Skip to main content
This is a pre-production deployment of Warehouse. Changes made here affect the production instance of PyPI (
Help us improve Python packaging - Donate today!

Not all nor any. Just the one.

Project Description

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.


>>> from one import one
>>> one((True, False, False))
>>> one((True, False, True))
>>> one((0, 0, 'a'))
>>> one((0, False, None))
>>> one((True, True))
>>> one(('', 1))
>>> one((10, 20, 30, 42), lambda i: i > 40)


pip install one


from one import one

Patterns 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)):

# using one
if one((a, b, c)):

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!

Release History

Release History

This version
History Node


History Node


History Node


Download Files

Download Files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

File Name & Checksum SHA256 Checksum Help Version File Type Upload Date
one-0.2.1-py2.py3-none-any.whl (4.2 kB) Copy SHA256 Checksum SHA256 2.7 Wheel Sep 24, 2014
one-0.2.1.tar.gz (5.0 kB) Copy SHA256 Checksum SHA256 Source Sep 24, 2014

Supported By

WebFaction WebFaction Technical Writing Elastic Elastic Search Pingdom Pingdom Monitoring Dyn Dyn DNS Sentry Sentry Error Logging CloudAMQP CloudAMQP RabbitMQ Heroku Heroku PaaS Kabu Creative Kabu Creative UX & Design Fastly Fastly CDN DigiCert DigiCert EV Certificate Rackspace Rackspace Cloud Servers DreamHost DreamHost Log Hosting