Most Pythonic way of creating an infinite loop with a 'while' statement

Question

Which one is the pythonic way of below two expressions.

1. while 1:
2. while True:

edit:Is there a pythonic way out of two above expressions?

Answer 1

I think this is mostly a matter of style. Both should be easily understandable as an infinite loop.

However, personally I prefer the second option. That's because it just takes a mental micro-step less to understand, especially for programmers without C background.

Answer 2

The first one will work also in those early versions where True is not yet defined.

Answer 3

This is only a matter of style, any programming beginner will understand either option.

But the second option will only work if True wasn't assigned to False, which was possible until Python 3:

>>> True = False
>>> True
False

Answer 4

IMO the second option is more obvious.

If you could get rid of the while and write more compact code, that might be more pythonic.
For example:

# Get the even numbers in the range 1..10
# Version 1
l = []
n = 1
while 1:
    if n % 2 == 0: l.append(n)
    n += 1
    if n > 10: break
print l

# Version 2
print [i for i in range(1, 11) if i % 2 == 0]

# Version 3
print range(2, 11, 2)

Answer 5

If you have an algorithm that is suppose to terminate in a finite time, I would recommend this, which is always safer than while True:

maxiter = 1000
for i in xrange(maxiter):
   # your code
   # on success:
     break
else:
   # that algorithm has not finished in maxiter steps! do something accordingly

Answer 6

The most pythonic way will always be the most readable. Use while True:

Answer 7

I believe the second expression is more explicit, and thus more pythonic.

Answer 8

The better way is "while True" with a conditional break out of the loop.

Answer 9

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.

Answer 10

It doesn't really matter. Neither is hard to read or understand, though personally I'd always use while True, which is a bit more explicit.

More generally, a whole lot of while–break loops people write in Python could be something else. Sometimes I see people write i = 0; while True: i += 1 ..., which can be replaced with for i in itertools.count() and people writing while True: foo = fun() if foo is None: break when this can be written for foo in iter(fun, None), which requires learning but has less boilerplate and opportunity for silly mistakes.