py
          ├── module4.py
          └── subpackage1
              └── __init__.py
              └── module5.py

| Let’s assume the following:

* package1/module2.py contains a function, *function1*.
* package2/__init__.py contains a class, *class1*.
* package2/subpackage1/module5.py contains another function, *function2*.

.. code-block:: python

  from package1 import module1       
  # There isn’t __init__.py file, so skip it. This will run whole module1.py.
  
  from package1.module2 import function1
  # This will run package1(__init__.py) and whole module2.py
  
  from package2 import class1
  # This will run package2’s __init__.py

  from package2.subpackage1.module5 import function2
  # This will skip package2’s __init__.py since it has been imported. It could be verified by sys.modules. It will run subpackage2’s __init__.py and then whole module5.py


__name__ == "__main__"
----------------------

Assuming we have files below.

.. code:: python

  # foo.py
  import bar
  print("foo.__name__ set to ", __name__)

.. code:: python

  # bar.py
  print("bar.__name__ set to ", __name__)

.. code:: python

  >>> python bar.py
  # Output: 
  # bar.__name__ set to __main__

  >>> python foo.py
  # Output:
  # bar.__name__ set to bar
  # foo.__name__ set to __main__

So if your module is first argument after ``python``, ``__name__`` will be set to ``__main__``. If module ran by ``import``, ``__name__`` is set to module name.