pythonshort-circuiting

python boolean expression not "short-circuit"?

For example:

def foo():
    print 'foo'
    return 1
if any([f() for f in [foo]*3]):
   print 'bar'

I thought the above code should output:

foo
bar

instead of :

foo
foo
foo
bar

Why ? how can I make the "short-circuit" effect ?

Solution

  • Deconstruct your program to see what is happening:

    >>> [f() for f in [foo]*3]
foo
foo
foo
[1, 1, 1]
>>>

    You are already creating a list and passing to any and have printed it 3 times.

    >>> any ([1, 1, 1])
True

    This is fed to if statement:

    >>> if any([1, 1, 1]):
...     print 'bar'
... 
bar
>>>

    Solution: Pass a generator to any

    >>> (f() for f in [foo]*3)
<generator object <genexpr> at 0x10041a9b0>