one, the missing Python function
Not all nor any: just the one.
one is a simple function to check if there is a unique value
that evaluates to True in an iterable, and return it. Optionally,
it receives a callable as the test function.
Examples:
>>> from one import one
>>> one((True, False, False))
True
>>> one((True, False, True))
False
>>> one((0, 0, 'a'))
'a'
>>> one((0, False, None))
False
>>> one((True, True))
False
>>> one(('', 1))
1
>>> one((10, 20, 30, 42), lambda i: i > 40)
42Install & usage
Install it via pip:
$ pip install one
from one import oneSince the version 0.6.3 one is also part of the wonderful library boltons. So, alternatively
you can install that:
$ pip install boltons
And import one
from boltons.iterutils import onePatterns and use cases
one could replace the following patterns:
true_values = [i for i in iterable if cmp(i)]
if len(true_values) == 1:
return true_values[0]
return False
# using one
return one(iterable, cmp)Another pattern, is in a complex condition
if ((a and not b and not c) or
(b and not a and not c) or
(c and not a and not b)):
do_something()
# using one
if one((a, b, c)):
do_something()A very frequent case is when you have values that must exclude each others.
class ShopStore(models.Model):
address = models.CharField(max_length=200, null=True, blank=True)
is_online = models.BooleanField(default=False)
def clean(self):
if not one((self.address, self.is_online)):
raise models.ValidationError(u'A shop must be online or physical, but not both')Send me your examples!
