Build complex rules, serialize them as JSON, and execute them in Python
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Latest commit 5fda912 Mar 31, 2017

README.md

json-logic-py

This parser accepts JsonLogic rules and executes them in Python.

This is a Python porting of the excellent GitHub project by jwadhams for JavaScript: json-logic-js.

All credit goes to him, this is simply an implementation of the same logic in Python (small differences below).

The JsonLogic format is designed to allow you to share rules (logic) between front-end and back-end code (regardless of language difference), even to store logic along with a record in a database. JsonLogic is documented extensively at JsonLogic.com, including examples of every supported operation and a place to try out rules in your browser.

The same format can also be executed in PHP by the library json-logic-php

Examples

Simple

from json_logic import jsonLogic
jsonLogic( { "==" : [1, 1] } )
# True

This is a simple test, equivalent to 1 == 1. A few things about the format:

  1. The operator is always in the "key" position. There is only one key per JsonLogic rule.
  2. The values are typically an array.
  3. Each value can be a string, number, boolean, array (non-associative), or null

Compound

Here we're beginning to nest rules.

jsonLogic(
  {"and" : [
    { ">" : [3,1] },
    { "<" : [1,3] }
  ] }
)
# True

In an infix language (like Python) this could be written as:

( (3 > 1) and (1 < 3) )

Data-Driven

Obviously these rules aren't very interesting if they can only take static literal data. Typically jsonLogic will be called with a rule object and a data object. You can use the var operator to get attributes of the data object:

jsonLogic(
  { "var" : ["a"] }, # Rule
  { a : 1, b : 2 }   # Data
)
# 1

If you like, we support syntactic sugar on unary operators to skip the array around values:

jsonLogic(
  { "var" : "a" },
  { a : 1, b : 2 }
)
# 1

You can also use the var operator to access an array by numeric index:

jsonLogic(
  {"var" : 1 },
  [ "apple", "banana", "carrot" ]
)
# "banana"

Here's a complex rule that mixes literals and data. The pie isn't ready to eat unless it's cooler than 110 degrees, and filled with apples.

rules = { "and" : [
  {"<" : [ { "var" : "temp" }, 110 ]},
  {"==" : [ { "var" : "pie.filling" }, "apple" ] }
] }

data = { "temp" : 100, "pie" : { "filling" : "apple" } }

jsonLogic(rules, data)
# True

Always and Never

Sometimes the rule you want to process is "Always" or "Never." If the first parameter passed to jsonLogic is a non-object, non-associative-array, it is returned immediately.

#Always
jsonLogic(True, data_will_be_ignored);
# True

#Never
jsonLogic(False, i_wasnt_even_supposed_to_be_here);
# False

Installation

The best way to install this library is via PIP:

pip install json-logic

If that doesn't suit you, and you want to manage updates yourself, the entire library is self-contained in json_logic.py and you can download it straight into your project as you see fit.

curl -O https://raw.githubusercontent.com/nadirizr/json-logic-py/master/json_logic.py