judge

judge is a Python library for choosing values based on a tree of conditions.

For example:

from judge import decide, rules, gt, lt, eq, is_number, otherwise

positivity = rules(
	(is_number, rules(
		(gt(0), 'positive'),
		(eq(0), 'zero'),
    	(lt(0), 'negative'),
	)),
	(otherwise, 'not a number')
)

decide(positivity, -10)     # 'negative'
decide(positivity, 0)       # 'zero'
decide(positivity, 10)      # 'positive'
decide(positivity, 'text')  # 'not a number'

Installation

$ pip install judge

Usage

judge.decide(rules_obj, *args, **kwargs)

Return first matching value from rules_obj by invoking each predicate within it with *args, **kwargs until a match is found. rules_obj must be an instance of rules.

If a non-rules object is passed as rules_obj, raise TypeError.

If no match is found, raise NoMatchingRule.

If no arguments are passed, then return a partially applied function that can be later used to choose a value using a the given set of rules.

judge.rules(*rules)

A list of predicates to be used in the decision process.

Each rule is a 2-tuple (predicate, value). predicate is a function that returns True or False depending on whether the given arguments match some condition. value is the value that will be returned if the predicate matches. Rules can be nested by setting value to another rules object.

Built-in predicates

Generic

Predicate	Matches
otherwise	Catch-all that matches anything

Type checks

Predicate	Matches
instancechecker(cls)	Instances of cls
subclasschecker(cls)	Subclasses of cls
is_string	Strings
is_number	Numbers (ints, floats, decimals...)
is_callable	Callables (functions, classes...)
is_sequence	Sequences (lists, tuples...)
is_mapping	Mappings (dicts)

Identity checks

Predicate	Matches
is_(other)	value is other
is_not(other)	value is not other

Comparisons

Predicate	Matches
eq(other)	value == other
ne(other)	value != other
gt(other)	value > other
ge(other)	value >= other
lt(other)	value < other
le(other)	value <= other

Collections

Predicate	Matches
contains(element)	element in value

Mathematical

Predicate	Matches
divisible_by(*ys)	Numbers that are divisible by all of the given divisors ys

Strings

Predicate	Matches
starts_with(sub)	value.startswith(sub)
ends_with(sub)	value.endswith(sub)

Examples

Declarative FizzBuzz

from judge import decide, rules, divisible_by, otherwise

fizzbuzz = decide(rules(
	(divisible_by(3, 5), "FizzBuzz"),
	(divisible_by(3), "Fizz"),
	(divisible_by(5), "Buzz"),
	(otherwise, None)
))

for number in range(1, 101):
	print(fizzbuzz(number) or number)