[Tutor] Generators

[Kent Johnson]

Tiger12506 wrote:
>> I am new to Python but not new to programming languages.  I have seen this
>> "while 1" a lot.  In fact in other languages I would expect "while 1" to 
>> be
>> the same as "while TRUE".

The predefined constants True and False were not added to Python until 
version 2.3. Before that it was common to write 'while 1'.

In fact 'while 1' is more efficient, too. Compare:

In [5]: dis.dis(compile('while 1: pass', '', 'exec'))
   1           0 SETUP_LOOP               3 (to 6)
         >>    3 JUMP_ABSOLUTE            3
         >>    6 LOAD_CONST               0 (None)
               9 RETURN_VALUE
In [6]: dis.dis(compile('while True: pass', '', 'exec'))
   1           0 SETUP_LOOP              12 (to 15)
         >>    3 LOAD_NAME                0 (True)
               6 JUMP_IF_FALSE            4 (to 13)
               9 POP_TOP
              10 JUMP_ABSOLUTE            3
         >>   13 POP_TOP
              14 POP_BLOCK
         >>   15 LOAD_CONST               0 (None)
              18 RETURN_VALUE

'while 1' is optimized to just a jump - the compiler recognizes that the 
test is not needed and skips it. 'while True' requires looking up the 
value of the name 'True' and testing it.

Old habits die hard, especially when they are more efficient and require 
less typing :-)

Kent