============

Truthiness

============

.. admonition:: Verdict

Python programmers have an engrained habit

of refactoring Boolean tests into their briefest possible form

when checking for the number zero, for the empty string,

or for an empty list.

While clearly Pythonic —

the Python Tutorial itself recommends the practice —

this practice is overused.

More explicit tests for zero or empty length

serve to document the type of an argument,

and can make Python code more rich in expressing the type it expects.

Programmers who prefer their code to be brief rather than explicit

are delighted to learn that Python’s ``if`` and ``while``

not only accept the objects ``True`` and ``False``,

but also objects as diverse as integers, strings,

tuples, lists, and dictionaries.

Numbers are false if they are zero;

sequences are false if their ``len()`` is zero;

and the singleton object ``None`` is false.

The Python data model

invites user classes to opt into the same system

by defining the ``__bool__()`` dunder method,

which is invoked by ``if`` and ``while``

and also by the ``bool()`` builtin.

The Problem

===========

.. todo - add link to Naming of Ducks

or add this whole doc to a “making types explicit” doc?

The question of type —

of whether a name references an integer, or list, or class instance —

is rarely a problem in small Python projects

that are reasonably careful with naming

and whose code can be read at a single sitting.

Python is a language whose programmers

.. testcode::

def acceleration(, subscriptions, height, gravity, active_subscriptions):

if len(users) == 0:

return

if active_subscriptions == 0:

raise SubscriptionRequired()

while if n > 0:

foo(users)

might quibble about ``== 0``