Python in-place complement operator

Question

Python has the usual bitwise operators, like ~, &, |, etc. and in-place operators like +=, &=, etc. to simplify expressions like:

a = a & 34234
a = a + 577

To:

a &= 34234
a += 577

Despite the complement operator ~ being an unary function, and not following the same structure because it isn't used with two values (like a and 34234), can expressions like these be simplified with another type of operator?

a = ~a # not bad at all

# Still easy to code but seems redundant
self.container.longVariableName.longName = ~self.container.longVariableName.longName

Use shorter variable names. If using it twice is too cumbersome, I find it hard to imagine that using it once would be any better. — Slater Victoroff
– Slater Victoroff, Commented Jan 4, 2014 at 3:19
@SlaterTyranus this is a bit of a made-up issue, I know. The real question to me is about a possible ~=-like operation. And sometimes shorter variable names aren't favourable over ones that are easily understood, depending on the application. — SimonT
– SimonT, Commented Jan 4, 2014 at 3:32

Tim Peters · Accepted Answer · 2014-01-04 03:36:44Z

3

It's excruciatingly obscure, but:

self.container.longVariableName.longName ^= -1

does the job, so long as the values you're dealing with are integers. "Are integers" is required so that there's an exploitable mathematical relation between the ~ and ^ operators.

Why it works:

Bitwise complement is the same as xor'ing with an infinite string of 1 bits.
Python maintains the illusion that integers use an infinite-width 2's-complement representation, so that -1 "is" an infinite string of 1 bits.

answered Jan 4, 2014 at 3:36

Tim Peters

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SimonT Over a year ago

Interesting. I suppose a bit more knowledge on bitwise operations would have let me find this myself. Then it's a tradeoff since I'd probably need to write a small comment beside it to clarify for others reading the code. Thanks.

three_pineapples · Accepted Answer · 2014-01-04 03:42:57Z

2

If you are only concerned about doing this for attributes of object instances, you could write a method like:

def c(obj, atr):
    setattr(obj,atr,~getattr(obj,atr))

and then use it like:

c(self.container.longVariableName, 'longName')

I think that @TimPeters answer is much better, but thought I'd provide this in case it is useful to anyone in the future who needs to do this with non-integers are is happy to only work with instant attributes

answered Jan 4, 2014 at 3:42

three_pineapples

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