.. currentmodule:: djem
.. versionadded:: 0.7
The :const:`UNDEFINED` constant is designed for use in argument default values.
Sometimes it is necessary to know whether any value was passed into a function or not, including values traditionally used as argument defaults (such as None, False, etc). Any given value could have an explicit meaning, with some default behaviour only performed if nothing was passed in.
Take the following contrived example:
def make_thing(name, label=None):
if not label:
label = name
...The make_thing function takes a required name argument and an optional label argument. The label defaults to the name unless overridden with something more meaningful/verbose. But what if the caller would like their thing to have no label? And what if None, False or an empty string could all indicate that? The function needs to differentiate those values from nothing being provided.
This example could be rewritten using :const:`UNDEFINED` in place of None as the argument default, allowing the caller to explicitly pass None to indicate that the label should not be given a default value:
from djem import UNDEFINED
def make_thing(name, label=UNDEFINED):
if label is UNDEFINED:
label = name
...:const:`UNDEFINED` is "falsey", so can also be used in more generic conditional statements:
from djem import UNDEFINED
value = UNDEFINED
if value:
print('truthy')
else:
print('falsey')
# output: 'falsey'