py-predicate 1.5

Module to create composable predicates

Introduction

py-predicate is a typed Python library to create composable predicates.

These predicates can be used for generating test-cases (property based testing) and run-time instrumenting of functions against a specification based on predicates.

Getting started

To get started, install the library with pip

pip install py-predicate

The full documentation can be found here. We give 2 small examples to show what the library can do.

Example 1

filtered = [x for x in range(10) if x >= 2 and x <= 3]

Version with predicates:

from predicate import ge_p, le_p

ge_2 = ge_p(2)
le_3 = le_p(3)

between_2_and_3 = ge_2 & le_3
filtered = [x for x in range(10) if between_2_and_3(x)]

Of course this example looks way more complicated than the original version. The point here is that you can build reusable predicates that can be used in multiple locations.

So lets do just that, reuse our predicate to create a generator. The generate_false will create an infinite series of integers for which the predicate between_2_and_3 is False. The generate_true will create an infinite (well, with lots of duplicates obviously) series of integers for which the predicate is True.

from predicate import generate_false, generate_true, ge_p, le_p
from more_itertools import take

ge_2 = ge_p(2)
le_3 = le_p(3)

between_2_and_3 = ge_2 & le_3

take(5, generate_true(between_2_and_3))

take(5, generate_false(between_2_and_3))

This might be useful for example in unit tests.

Example 2

A unique (?) py-predicate feature is that you can define self referencing predicates. This makes it easy to apply predicates to arbitrarily nested structures, like JSON data.

In the next example we define a predicate, that tests if a given data structure is either a string, or a list of data that can again either be a string or a list of data. Ad infinitum.

from predicate import all_p, is_list_p, is_str_p, root_p

str_or_list_of_str = is_str_p | (is_list_p & all_p(root_p))

Using plain Python, the above one-liner would have to be coded as a (recursive) function.