Stupid Python Ideas: January 2015

Statements and Expressions

On the Python-ideas list, in yet another thread on a way to embed statements in expressions, I raised the issue that the statement-expression distinction, and not having a way to escape it, is important to why Python is so readable. But I couldn't explain exactly why.

The question ultimately comes down to: why have statements in the first place? After all, there's no reason you can't make (almost) everything an expression, even in an imperative language (Ruby and CoffeeScript do so), and use significant indentation within expressions (again, CoffeeScript does so).

Guido's answer

Hm... Practically every language I knew before I designed Python had this distinction built right into the grammar and other assumptions: Algol-60, Fortran, Pascal, C, ABC. Even Basic.

Jan

An Abbreviated Table of binary64 Values

In IEEE Floats and Python, I chose to use a tiny "binary6" type because it's easy to show a table of values that helps clarify things.

Someone suggested that you could just as easily write a table of binary64 values, ellipsizing large chunks of it, and it would also be useful for clarifying things. And I think he's right.

Jan

IEEE Floats and Python

Everybody knows that "floating point numbers cause problems." Many people have misconceptions about what those problems are, and how to deal with them. There are some great references for people who want to understand things, the (justly) most famous being What Every Computer Scientist Should Know About Floating-Point Arithmetic by David Goldberg.

But many Python programmers like to learn by experimenting.

Jan

Subtyping and Ducks

Many tutorials on object-oriented programming conflate inheritance and subtyping. In fact, it's often considered part of the OO dogma that they should be conflated.

This is wrong for Python in a wide variety of ways.

Substitutability

Let's start with the Liskov Substitution Principle: A is a subtype of B iff instances of type A can substitute for instances of type B without affecting correctness or semantics.

Jan

Greenlets, threads, and processes

It's very common in a program to want to do two things at once: repaginate a document while still responding to user input, or handle requests from two (or 10000) web browsers at the same time. In fact, pretty much any GUI application, network server, game, or simulator needs to do this.

It's possible to write your program to explicitly switch off between different tasks, and there are many higher-level approaches to this, which I've covered in previous posts.

Jan

Why don't you want getters and setters?

If you've migrated to Python from C++ or one of its descendants (Java, C#, D, etc.), or to a lesser extent from other OO languages (Objective C, Ruby, etc.), the first time you asked for help on StackOverflow or CodeReview or python-list or anywhere else, the first response you got was probably: "Get rid of those getters and setters."

If you asked why, you probably got no more of an answer than "You don't need them in Python."

That's exactly the response you should get—but that doesn't mean it