.. title: The Story of None: Part 4 - Guard Clauses
.. slug: none_04_guard_clauses
.. date: 2013/02/01 17:00
.. tags: python, patterns, pythonic, planetpython
.. description: 

The Story of None: Part 4 - Guard Clauses
=========================================

`part 1 <http://blog.startifact.com/posts/none_01_the_beginning.html>`_ `part 2 <http://blog.startifact.com/posts/none_02_recognizing.html>`_ `part 3 <http://blog.startifact.com/posts/none_03_handling_it.html>`_ `part 4 <http://blog.startifact.com/posts/none_04_guard_clauses.html>`_ `part 5 <http://blog.startifact.com/posts/none_05_more_on_guarding.html>`_ `part 6 <http://blog.startifact.com/posts/none_06_avoiding_it.html>`_

Last time...
-------------

`Last time`_ in the Story of None we ended up with this validation
function::

  def validate_end_date_later_than_start(start_date, end_date):
      if start_date is None and end_date is None:
          return
      if start_date is None:
          return
      if end_date is None:
          return
      if end_date <= start_date:
          raise ValidationError(
              "The end date should be later than the start date.")

.. _`Last time`: http://blog.startifact.com/posts/none_03_handling_it

So let's apply some boolean logic and rewrite all this to be slightly
shorter (but still readable)::

  def validate_end_date_later_than_start(start_date, end_date):
      if start_date is None or end_date is None:
          return
      if end_date <= start_date:
          raise ValidationError(
              "The end date should be later than the start date.")

Guard Clauses
-------------

What we've written above with the ``if ... return`` statement is a
`guard clause`_: a check at the beginning of a function that returns
early from the function if the condition is true.

.. _`guard clause`: http://c2.com/cgi/wiki?GuardClaus

Instead we could have written the function without guard clauses, like
this::

  def validate_end_date_later_than_start(start_date, end_date):
      if start_date is not None and end_date is not None:
          if end_date <= start_date:
             raise ValidationError(
                "The end date should be later than the start date.")
  
or like this::

  def validate_end_date_later_than_start(start_date, end_date):
      if (start_date is not None and 
          end_date is not None and
          end_date <= start_date):
             raise ValidationError(
                "The end date should be later than the start date.")

I think both alternatives are a lot less clear than the one with the
guard clause in it. 

With a function it's better if the main thing that it is doing, in
this case comparing ``end_date`` with ``start_date``, is also the main
unindented block. This makes the function easier to read. If you make
the main behavior of the function be inside some other ``if`` clause,
it becomes slightly harder to read.

The second alternative doesn't create this second level of
indentation, but still makes the comparison between start and end date
rather hidden among the ``is not None`` clauses. This makes the
expression a bit harder to think about compared to the version of the
function that uses the guard clause.

Guard and Stop Worrying
-----------------------

The great advantage of a guard clause is that you can *forget* about
the stuff you're guarding against in the code you write after the
guard clause. So once we have passed the guards, we can entirely stop
worrying about ``start_date`` or ``end_date`` being ``None``; we've
already guarded against those cases and handled them. 

This is great! We can compare ``start_date`` and ``end_date`` freely.
We can call methods on ``start_date`` and ``end_date`` safely. We can
pass ``start_date`` and ``end_date`` to other functions and *they*
don't need to worry about them being ``None`` either, unless of course
such functions can be called from somewhere else where it could be
``None``.

We worry about ``None`` so we can *stop* worrying about ``None`` when
it matters. Ah, such a relief! Thank you guard clauses!

We'll talk a bit more about guard clauses next_.

.. _next: http://blog.startifact.com/posts/none_05_more_on_guarding.html

`part 1 <http://blog.startifact.com/posts/none_01_the_beginning.html>`_ `part 2 <http://blog.startifact.com/posts/none_02_recognizing.html>`_ `part 3 <http://blog.startifact.com/posts/none_03_handling_it.html>`_ `part 4 <http://blog.startifact.com/posts/none_04_guard_clauses.html>`_ `part 5 <http://blog.startifact.com/posts/none_05_more_on_guarding.html>`_ `part 6 <http://blog.startifact.com/posts/none_06_avoiding_it.html>`_