Explicit return in __init__
===========================

``__init__`` is a `special Python method <https://docs.python.org/2/reference/datamodel.html#special-method-names>`_ that is automatically called when memory is allocated for a new object. The sole purpose of ``__init__`` is to initialize the values of instance members for the new object. Using ``__init__`` to return a value implies that a program is using ``__init__`` to do something other than initialize the object. This logic should be moved to another instance method and called by the program later, after initialization.

Anti-pattern
------------

The ``__init__`` method of the ``Rectangle`` class below attempts to return the area of the rectangle within the ``__init__`` method. This violates the rule of only using ``__init__`` to initialize instance members.

.. code:: python

    class Rectangle:
        def __init__(self, width, height):
            self.width = width
            self.height = height
            self.area = width * height
            # causes "Explicit return in __init__" error
            return self.area

Best practices
--------------

Remove the `return` statement from the ``__init__`` method
..........................................................

Remove the ``return`` statement in the ``__init__`` method that is returning a value.

.. code:: python

    class Rectangle:
        def __init__(self, width, height):
            self.width = width
            self.height = height
            self.area = width * height
            # return statement removed from here

Move the program logic to another instance method
.................................................

There is no reason why the ``Rectangle`` class MUST return the area immediately upon initialization. This program logic should be moved to a separate method of the ``Rectangle`` class. The program can call the method later, after the object has successfully initialized.

.. code:: python

    class Rectangle(object):
        def __init__(self, width, height):
            self.width = width
            self.height = height
            self._area = width * height

        @property
        # moved the logic for returning area to a separate method
        def area(self):
            return self._area

Note that the class must inherit from ``object`` now, since the ``property`` decorator only works for new style classes.

References
----------

- PyLint - E0101, return-in-init
- `Python Language Reference - object.__init__(self[, ...]) <https://docs.python.org/2/reference/datamodel.html#object.__init__>`_