Stupid Python Ideas: December 2015

How lookup works

The documentation does a great job explaining how things normally get looked up, and how you can hook them.

But to understand how the hooking works, you need to go under the covers to see how that normal lookup actually happens.

When I say "Python" below, I'm mostly talking about CPython 3.5. Other implementations vary to differing degrees in how they implement things; they just have to result in the same rules being followed.

Variables: LEGB

Python has simple lexical scoping, like most other languages. (Unlike many languages, only module, function, and class definitions create new scopes, not every suite, but that's not too important here.

Dec

How functions work

In Python (I'm mostly talking about CPython here, but other implementations do similar things), when you write the following:

def spam(x): return x+1 spam(3) What happens?

Really, it's not that complicated, but there's no documentation anywhere that puts it all together. Anyone who's tried hacking on the eval loop, understands it, but explaining it someone else is very difficult.

Dec

Why you can't have exact decimal math

I've seen a number of people ask why, if you can have arbitrary-sized integers that do everything exactly, you can't do the same thing with floats, avoiding all the rounding problems that they keep running into.

If we compare the two, it should become pretty obvious at some point within the comparison.

Integers

Let's start with two integers, a=42 and b=2323, and add them. How many digits do I need? Think about how you add numbers: you line up the columns, and at worst carry one extra column.

Dec

Can you customize method resolution order?

In a recent thread on python-ideas, Stephan Sahm suggested, in effect, changing the method resolution order (MRO) from C3-linearization to a simple depth-first search a la old-school Python or C++. I don't think he realized that's what he was proposing at first, but the key idea is that he wants Mixin to override A in B as well as overriding object in A in this code:

class Mixin: pass class A(Mixin): pass class B(Mixin, A): pass

In other words, the MRO should be B-Mixin-A-Mixin-object.

Dec

Prototype inheritance is inheritance

Note: This post doesn't talk about Python that much, except as a point of comparison for JavaScript.

Most object-oriented languages out there, including Python, are class-based. But JavaScript is instead prototype-based. Over the years, this has led to a lot of confusion, and even more attempts to resolve that confusion, either by faking classes, or by explaining why prototypes are good. Unfortunately, most of the latter attempts are completely wrong, and only add to the confusion.

Dec

Pattern matching again

About a year and a half ago, I wrote a blog post on the idea of adding pattern matching to Python.

I finally got around to playing with Scala semi-seriously, and I realized that they pretty much solved the same problem, in a pretty similar way to my straw man proposal, and it works great. Which makes me think that it's time to revisit this idea. For one thing, using Scala terminology that people may be familiar with (and can look up) can't hurt.