while (1) vs. while(True) -- Why is there a difference (in python 2 bytecode)?

Question

Intrigued by this question about infinite loops in perl: while (1) Vs. for (;;) Is there a speed difference?, I decided to run a similar comparison in python. I expected that the compiler would generate the same byte code for while(True): pass and while(1): pass, but this is actually not the case in python2.7.

The following script:

import dis

def while_one():
    while 1:
        pass

def while_true():
    while True:
        pass

print("while 1")
print("----------------------------")
dis.dis(while_one)

print("while True")
print("----------------------------")
dis.dis(while_true)

produces the following results:

while 1
----------------------------
  4           0 SETUP_LOOP               3 (to 6)

  5     >>    3 JUMP_ABSOLUTE            3
        >>    6 LOAD_CONST               0 (None)
              9 RETURN_VALUE        
while True
----------------------------
  8           0 SETUP_LOOP              12 (to 15)
        >>    3 LOAD_GLOBAL              0 (True)
              6 JUMP_IF_FALSE            4 (to 13)
              9 POP_TOP             

  9          10 JUMP_ABSOLUTE            3
        >>   13 POP_TOP             
             14 POP_BLOCK           
        >>   15 LOAD_CONST               0 (None)
             18 RETURN_VALUE        

Using while True is noticeably more complicated. Why is this?

In other contexts, python acts as though True equals 1:

>>> True == 1
True

>>> True + True
2

Why does while distinguish the two?

I noticed that python3 does evaluate the statements using identical operations:

while 1
----------------------------
  4           0 SETUP_LOOP               3 (to 6) 

  5     >>    3 JUMP_ABSOLUTE            3 
        >>    6 LOAD_CONST               0 (None) 
              9 RETURN_VALUE         
while True
----------------------------
  8           0 SETUP_LOOP               3 (to 6) 

  9     >>    3 JUMP_ABSOLUTE            3 
        >>    6 LOAD_CONST               0 (None) 
              9 RETURN_VALUE         

Is there a change in python3 to the way booleans are evaluated?

Answer 1

In Python 2.x, True is not a keyword, but just a built-in global constant that is defined to 1 in the bool type. Therefore the interpreter still has to load the contents of True. In other words, True is reassignable:

Python 2.7 (r27:82508, Jul  3 2010, 21:12:11) 
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> True = 4
>>> True
4

---

In Python 3.x it truly becomes a keyword and a real constant:

Python 3.1.2 (r312:79147, Jul 19 2010, 21:03:37) 
[GCC 4.2.1 (Apple Inc. build 5664)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> True = 4
  File "<stdin>", line 1
SyntaxError: assignment to keyword

thus the interpreter can replace the while True: loop with an infinite loop.

Answer 2

It isn't quite right, to call it an "infinite loop"

thus the interpreter can replace the while True: loop with an infinite loop.

because one can still break out of such a while True: loop. But such a loop's else clause would never be accessed in Python 3.

And Python 3's simplifying of the value lookup for True makes it run just as quickly as while 1 in Python 2.

Performance Comparison

Demonstrating the difference in time for a somewhat nontrivial while loop:

Setup

def while1():
    x = 0
    while 1:
        x += 1
        if x == 10:
            break
            
def whileTrue():
    x = 0
    while True:
        x += 1
        if x == 10:
            break

Python 2

>>> import timeit
>>> min(timeit.repeat(while1))
0.49712109565734863
>>> min(timeit.repeat(whileTrue))
0.756627082824707

Python 3

>>> import timeit
>>> min(timeit.repeat(while1))
0.6462970309949014
>>> min(timeit.repeat(whileTrue))
0.6450748789939098

Explanation

To explain the difference, in Python 2:

>>> import keyword
>>> 'True' in keyword.kwlist
False

but in Python 3:

>>> import keyword
>>> 'True' in keyword.kwlist
True
>>> True = 'true?'
  File "<stdin>", line 1
SyntaxError: can't assign to keyword

Since True is a keyword in Python 3, the interpreter doesn't have to look up the value to see if someone replaced it with some other value. But since one can assign True to another value in Python 2, the interpreter has to look it up every time.

Conclusion for Python 2

If you have a tight, long-running loop in Python 2, you probably should use while 1: instead of while True:.

Conclusion for Python 3

Use while True: if you have no condition for breaking out of your loop.

Answer 3

This is a 7-year-old question that already has a great answer, but a misconception in the question, which isn't addressed in any of the answers, makes it potentially confusing for some of the other questions marked as duplicates.

In other contexts, python acts as though True equals 1:

>>> True == 1
True

>>> True + True
2

Why does while distinguish the two?

In fact, while isn't doing anything different here at all. It distinguishes 1 and True in exactly the same way that the + example does.

---

Here's 2.7:

>>> dis.dis('True == 1')
  1           0 LOAD_GLOBAL              0 (True)
              3 LOAD_CONST               1 (1)
              6 COMPARE_OP               2 (==)
              9 RETURN_VALUE

>>> dis.dis('True == 1')
  1           0 LOAD_GLOBAL              0 (True)
              3 LOAD_GLOBAL              0 (True)
              6 BINARY_ADD
              9 RETURN_VALUE

Now compare:

>>> dis.dis('1 + 1')
  1           0 LOAD_CONST               1 (2)
              3 RETURN_VALUE

It's emitting a LOAD_GLOBAL (True) for each True, and there's nothing the optimizer can do with a global. So, while distinguishes 1 and True for the exact same reason that + does. (And == doesn't distinguish them because the optimizer doesn't optimize out comparisons.)

---

Now compare 3.6:

>>> dis.dis('True == 1')
  1           0 LOAD_CONST               0 (True)
              2 LOAD_CONST               1 (1)
              4 COMPARE_OP               2 (==)
              6 RETURN_VALUE

>>> dis.dis('True + True')
  1           0 LOAD_CONST               1 (2)
              2 RETURN_VALUE

Here, it's emitting a LOAD_CONST (True) for the keyword, which the optimizer can take advantage of. So, True + 1 doesn't distinguish, for exactly the same reason while True doesn't. (And == still doesn't distinguish them because the optimizer doesn't optimize out comparisons.)

---

Meanwhile, if the code isn't optimized out, the interpreter ends up treating True and 1 exactly the same in all three of these cases. bool is a subclass of int, and inherits most of its methods from int, and True has an internal integer value of 1. So, whether you're doing a while test (__bool__ in 3.x, __nonzero__ in 2.x), a comparison (__eq__), or arithmetic (__add__), you're calling the same method whether you use True or 1.