Why are there no ++ and --​ operators in Python?


Question

Why are there no ++ and -- operators in Python?

1
418
12/9/2018 1:36:10 PM

Accepted Answer

It's not because it doesn't make sense; it makes perfect sense to define "x++" as "x += 1, evaluating to the previous binding of x".

If you want to know the original reason, you'll have to either wade through old Python mailing lists or ask somebody who was there (eg. Guido), but it's easy enough to justify after the fact:

Simple increment and decrement aren't needed as much as in other languages. You don't write things like for(int i = 0; i < 10; ++i) in Python very often; instead you do things like for i in range(0, 10).

Since it's not needed nearly as often, there's much less reason to give it its own special syntax; when you do need to increment, += is usually just fine.

It's not a decision of whether it makes sense, or whether it can be done--it does, and it can. It's a question of whether the benefit is worth adding to the core syntax of the language. Remember, this is four operators--postinc, postdec, preinc, predec, and each of these would need to have its own class overloads; they all need to be specified, and tested; it would add opcodes to the language (implying a larger, and therefore slower, VM engine); every class that supports a logical increment would need to implement them (on top of += and -=).

This is all redundant with += and -=, so it would become a net loss.

428
9/6/2010 11:57:42 PM

This original answer I wrote is a myth from the folklore of computing: debunked by Dennis Ritchie as "historically impossible" as noted in the letters to the editors of Communications of the ACM July 2012 doi:10.1145/2209249.2209251


The C increment/decrement operators were invented at a time when the C compiler wasn't very smart and the authors wanted to be able to specify the direct intent that a machine language operator should be used which saved a handful of cycles for a compiler which might do a

load memory
load 1
add
store memory

instead of

inc memory 

and the PDP-11 even supported "autoincrement" and "autoincrement deferred" instructions corresponding to *++p and *p++, respectively. See section 5.3 of the manual if horribly curious.

As compilers are smart enough to handle the high-level optimization tricks built into the syntax of C, they are just a syntactic convenience now.

Python doesn't have tricks to convey intentions to the assembler because it doesn't use one.


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