Wow, thanks! That explains it.

Wow, this brings me back :-)

Checked the code, the difference is still present, and the first option is faster.

I'm not sure why exactly, checked the opcodes using dis.dis, and figured that the only difference is that

LOAD_GLOBAL              0 (sqrt)
LOAD_FAST                0 (sq)
CALL_FUNCTION            1

is called for the first one, and

STORE_FAST               1 (s)
LOAD_FAST                1 (s)
LOAD_FAST                1 (s)

for the second function (same opcodes omitted).

So it looks like storing a value is more expensive then calling a function.