Not using ``else`` where appropriate in a loop
==============================================

The Python language provides a built-in ``else`` clause for ``for`` loops. If a ``for`` loop completes without being prematurely interrupted by a ``break`` or ``return`` statement, then the ``else`` clause of the loop is executed.

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

The code below searches a list for a magic number. If the magic number is found in the list, then the code prints ``Magic number found``. If the magic number is not found, then the code prints ``Magic number not found``.

The code uses a flag variable called ``found`` to keep track of whether or not the magic number was found in the list.

The logic in this code is valid; it will accomplish its task. But the Python language has built-in language constructs for handling this exact scenario and which can express the same idea much more concisely and without the need for flag variables that track the state of the code.

.. code:: python

    l = [1, 2, 3]
    magic_number = 4
    found = False

    for n in l:
        if n == magic_number:
            found = True
            print("Magic number found")
            break

    if not found:
        print("Magic number not found")

Best practice
-------------

Use ``else`` clause with ``for`` loop
.....................................

In Python, you can declare an ``else`` loop in conjunction with a ``for`` loop. If the ``for`` loop iterates to completion without being prematurely interrupted by a ``break`` or ``return`` statement, then Python executes the ``else`` clause of the loop.

In the modified code below, the ``for`` loop will iterate through all three items in the list. Because the magic number is not contained in the list, the ``if`` statement always evaluates to ``False``, and therefore the ``break`` statement is never encountered. Because Python never encounters a ``break`` statement while iterating over the loop, it executes the ``else`` clause.

The modified code below is functionally equivalent to the original code above, but this modified code is more concise than the original code and does not require any flag variables for monitoring the state of the code.

.. code:: python

    l = [1, 2, 3]
    magic_number = 4

    for n in l:
        if n == magic_number:
            print("Magic number found")
            break
    else:
        print("Magic number not found")


.. note:: Since ``else`` on a ``for`` loop is so unintuitive and error-prone, even some experienced Python developers suggest not using this feature at all.

References
----------

- `Python Language Reference - else Clauses on Loops <https://docs.python.org/2/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops>`_