if statement vs. expression

Question

What do you prefer concerning performance (memory consumption, speed) / readability, if you need to switch variable value between 0 and 1 in a cycle:

x=get(x) 
for i in range (start, end):
    if x==0:
        x=1
    else:
        x=0

or

x=get(x)
for i in range (start, end):
    x = ((x + 1) % 2)

How the python compiler interprets it?

Have a look at the [dis](http://docs.python.org/2.7/library/dis.html#module-dis) module. Use it, look at the bytecode and come back here if it's something that you don't understand. — Fredrik Pihl, Sep 30 '13 at 09:00
..and for a better approach to toggle between values, see http://stackoverflow.com/questions/10986970/python-how-to-toggle-between-two-values — Fredrik Pihl, Sep 30 '13 at 09:01
Thanks for hints, the issue is that initial value of x may not be only 0/1. And what about the performance comparison? — Petr Krampl, Sep 30 '13 at 09:23

Martijn Pieters · Accepted Answer · 2013-09-30T09:24:40.807

There are a multitude of options beyond the if statement and expression you used.

Use subtraction:
```
x = 1 - x
```

Use ^ (XOR):

for i in range (start, end):
    x ^= 1

This toggles between 0 and 1:

>>> x = 1
>>> x ^ 1
0
>>> x ^= 1
>>> x ^ 1
1

You could use itertools.cycle():

from itertools import cycle

x_values = cycle((0, 1))

for i in range (start, end):
    x = next(x_values)

cycle() is especially useful for swapping between more than one value, especially when there is no easy mathematical or logical relationship between them:

>>> from itertools import cycle
>>> x_values = cycle((1, 'spam', 3.14))
>>> next(x_values)
1
>>> next(x_values)
'spam'
>>> next(x_values)
3.14
>>> next(x_values)
1

But to figure out which works best for you, pick a metric and stick to that. For performance, use the timeit module for comparative trials.

Using x - 1 is by far faster than cycle() or your expression, for example:

>>> from timeit import timeit
>>> timeit('x = 1 - x', 'x = 0')
0.044335126876831055
>>> timeit('x ^= 1', 'x = 0')
0.05133986473083496
>>> timeit('x = ((x + 1) % 2)', 'x = 0')
0.11392998695373535
>>> timeit('x = next(x_values)', 'from itertools import cycle; x_values = cycle((0, 1))')
0.1254570484161377

The Python compiler interprets code pretty straightforward, mostly. Your if version would not be optimized, for example. Use the dis.dis() function to see a disassembly of the compiler output:

>>> import dis
>>> def foo():
...     if x==0:
...         x=1
...     else:
...         x=0
... 
>>> dis.dis(foo)
  2           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               1 (0)
              6 COMPARE_OP               2 (==)
              9 POP_JUMP_IF_FALSE       21

  3          12 LOAD_CONST               2 (1)
             15 STORE_FAST               0 (x)
             18 JUMP_FORWARD             6 (to 27)

  5     >>   21 LOAD_CONST               1 (0)
             24 STORE_FAST               0 (x)
        >>   27 LOAD_CONST               0 (None)
             30 RETURN_VALUE

As you can see the full branching structure is still there. Using an expression leads to very different bytecode:

>>> def bar():
...     x = ((x + 1) % 2)
... 
>>> dis.dis(bar)
  2           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               1 (1)
              6 BINARY_ADD          
              7 LOAD_CONST               2 (2)
             10 BINARY_MODULO       
             11 STORE_FAST               0 (x)
             14 LOAD_CONST               0 (None)
             17 RETURN_VALUE

score 0 · Answer 2 · answered Sep 30 '13 at 09:09

0

You can use xor operator ^ to toggle between 0 and 1:

x ^= 1

answered Sep 30 '13 at 09:09

mvp

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if statement vs. expression

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