Stupid Python Ideas: August 2014

ADTs for Python

Recently, as an aside to the proposal for static type annotations in Python, Guido offhandedly noted another suggested improvement to Python, adding ADTs (Algebraic Data Types). This has also come up in discussions about pattern matching, and about existing run-time and compile-time static-typing libraries.

What is an ADT?

Instead of trying to fully define an ADT in formal terms (see Wikipedia, or, better, work through a Haskell tutorial, for that), I'll try to define it in terms of values as you might use them in Python.

Basically, an ADT is either a simple type, a product of ADTs, or a sum of ADTs. That's what makes it "algebraic".

Aug

A pattern-matching case statement for Python

The idea of a pattern-matching case statement has come up a few times recently (first in a proposal to add a C-style case statement, more recently as part of a the proposal to add static type annotations), but the discussion ends as soon as people realize that it's not quite as easy as it appears.

The core problem is that the key to pattern matching is unpacking constructors of algebraic data types, and Python has arbitrary OO-style classes, not ADTs.

Aug

How strongly typed is Python?

Weak typing

A language with weak typing is one where programs can escape the type system. Another way to describe it is that values can change types.

For example, in C, I can create a pointer to characters, then tell the compiler I want to use it as a pointer to integers:

char sz[] = "abcdefg"; int *i = (int *)sz; On a little-endian platform with 32-bit integers, this makes i into an array of the numbers 0x64636261 and 0x00676665.

Aug

How do comprehensions work?

The official tutorial does a great job explaining list comprehensions, iterators, generators, and generator expressions at a high level. Since some people don't want to read even that much, I wrote a post on comprehensions for dummies to summarize it.

And if you want to know the lowest level nitty gritty of how they work, there's documentation to point you to the right place, and then the code is relatively readable, at least if you understand C well and know the basics of how CPython works.