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Why is early return slower than else?

Posted by: admin November 1, 2017 Leave a comment


This is a follow-up question to an answer I gave a few days back. Edit: it seems that the OP of that question already used the code I posted to him to ask the same question, but I was unaware of it. Apologies. The answers provided are different though!

Substantially I observed that:

>>> def without_else(param=False):
...     if param:
...         return 1
...     return 0
>>> def with_else(param=False):
...     if param:
...         return 1
...     else:
...         return 0
>>> from timeit import Timer as T
>>> T(lambda : without_else()).repeat()
[0.3011460304260254, 0.2866089344024658, 0.2871549129486084]
>>> T(lambda : with_else()).repeat()
[0.27536892890930176, 0.2693932056427002, 0.27011704444885254]
>>> T(lambda : without_else(True)).repeat()
[0.3383951187133789, 0.32756996154785156, 0.3279120922088623]
>>> T(lambda : with_else(True)).repeat()
[0.3305950164794922, 0.32186388969421387, 0.3209099769592285]

…or in other words: having the else clause is faster regardless of the if condition being triggered or not.

I assume it has to do with different bytecode generated by the two, but is anybody able to confirm/explain in detail?

EDIT: Seems not everybody is able to reproduce my timings, so I thought it might be useful to give some info on my system. I’m running Ubuntu 11.10 64 bit with the default python installed. python generates the following version information:

Python 2.7.2+ (default, Oct  4 2011, 20:06:09) 
[GCC 4.6.1] on linux2

Here are the results of the disassembly in Python 2.7:

>>> dis.dis(without_else)
  2           0 LOAD_FAST                0 (param)
              3 POP_JUMP_IF_FALSE       10

  3           6 LOAD_CONST               1 (1)
              9 RETURN_VALUE        

  4     >>   10 LOAD_CONST               2 (0)
             13 RETURN_VALUE        
>>> dis.dis(with_else)
  2           0 LOAD_FAST                0 (param)
              3 POP_JUMP_IF_FALSE       10

  3           6 LOAD_CONST               1 (1)
              9 RETURN_VALUE        

  5     >>   10 LOAD_CONST               2 (0)
             13 RETURN_VALUE        
             14 LOAD_CONST               0 (None)
             17 RETURN_VALUE        

This is a pure guess, and I haven’t figured out an easy way to check whether it is right, but I have a theory for you.

I tried your code and get the same of results, without_else() is repeatedly slightly slower than with_else():

>>> T(lambda : without_else()).repeat()
[0.42015745017874906, 0.3188967452567226, 0.31984281521812363]
>>> T(lambda : with_else()).repeat()
[0.36009842032996175, 0.28962249392031936, 0.2927151355828528]
>>> T(lambda : without_else(True)).repeat()
[0.31709728471076915, 0.3172671387005721, 0.3285821242644147]
>>> T(lambda : with_else(True)).repeat()
[0.30939889008243426, 0.3035132258429485, 0.3046679117038593]

Considering that the bytecode is identical, the only difference is the name of the function. In particular the timing test does a lookup on the global name. Try renaming without_else() and the difference disappears:

>>> def no_else(param=False):
    if param:
        return 1
    return 0

>>> T(lambda : no_else()).repeat()
[0.3359846013948413, 0.29025818923918223, 0.2921801513879245]
>>> T(lambda : no_else(True)).repeat()
[0.3810395594970828, 0.2969634408842694, 0.2960104566362247]

My guess is that without_else has a hash collision with something else in globals() so the global name lookup is slightly slower.

Edit: A dictionary with 7 or 8 keys probably has 32 slots, so on that basis without_else has a hash collision with __builtins__:

>>> [(k, hash(k) % 32) for k in globals().keys() ]
[('__builtins__', 8), ('with_else', 9), ('__package__', 15), ('without_else', 8), ('T', 21), ('__name__', 25), ('no_else', 28), ('__doc__', 29)]

To clarify how the hashing works:

__builtins__ hashes to -1196389688 which reduced modulo the table size (32) means it is stored in the #8 slot of the table.

without_else hashes to 505688136 which reduced modulo 32 is 8 so there’s a collision. To resolve this Python calculates:

Starting with:

j = hash % 32
perturb = hash

Repeat this until we find a free slot:

j = (5*j) + 1 + perturb;
perturb >>= 5;
use j % 2**i as the next table index;

which gives it 17 to use as the next index. Fortunately that’s free so the loop only repeats once. The hash table size is a power of 2, so 2**i is the size of the hash table, i is the number of bits used from the hash value j.

Each probe into the table can find one of these:

  • The slot is empty, in that case the probing stops and we know the
    value is not in the table.

  • The slot is unused but was used in the past in which case we go try
    the next value calculated as above.

  • The slot is full but the full hash value stored in the table isn’t
    the same as the hash of the key we are looking for (that’s what
    happens in the case of __builtins__ vs without_else).

  • The slot is full and has exactly the hash value we want, then Python
    checks to see if the key and the object we are looking up are the
    same object (which in this case they will be because short strings
    that could be identifiers are interned so identical identifiers use
    the exact same string).

  • Finally when the slot is full, the hash matches exactly, but the keys
    are not the identical object, then and only then will Python try
    comparing them for equality. This is comparatively slow, but in the
    case of name lookups shouldn’t actually happen.