Is there a function that takes a number with binary numeral a, and does the NOT?

(For example, the function's value at 18 [binary 10010] would be 13 [binary 01101].) I thought this was what the tilde operator (~) did, but it only adds a minus sign to 18, which is two's complement of that, instead of getting 13.

As mentioned in the comments ~ is the bitwise NOT.

If you want a 5 bit unsigned bitwise NOT you can use an XOR with a mask:

>>> n = 0b10010 # 18
>>> m = 0b11111
>>> n ^ m
13