python - What does "while False" mean?

Question

I don't undestand how this code works:

i = 1
while False:
    if i % 5 == 0:
        break
    i = i + 2
print(i)

what does while False? What does it have to be false? I don't get it...

Answer 1

A while loop checks the condition (well, the expression) behind the while before each iteration and stops executing the loop body when the condition is False.

So while False means that the loop body will never execute. Everything inside the loop is "dead code". Python-3.x will go so far that it "optimizes" the while-loop away because of that:

def func():
    i = 1
    while False:
        if i % 5 == 0:
            break
        i = i + 2
    print(i)

import dis

dis.dis(func)

Gives the following:

  Line        Bytecode

  2           0 LOAD_CONST               1 (1)
              3 STORE_FAST               0 (i)

  7           6 LOAD_GLOBAL              0 (print)
              9 LOAD_FAST                0 (i)
             12 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             15 POP_TOP
             16 LOAD_CONST               0 (None)
             19 RETURN_VALUE

That means the compiled function won't even know there has been a while loop (no instructions for line 3-6!), because there is no way that the while-loop could be executed.