Debug python, determine which condition was true

Question

I have a fairly long list of conditions that might be true in order to satisfy a particular if statement. I'm trying to debug and find out which condition was met since it shouldn't be, is there any better way than testing each condition individually? something like:

if (xpr1 or xpr2 or xpr3 .......):
     print "xpr# was true"

OR better.

     print "all variables" from true condition

Python 2.7

what about any(xpr1, xpr2, ...)? to debug you could use [bool(x) for x in (xp1, xpr2, ...)]. — hiro protagonist
– hiro protagonist, Commented Dec 22, 2016 at 8:03
@hiro all() returns True, if all elements are True, if (...) from UserZer0 schould return True, if at least one element is True. — Humbalan
– Humbalan, Commented Dec 22, 2016 at 8:07

kmaork · Accepted Answer · 2016-12-22 08:13:46Z

2

>>> a = False
>>> b = True
>>> c = False
>>> print [name for name in ['a', 'b', 'c'] if locals()[name]]
['b']

answered Dec 22, 2016 at 8:13

kmaork

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3 Comments

Alex Hall Over a year ago

Alternatively, if typing out quotes and commas gets annoying for many variables, you can instead just write 'a b c'.split().

UserZer0 Over a year ago

@AlexHall your comment massively improves the usefulness of this answer! Would not have thought about that!

Rob Over a year ago

Or, since they are single character names: list('abc').

PM 2Ring · Accepted Answer · 2016-12-22 08:44:45Z

There isn't really any better way than testing each condition individually, but you can do that compactly in a list comprehension, eg:

conditions = (xpr0, xpr1, xpr2, xpr3)
print [i for i, xpr in enumerate(conditions) if xpr]

will print a list of the indices of expressions that are true. However, this won't perform the short-circuiting that you get with or.

Of course, you can force short-circuiting by using a "traditional" for loop, with a break statement, eg:

for i, xpr in enumerate(conditions):
    if xpr:
        print i, xpr
        break

although this isn't exactly the same as the short-circuiting performed by

if xpr0 or xpr1 or xpr2 or xpr3:

because we've already pre-computed all the expressions in the conditions list.

As hiro protagonist mentions in the comments, you can test if any of those expressions are true with

any(conditions)

and that will short-circuit in the same way as the traditional for loop code above.

FWIW, normally, one uses any and all on a generator expression rather than a list comp, since that avoids unnecessary evaluation of expressions after a short-circuit point is reached, but that's not applicable here.

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