Python Introduction: Functions

Functions help extract out common code blocks.

Let's define a function print_greeting().

In [1]:
def print_greeting():
    print("Hi there, how are you?")
    print("Long time no see.")

And call it:

In [2]:
print_greeting()

Hi there, how are you?
Long time no see.

That's a bit impersonal.

In [3]:
def print_greeting(name):

    print("Hi there, {0}, how are you?".format(name))

    print("Long time no see.")
In [4]:
print_greeting("Andreas")

Hi there, Andreas, how are you?
Long time no see.
In [5]:
print_greeting()

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-da6a1dd1a775> in <module>()
----> 1 print_greeting()

TypeError: print_greeting() missing 1 required positional argument: 'name'

But we might not know their name. So we can set a default value for parameters.

(And we just changed the interface of print_greeting!)

In [6]:
def print_greeting(name="my friend"):

    print("Hi there, {0}, how are you?".format(name))

    print("Long time no see.")
In [8]:
print_greeting("Tim")

Hi there, Tim, how are you?
Long time no see.

Note that the order of the parameters does not matter

In [11]:
def printinfo( name , age ):
    print("Name: ", name)
    print("Age: ", age)

printinfo(40,"Mariana")

Name:  40
Age:  Mariana
In [12]:
printinfo( age=8, name="Julia" )

Name:  Julia
Age:  8

However the parameters "age" and "name" are both required above. What if we want to have optional parameters (without setting default values)?

In [16]:
def printinfo( firstvar , *othervar ):
    print("First parameter:", firstvar)
    print("List of other parameters:")
    for var in othervar:
        print(var)

# Now you can call printinfo function
printinfo(10,20,30,50,60)

First parameter: 10
List of other parameters:
20
30
50
60

A function can also return more than one parameter, and the results appear as a tuple:

In [ ]:
def average_total(a,b):
    totalsum = a + b
    average = totalsum/2
    return average,totalsum

average_total(2,3)

Remember mutable and immutable types...

Function parameters work like variables. So what does this do?

In [ ]:
def my_func(my_list):
    my_list.append(5)
    print("List printed inside the function: ",my_list)
    
numberlist = [1,2,3]
print("List before function call: ",numberlist)
my_func(numberlist)
print("List after function call: ",numberlist)

Can be very surprising! Here, we are maintaining reference of the passed object and appending values in the same object.

Define a better function my_func_2:

In [ ]:
def my_func_2(my_list):
    
    my_list = my_list + [5]
    print("List printed inside the function: ",my_list)

    return my_list


numberlist = [1,2,3]
print("List before function call: ",numberlist)
new_list = my_func_2(numberlist)
#inside the function my_list = [1,2,3,5]
print("List after function call: ",numberlist)
print("Modified list after function call: ",new_list)

Note that the parameter my_list is local to the function.

In [ ]:
def change_fruits(fruit):
    fruit='apple'
    print("I'm changing the fruit to %s" % fruit)
In [ ]:
myfruit = 'banana'
change_fruits(myfruit)
print("The fruit is %s " % myfruit)

What happened?! Remember that the input, fruit to change_fruits is bound to an object within the scope of the function:

  • If the object is mutable, the object will change
  • If the object is immutable (like a string!), then a new object is formed, only to live within the function scope.

Iclicker question:

In [18]:
def do_stuff(a,b):
    a +=  [5]
    b += [8]
In [28]:
In [30]:
John = ['computer_science']
Tim = John
print(Tim,John)
add_minor(Tim)
print(Tim,John)
John = switch_majors(John)
print(Tim,John)

['computer_science'] ['computer_science']
['computer_science', 'math'] ['computer_science', 'math']
['physics', 'economics']
['computer_science', 'math'] ['physics', 'economics']