Ignore python multiple return value

Mat_42974 0
Viewed 270870

Say I have a Python function that returns multiple values in a tuple:

def func():
    return 1, 2

Is there a nice way to ignore one of the results rather than just assigning to a temporary variable? Say if I was only interested in the first value, is there a better way than this:

x, temp = func()
tuples function python
11 Answers

You can use x = func()[0] to return the first value, x = func()[1] to return the second, and so on.

If you want to get multiple values at a time, use something like x, y = func()[2:4].

Pourquoi_Litytestdata_485030

One common convention is to use a "_" as a variable name for the elements of the tuple you wish to ignore. For instance:

def f():
    return 1, 2, 3

_, _, x = f()
Brian_Clapper_534950

If you're using Python 3, you can you use the star before a variable (on the left side of an assignment) to have it be a list in unpacking.

# Example 1: a is 1 and b is [2, 3]

a, *b = [1, 2, 3]

# Example 2: a is 1, b is [2, 3], and c is 4

a, *b, c = [1, 2, 3, 4]

# Example 3: b is [1, 2] and c is 3

*b, c = [1, 2, 3]       

# Example 4: a is 1 and b is []

a, *b = [1]
Admin1

Remember, when you return more than one item, you're really returning a tuple. So you can do things like this:

def func():
    return 1, 2

print func()[0] # prints 1
print func()[1] # prints 2
Evan_Fosmark_497010

Three simple choices.

Obvious

x, _ = func()

x, junk = func()

Hideous

x = func()[0]

And there are ways to do this with a decorator.

def val0( aFunc ):
    def pick0( *args, **kw ):
        return aFunc(*args,**kw)[0]
    return pick0

func0= val0(func)
SLott_106610

This seems like the best choice to me:

val1, val2, ignored1, ignored2 = some_function()

It's not cryptic or ugly (like the func()[index] method), and clearly states your purpose.

Mosor_338410

If this is a function that you use all the time but always discard the second argument, I would argue that it is less messy to create an alias for the function without the second return value using lambda. 

def func():
    return 1, 2

func_ = lambda: func()[0] 

func_()  # Prints 1
user_24905410

When you have many output from a function and you don't want to call it multiple times, I think the clearest way for selecting the results would be :

results = fct()
a,b = [results[i] for i in list_of_index]

As a minimum working example, also demonstrating that the function is called only once :

def fct(a):
    b=a*2
    c=a+2
    d=a+b
    e=b*2
    f=a*a
    print("fct called")
    return[a,b,c,d,e,f]

results=fct(3)
> fct called

x,y = [results[i] for i in [1,4]]

And the values are as expected :

results
> [3,6,5,9,12,9]
x
> 6
y
> 12

For convenience, Python list indexes can also be used :

x,y = [results[i] for i in [0,-2]]

Returns : a = 3 and b = 12

user_74709250
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