[Python-ideas] Reverse assignment operators (=+, =-, =*, =/, =//, =**, =%)

Steven D'Aprano steve at pearwood.info
Mon Nov 14 06:16:27 EST 2016


On Mon, Nov 14, 2016 at 01:19:30AM +0100, Mikhail V wrote:

[...]
> >> A good syntax example:
> >>
> >> a = sum (a, c)

There is a reason why mathematicians and accountants use symbols as a 
compact notation for common functions, instead of purely functional 
notation.

Here is a real-world example, the formula for compound interest:

A = P*(1+r/100)**n

Compare that to:

A = mul(P, pow(sum(1, div(r, 100)), n))

Even Reverse Polish Notation is easier to read than that series of 
functions:

P 1 r 100 / + n ** *


Don't misunderstand me, functions are important, and over-use of cryptic 
symbols that are only meaningful to an expert will hurt readability and 
maintainability of code. But people have needed to do arithmetic for 
over six thousand years, and there is no good substitute for compact 
operators for basic operations.


[...]
> It is kind of clear from the context, that I am speaking of syntax and
> not how things are working under the hood, or?
> If a compiler cannot optimize "a = a + 1" into an in-place operation,
> that is misfortune.

That's not how Python works. Or at least, not without an extremely 
powerful and smart compiler, like PyPy.

In Python, integers (and floats) are immutable objects. They have to be 
immutable, otherwise you would have things like this:

x = 1
y = x
x = x + 1  # changes the object 1 in place
print(y*10)  # expect 10, but get 20

That's how lists work, because they are mutable:

py> x = [1]
py> y = x
py> x[0] = x[0] + 1
py> print(y[0]*10)  # expect 1*10 = 10
20


A "sufficiently smart" compiler can work around this, as PyPy does under 
some circumstances, but you shouldn't expect this optimization to be 
simple.

[...]
> A better option would be to support unary operators so the user
> can write directly without assignment:
> 
> inc (a, 1)
> 
> Would mean in-place increment "a" with 1

That is impossible with Python's executation model, in particular the 
"pass by object sharing" calling convention. I expect that this would 
require some extremely big changes to the way the compiler works, 
possibly a complete re-design, in order to allow pass by reference 
semantics.

The biggest problem is that even if the compiler could choose between 
calling conventions, it would have to make that decision at runtime. It 
won't know until runtime that inc() requires pass by reference. That 
would make function calls even slower than they are now.


-- 
Steve


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