Python Question

What's the difference between "while 1" and "while True"?

I've seen two ways to create an infinite loop in Python:

  1. while 1:

  2. while True:

Is there any difference between these? Is one more pythonic than the other?

gz. gz.
Answer Source

Fundamentally it doesn't matter, such minutiae doesn't really affect whether something is 'pythonic' or not.

If you're interested in trivia however, there are some differences.

  1. The builtin boolean type didn't exist till Python 2.3 so code that was intended to run on ancient versions tends to use the while 1: form. You'll see it in the standard library, for instance.

  2. The True and False builtins are not reserved words prior to Python 3 so could be assigned to, changing their value. This helps with the case above because code could do True = 1 for backwards compatibility, but means that the name True needs to be looked up in the globals dictionary every time it is used.

  3. Because of the above restriction, the bytecode the two versions compile to is different in Python 2 as there's an optimisation for constant integers that it can't use for True. Because Python can tell when compiling the 1 that it's always non-zero, it removes the conditional jump and doesn't load the constant at all:

    >>> import dis
    >>> def while_1():
    ...     while 1:
    ...         pass
    >>> def while_true():
    ...     while True:
    ...         pass
    >>> dis.dis(while_1)
      2           0 SETUP_LOOP               5 (to 8)
      3     >>    3 JUMP_ABSOLUTE            3
                  6 POP_TOP
                  7 POP_BLOCK
            >>    8 LOAD_CONST               0 (None)
                 11 RETURN_VALUE
    >>> dis.dis(while_true)
      2           0 SETUP_LOOP              12 (to 15)
            >>    3 LOAD_GLOBAL              0 (True)
                  6 JUMP_IF_FALSE            4 (to 13)
                  9 POP_TOP
      3          10 JUMP_ABSOLUTE            3
            >>   13 POP_TOP
                 14 POP_BLOCK
            >>   15 LOAD_CONST               0 (None)
                 18 RETURN_VALUE

So, while True: is a little easier to read, and while 1: is a bit kinder to old versions of Python. As you're unlikely to need to run on Python 2.2 these days or need to worry about the bytecode count of your loops, the former is marginally preferable.

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