Unexpected python function return behavior

Question

I am working on a function that usually returns 1 value, but sometimes returns 2 for similar reasons to this post, and noticed some unexpected behavior best illustrated by this example:

def testfcn1(return_two=False):
    a = 10
    return a, a*2 if return_two else a

def testfcn2(return_two=False):
    a = 10
    if return_two:
        return a, a*2
    return a

I would expect both functions to behave the same way. testfcn2 works as expected:

testfcn2(False)
10

testfcn2(True)
(10, 20)

However, testfcn1 always returns two values, and just returns the first value twice if return_two is False:

testfcn1(False)
(10, 10)

testfcn1(True)
(10, 20)

Is there a rationale for this kind of behavior?

But do you really want a function that sometimes returns an int & sometimes returns a tuple? That can get messy. If the number of values to be returned can vary, then always return a tuple & let the caller call the tuple's len method. A possible exception to this pattern is returning None instead of a tuple, but even then you can return an empty tuple. — PM 2Ring
– PM 2Ring, Commented Sep 18, 2015 at 14:58

Anand S Kumar · Accepted Answer · 2015-09-18 14:58:29Z

5

In your testfcn1 , the expressions are grouped as -

(a, (a*2 if return_two else a))           #This would always return a tuple of 2 values.

And not (what you thought it would be) -

(a, a*2) if return_two else a             #This can return a tuple if return_two is True otherwise a single value `a` .

If you wanted the second grouping of expressions, you have to use brackets as I have used above.

Example to show the difference -

>>> 10, 20 if True else 10
(10, 20)
>>> 10, 20 if False else 10
(10, 10)
>>>
>>>
>>> (10, 20) if False else 10
10
>>> (10, 20) if True else 10
(10, 20)
>>>
>>>
>>> 10, (20 if False else 10)
(10, 10)
>>> 10, (20 if True else 10)
(10, 20)

edited Sep 18, 2015 at 14:58

answered Sep 18, 2015 at 14:48

Anand S Kumar

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Comments

Łukasz Rogalski · Accepted Answer · 2015-09-18 14:49:35Z

3

It a simple issue of operator precedence. return a, a*2 if return_two else a if interpreted as return a, (a*2 if return_two else a). You should use parenthesis in order to change precedence.

def testfcn1(return_two=False):
    a = 10
    return (a, a*2) if return_two else a

answered Sep 18, 2015 at 14:49

Łukasz Rogalski

23.3k10 gold badges63 silver badges93 bronze badges

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