The rel\_imp utility
====================

Enabling explicit relative imports in main modules the easy way.

For enabling relative import in ``__main__`` module PEP 366 presents a
workaround like:

.. code:: python

    if __name__ == "__main__" and __package__ is None:
        __package__ = "my_pkg"

    from .foo import bar

With ``rel_imp`` you can rewrite it as:

.. code:: python

    import rel_imp; rel_imp.init()
    from .foo import bar

Cleaner, faster and less coupled. (you don't need to specify the package
manually)

**Note:** In order to use ``rel_imp`` the module you are coding must be
inside a package or a sub-package.

Why using explicit relative imports?
------------------------------------

Python 2 still supports implicit relative import and will be deprecated
in 3, so you will want to migrate those scripts using implicit relative
import to explicit relative import. Check more on the `PEP
404 <http://legacy.python.org/dev/peps/pep-0404/#id18>`__.

Explicit relative imports makes your code less coupled. As `PEP
328 <http://legacy.python.org/dev/peps/pep-0328/#rationale-for-relative-imports>`__
says:

    Several use cases were presented, the most important of which is
    being able to rearrange the structure of large packages without
    having to edit sub-packages. In addition, a module inside a package
    can't easily import itself without relative imports.

Although it is still a matter of taste. (I personally prefer less code
to express the same)

Why running sub-modules as main?
--------------------------------

Some reasons:

1. A submodule can become a command line tool if called as main.
2. You can have unit test or smoke test within the module.
3. You simply want to run it without any explicit test to see if at
   least it imports everything it needs.

Installation and Uninstallation
-------------------------------

Install via pip
~~~~~~~~~~~~~~~

::

    pip install rel_imp

Remove it with:

::

    pip uninstall rel_imp

Install downloading the file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Download the ``rel_imp.py`` in one your Python's search path.

::

    wget https://raw.githubusercontent.com/joaduo/rel_imp/master/rel_imp.py

Example
-------

Imagine you have 2 modules inside a package called "my\_pkg".

That would be:

-  ``my_pkg/__init__.py``
-  ``my_pkg/math_lib.py``
-  ``my_pkg/test.py``

So in test.py we could have

.. code:: python

    from .math_lib import factorize

    def print_factorize(number):
        num = factorize(number)
        print num

    if __name__ == '__main__':
        #Small smoke test
        factorize_and_print(10)
        factorize_and_print(0)
        factorize_and_print(-10)

If you do ``python my_pkg/test.py`` it will throw an exception because
of the relative import at the first line.

PEP 366 presents a workaround like:

.. code:: python

    if __name__ == "__main__" and __package__ is None:
        __package__ = "my_pkg"

    from .math_lib import factorize

This will make the code work, but it is not an elegant solution.

So you can use ``rel_imp`` to make your code look nicer. Simply do:

.. code:: python

    import rel_imp; rel_imp.init()
    from .math_lib import factorize

It is equivalent as the prior solution but you don't have to worry about
keeping in sync ``__package__``'s value.

Notes on Windows
----------------

For some reason when executing - at least in wine - seems python won't
add the current directory to the ``sys.path`` list, but the path of the
ran script. For example running:

::

    python my_pkg\test.py

Will add the path to ``my_pkg`` to ``sys.path`` and not the current path
to ``sys.path``. This differ from the linux's behavior - which adds
current working dir to sys.path -.

To fix this behavior make sure you have set the correct ``sys.path`` or
you can add this ugly hack before importing ``rel_imp``.

.. code:: python

    import sys, os
    sys.path.append(os.path.abspath(os.curdir))

How does it work?
-----------------

It uses the same technique in PEP 366 but ``__package__``'s value is set
through dynamic inspection of the stack. To solve the value of
``__package__`` it compares the current ``__main__``'s file with search
paths in sys.path - or, optionally, a list of paths given in the
settings -.

For example, for a file in
``/home/user/projects/python/math/my_pkg/test.py`` given the following
paths in sys.path:

.. code:: python

    [
    '/home/user/projects/python/',
    '/home/user/projects/python/math/',
    '/home/user/projects/python/math/my_pkg/',
    ]

It will pick the closest path to the ``__main__``'s file that is not the
``__main__``'s file's directory.

Then the base path use to solve ``__package__`` variable will be
``/home/user/projects/python/math/``

Disabling rel\_imp
------------------

Relative import shouldn't have any non-desired side effect, but if for
some reason you want to disable you can:

1. Replace the ``def init()`` function in the ``rel_imp.py`` file (if
   you downloaded it)
2. Create an empty ``rel_imp.py`` with a dummy ``init()`` in a
   PYTHONPATH with higher priority than the installed one (or optionally
   uninstall the original one)

``rel_imp`` is really lightweight so you won't gain any performance
difference disabling it.

Feedback or bugs reporting
--------------------------

File an issue through github's `issue tracking
system <https://github.com/joaduo/rel_imp/issues>`__.

You can optionally contact me at joaduo gmail com.

Related PEPs
------------

-  http://legacy.python.org/dev/peps/pep-0328/
-  http://legacy.python.org/dev/peps/pep-0366/

