Return multiple lists in Python function

Question

I am new to Python, and am trying to create a function which creates lists with different material parameters from user inputs as shown in the code below.

def material():
    layers = int(raw_input("State the number of material layers in the wall (0 for default material): "))

    rho = [] # Density [kg/m3]
    c = [] # Heat capacity [J/(kg K)] 
    k = [] # Heat conductivity [W/(m K)]
    #a = [] # Thermal diffusivity [m2/s]
    d = [] # Thickness of material [m]

    # Saveing material properties
    if layers == 0:
        rho.append(2300)
        c.append(900)
        k.append(1.6)
        d.append(3.2)
        layers = 1

    else:
        for i in range(layers):
            print "\n" "Define thermal properties for material", i+1,"(starting from left)"
            rho.append(float(raw_input("Density [kg/m3]: ")))
            c.append(float(raw_input("Heat capacity [J/(kg K)]: ")))
            k.append(float(raw_input("Heat conductivity [W/(m K)]: ")))
            d.append(float(raw_input("Thickness [m]: ")))

    return ???

How should I return rho, c, k, d and layers so I am able to e.g. print – or use the value of – e.g. the second item in the list of d?

print d[1]

DSM · Accepted Answer · 2013-10-19 18:48:44Z

15

How should I return rho, c, k, d and layers [...] ?

Simply do it:

return rho, c, k, d, layers

And then you'd call it like

rho, c, k, d, layers = material()
print d[1]

Note that the more stuff you're returning, the more likely it is you're going to want to wrap it all together into some structure like a dict (or namedtuple, or class, etc.)

answered Oct 19, 2013 at 18:48

DSM

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3 Comments

OdraEncoded Over a year ago

Obs.: This feature is actually returning a tuple and then doing a multiple assignment statement. You can also use S = material(); print S[3][1] to achieve the same result for example.

DSM Over a year ago

@OdraEncoded: very true. I tend not to use tuple indexing for heterogeneous return results because it's too easy for me to mix up which variable correponds to which index.

Christoffer Over a year ago

God, you are fast out there. Thank you so much. I can see that my problem was more about how to call the function – but I got it now. Thanks again, everyone. Cheers.

score 6 · Accepted Answer · 2013-10-19 18:54:26Z

6

return can return multiple values if you separate them with commas:

return rho, c, k, d, layers

This will make material return a tuple containing rho, c, k, d, and layers.

Once this is done, you can access the values returned by material through unpacking:

rho, c, k, d, layers = material()

Here is a demonstration:

>>> def func():
...     return [1, 2], [3, 4], [5, 6]
...
>>> a, b, c = func()
>>> a
[1, 2]
>>> b
[3, 4]
>>> c
[5, 6]
>>> a[1]
2
>>>

edited Oct 19, 2013 at 18:54

answered Oct 19, 2013 at 18:48

user2555451

Comments

santosh singh · Accepted Answer · 2013-10-19 18:49:52Z

0

def return_values():
    # your code
    return list1, list2

and get the value like below

value1, value2 = return_values()

answered Oct 19, 2013 at 18:49

santosh singh

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Comments

Archit Jain · Accepted Answer · 2013-10-19 18:50:10Z

0

def function_returning_multiple_list():
  l1=[]
  l2=[]
  l3=[]
  return (l1,l2,l3)


def another_fucntion():
  list1,list2,list3=function_returning_multiple_list()
  print list2

answered Oct 19, 2013 at 18:50

Archit Jain

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1 Comment

user2555451 Over a year ago

Note that there is no need for the parenthesis in the return statement.

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