A Python implementation of the Maybe pattern.
pip install pymaybe
from pymaybe import maybe
first_name = maybe(deep_hash)['account']['user_profile']['first_name'].or_else("<unknown>")
Maybe monad is a programming pattern that allows to treat None values that same way as non-none values. This is done by wrapping the value, which may or may not be None to, a wrapper class.
The implementation includes two classes: Maybe and Something. Something represents a value while Nothing represents a None value. There's also a method maybe which wraps a regular value and and returns Something or Nothing instance.
>>> maybe("I'm a value")
"I'm a value"
>>> maybe(None);
None
Both Something and Nothing implement 4 methods allowing you to test their real value: is_some, is_none, get and or_else
>>> maybe("I'm a value").is_some()
True
>>> maybe("I'm a value").is_none()
False
>>> maybe(None).is_some()
False
>>> maybe(None).is_none()
True
>>> maybe("I'm a value").get()
"I'm a value"
>>> maybe("I'm a value").or_else(lambda: "No value")
"I'm a value"
>>> maybe(None).get()
Traceback (most recent call last):
...
Exception: No such element
>>> maybe(None).or_else(lambda: "value")
'value'
>>> maybe(None).or_else("value")
'value'
In addition, Something and Nothing implement the Python magic methods allowing you to treat them as dictionaries:
>>> nested_dict = maybe(nested_dict)
>>> nested_dict['store']['name']
'MyStore'
>>> nested_dict['store']['address']
None
>>> nested_dict['store']['address']['street'].or_else('No Address Specified')
'No Address Specified'
All other method calls on Something are forwarded to its real value:
>>> maybe('VALUE').lower()
'value'
>>> maybe(None).invalid().method().or_else('unknwon')
'unknwon'
The Maybe pattern helps you avoid nasty try..except blocks. Consider the following code:
try:
url = rss.load_feeds()[0].url.domain
except (TypeError, IndexError, KeyError, AttributeError):
url = "planetpython.org"
With Maybe you could simply do:
url = maybe(rss).load_feeds()[0]['url'].domain.or_else("planetpython.org")
Getting the current logged in user's name. Without maybe:
def get_user_zipcode():
address = getattr(request.user, 'address', None)
if address:
return getattr(address, 'zipcode', '')
return ''
With maybe:
def get_user_zipcode():
return maybe(request.user).address.zipcode.or_else('')
Copyright 2015 - Eran Kampf
- Free software: BSD license
- Documentation: https://pymaybe.readthedocs.org.
- Code is hosted on GitHub