Python: The Fake '-=-' Operator

While writing some Python, I accidentally swapped two characters:

a =- b       # What I wrote
a -= b       # What I intended to write

That’s “equals minus” instead of “minus equals”. I was initially suprised that the interpreter allowed it. Digging in, I found that a =- b, is equivalent to a = -b (spaces don’t matter), which is the same as a = -1 * b. Rather than decrementing a by the amount b, I was assigning a to be the inverse of b.

This led my friend Ben and I to figure out that the following weird-looking statements are both syntactically correct and equivalent to one another.

a -=- b
a +=+ b

What they do and why they are equivalent are left as an exercise for the reader.