How to toggle a value?

What is the most efficient way to toggle between 0 and 1?

Answer

Solution using NOT

If the values are boolean, the fastest approach is to use the not operator:

>>> x = True
>>> x = not x        # toggle
>>> x
False
>>> x = not x        # toggle
>>> x
True
>>> x = not x        # toggle
>>> x
False

Solution using subtraction

If the values are numerical, then subtraction from the total is a simple and fast way to toggle values:

>>> A = 5
>>> B = 3
>>> total = A + B
>>> x = A
>>> x = total - x    # toggle
>>> x
3
>>> x = total - x    # toggle
>>> x
5
>>> x = total - x    # toggle
>>> x
3

Solution using XOR

If the value toggles between 0 and 1, you can use a bitwise exclusive-or:

>>> x = 1
>>> x ^= 1
>>> x
0
>>> x ^= 1
>>> x
1

The technique generalizes to any pair of integers. The xor-by-one step is replaced with a xor-by-precomputed-constant:

>>> A = 205
>>> B = -117
>>> t = A ^ B        # precomputed toggle constant
>>> x = A
>>> x ^= t           # toggle
>>> x
-117
>>> x ^= t           # toggle
>>> x
205
>>> x ^= t           # toggle
>>> x
-117

(This idea was submitted by Nick Coghlan and later generalized by @zxxc.)

Solution using a dictionary

If the values are hashable, you can use a dictionary:

>>> A = 'xyz'
>>> B = 'pdq'
>>> d = {A:B, B:A}
>>> x = A
>>> x = d[x]         # toggle
>>> x
'pdq'
>>> x = d[x]         # toggle
>>> x
'xyz'
>>> x = d[x]         # toggle
>>> x
'pdq'

Solution using a conditional expression

The slowest way is to use a conditional expression:

>>> A = [1,2,3]
>>> B = [4,5,6]
>>> x = A
>>> x = B if x == A else A
>>> x
[4, 5, 6]
>>> x = B if x == A else A
>>> x
[1, 2, 3]
>>> x = B if x == A else A
>>> x
[4, 5, 6]

Solution using itertools

If you have more than two values, the itertools.cycle() function provides a generic fast way to toggle between successive values:

>>> import itertools
>>> toggle = itertools.cycle(['red', 'green', 'blue']).next
>>> toggle()
'red'
>>> toggle()
'green'
>>> toggle()
'blue'
>>> toggle()
'red'
>>> toggle()
'green'
>>> toggle()
'blue'

Note that in Python 3 the next() method was changed to __next__(), so the first line would be now written as toggle = itertools.cycle(['red', 'green', 'blue']).__next__