Python if statement efficiency
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Chapters
00:00 Python If Statement Efficiency
01:04 Accepted Answer Score 52
01:28 Answer 2 Score 9
01:40 Answer 3 Score 62
02:06 Answer 4 Score 20
02:57 Thank you
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Full question
https://stackoverflow.com/questions/2539...
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Content licensed under CC BY-SA
https://meta.stackexchange.com/help/lice...
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Tags
#python #performance
#avk47
ANSWER 1
Score 62
I would say the single test is as fast as the separate tests. Python also makes use of so called short-circuit evaluation.
That means for (a and b and c), that b or c would not be tested anymore if a is false.
Similar, if you have an OR expression (a or b) and a is true, b is never evaluated.
So to sum up, the clauses don't fail faster with separation.
ACCEPTED ANSWER
Score 52
if statements will skip everything in an else bracket if it evaluates to true. It should be noted that worrying about this sort of problem, unless it's done millions of times per program execution, is called "premature optimization" and should be avoided. If your code is clearer with three if (a and b and c) statements, they should be left in.
ANSWER 3
Score 20
Code:
import dis
def foo():
if ( a and b and c):
pass
else:
pass
def bar():
if a:
if b:
if c:
pass
print 'foo():'
dis.dis(foo)
print 'bar():'
dis.dis(bar)
Output:
foo():
4 0 LOAD_GLOBAL 0 (a)
3 JUMP_IF_FALSE 18 (to 24)
6 POP_TOP
7 LOAD_GLOBAL 1 (b)
10 JUMP_IF_FALSE 11 (to 24)
13 POP_TOP
14 LOAD_GLOBAL 2 (c)
17 JUMP_IF_FALSE 4 (to 24)
20 POP_TOP
5 21 JUMP_FORWARD 1 (to 25)
>> 24 POP_TOP
7 >> 25 LOAD_CONST 0 (None)
28 RETURN_VALUE
bar():
10 0 LOAD_GLOBAL 0 (a)
3 JUMP_IF_FALSE 26 (to 32)
6 POP_TOP
11 7 LOAD_GLOBAL 1 (b)
10 JUMP_IF_FALSE 15 (to 28)
13 POP_TOP
12 14 LOAD_GLOBAL 2 (c)
17 JUMP_IF_FALSE 4 (to 24)
20 POP_TOP
13 21 JUMP_ABSOLUTE 29
>> 24 POP_TOP
25 JUMP_ABSOLUTE 33
>> 28 POP_TOP
>> 29 JUMP_FORWARD 1 (to 33)
>> 32 POP_TOP
>> 33 LOAD_CONST 0 (None)
36 RETURN_VALUE
So, although the setup is the same, the cleanup for the combined expression is faster since it leaves only a single value on the stack.
ANSWER 4
Score 9
At least in python, efficiency is second to readability and "Flat is better than nested".