py-predicate 0.3

Module to create composable predicates

Introduction

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

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, lazy_p

str_or_list_of_str = is_str_p | (is_list_p & all_p(lazy_p("str_or_list_of_str")))

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