Alec - 1 year ago 230
Python Question

# Applications of '~' (tilde) operator in Python

I just discovered the bitwise complement unary operation in Python via this question and have been trying to come up with an actual application for it, and if not, to determine if it's generally safe to overload the operator (by overriding the

__invert__
method) for other uses. The example given in the question fails with a
TypeError
, and the link provided seems pretty intimidating. Here's some fiddling around to see
~
in use:

from bitstring import BitArray

x = 7

print(~x)
# -8

print(BitArray(int=x, length=4).bin)
# '0111'

print(BitArray(int=~x, length=4).bin)
# '1000'

print(~~True, ~~False)
# 1 0

for i in range(-100, 100):
assert i + ~i == -1
assert i ^ ~i == -1
assert bool(i) == ~~bool(i)

Are there any examples of valid use-cases for this operator that I should be aware of? And even if there are, is it generally acceptable to override this operator for types other than
int
?

The standard use cases for the bitwise NOT operator are bitwise operations, just like the bitwise AND &, the bitwise OR |, the bitwise XOR ^, and bitwise shifting << and >>. Although they are rarely used in higher level applications, there are still some times where you need to do bitwise manipulations, so that’s why they are there.

Of course, you may overwrite these for custom types, and in general you are not required to follow any specific semantics when doing so. Just choose what makes sense for your type and what still fits the operator in some way.

If the operation is obscure and better explained with a word or two, then you should use a standard method instead. But there are some situations, especially when working with number related types, that could have some mathematical-like operations which fit the bitwise operators, and as such are fine to use those.

Just like you would overwrite standard operators like + and - only for meaningful operations, you should try to do the same for bitwise operators.

The reason ~~True, ~~False gives you (1, 0) is because the bool type does not define its own __invert__ operation. However, int does; and bool is actually a subtype of int. So bool actually inherits the logic of all bitwise and arithmetical operators. That’s why True + True == 2 etc.

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download