intensional 0.106

Intensional sets in Python

Intensional (rule-defined) sets for Python.

Overview

There are two ways of defining a set: intensional and extensional. Extensional sets like set([1,3,5,'daisy']) enumerate every member of the set explicitly.

Intensional sets, in contrast, are defined by rules. For example “the set of all prime numbers” or “every word beginning with 'a' and ending with 't'. Intensional sets are often infinite. It may be possible to generate a list of their members, but it’s not as simple as a “give me everything you’ve got!” for loop.

Once you know what you’re looking for, intensional sets are everywhere. Python doesn’t represent them directly, but regular expressions, many list comprehensions, and all manner of testing and filtering operations are really faces of the intensional set concept. Many functions test whether something ‘qualifies’. os.path.isdir(d) for example, tests whether d is in the set of legitimate directories, and isinstance(s, str) tests whether s is a member of the set of str objects. Even the core if conditional can be construed as testing for membership in an intensional set–the set of all items that pass the test.

Many such tests have a temporal aspect–they determine whether a value is a member right now. The answer may change in the future, if conditions change. Others tests are invariant over time. %%734 will never be a valid Python identifier, no matter how many times it’s tested–unless the rules of the overall Python universe change, that is.

Intensional sets are part and parcel of all programming, even if they’re not explicitly represented or called by that name.``intensional`` helps Python programs represent intensional sets directly.

NB Active construciton zone.

Installation

pip install intensional

(You may need to prefix this with “sudo “ to authorize installation.)