Why must Python list addition be homogenous?

Question

Can anyone familiar with Python\'s internals (CPython, or other implementations) explain why list addition is required to be homogenous:

In [1]: x = [1]

In          


        

Answer 1

My guess is that Python is strongly typed, and there's not a clear indication of the right thing to do here. Are you asking Python to append the string itself, or to cast the string to a list (which is what you indicated you'd like it to do)?

Remember explicit is better than implicit. In the most common case, neither of those guesses is correct and you're accidentally trying to do something you didn't intent. Raising a TypeError and letting you sort it out is the safest, most Pythonic thing to do here.

Answer 2

From the Zen of Python:

In the face of ambiguity, refuse the temptation to guess.

Let's look at what happens here:

x + y

This gives us a value, but of what type? When we add things in real life, we expect the type to be the same as the input types, but what if they are disparate? Well, in the real world, we refuse to add 1 and "a", it doesn't make sense.

What if we have similar types? In the real world, we look at context. The computer can't do this, so it has to guess. Python picks the left operand and lets that decide. Your issue occurs because of this lack of context.

Say a programmer wants to do ["a"] + "bc" - this could mean they want "abc" or ["a", "b", "c"]. Currently, the solution is to either call "".join() on the first operand or list() on the second, which allows the programmer to do what they want and is clear and explicit.

Your suggestion is for Python to guess (by having a built-in rule to pick a given operand), so the programmer can do the same thing by doing the addition - why is that better? It just means it's easier to get the wrong type by mistake, and we have to remember an arbitrary rule (left operand picks type). Instead, we get an error so we can give Python the information it needs to make the right call.

So why is += different? Well, that's because we are giving Python that context. With the in-place operation we are telling Python to modify a value, so we know that we are dealing with something of the type the value we are modifying is. This is the context Python needs to make the right call, so we don't need to guess.

When I talk about guessing, I'm talking about Python guessing the programmer's intent. This is something Python does a lot - see division in 3.x. / does float division, correcting the error of it being integer division in 2.x.

This is because we are implicitly asking for float division when we try to divide. Python takes this into account and it's operations are done according to that. Likewise, it's about guessing intent here. When we add with + our intent is unclear. When we use +=, it is very clear.

Answer 3

These bug reports suggest that this design quirk was a mistake.

Issue12318:

Yes, this is the expected behavior and yes, it is inconsistent.

It's been that way for a long while and Guido said he wouldn't do it again (it's in his list of regrets). However, we're not going to break code by changing it (list.__iadd__ working like list.extend).

Issue575536:

The intent was that list.__iadd__ correspond exactly to list.extend(). There's no need to hypergeneralize list.__add__() too: it's a feature that people who don't want to get surprised by Martin-like examples can avoid them by using plain + for lists.

(Of course, there are those of us who find this behaviour quite surprising, including the developer who opened that bug report).

(Thanks to @Mouad for finding these).

Answer 4

I believe Python designers made addition this way so that '+' operator stays consistently commutative with regard to result type: type(a + b) == type(b + a)

Everybody expects that 1 + 2 has the same result as 2 + 1. Would you expect [1] + 'foo' to be the same as 'foo' + [1]? If yes, what should be the result?

You have 3 choices, you either pick left operand as result type, right operand as result type, or raise an error.

+= is not commutative because it contains assignment. In this case you either pick left operand as result type or throw. The surprise here is that a += b is not the same as a = a + b. a += b does not translate in English to "Add a to b and assign result to a". It translates to "Add a to b in place". That's why it doesn't work on immutables such as string or tuple.

Thanks for the comments. Edited the post.