From Computer Science Wiki
A function f takes an input x, and returns a single output f(x). One metaphor describes the function as a "machine" or "black box" that for each input returns a corresponding output.[1]


We can control the flow of a program by calling a function. If we call a function, it executes, returns a value, and then resumes the program where it was called.

In programming, a named section of a program that performs a specific task is called a function. In this sense, a function is a type of procedure or routine. Some programming languages make a distinction between a function, which returns a value, and a procedure, which performs some operation but does not return a value.

Most programming languages come with a prewritten set of functions that are kept in a library. You can also write your own functions to perform specialized tasks. [2]

We use function so we don't need to repeat ourselves. Please watch the video below and remember the content.

The difference between returning and printing

Students often print from within a function. Please understand the difference between printing a result from a function and returning a result from a function.

print: gives the value to the user as an output string. print(3) would give a string '3' to the screen for the user to view. The program would lose the value.

return: gives the value to the program. Callers of the function then have the actual data and data type (bool, int, etc...) return 3 would have the value 3 put in place of where the function was called.[3]

In general, you should return a value from a function and not directly print from a function.

Example of a function

# this is a simple function that remembers who like hamburgers and who doesn't like hamburgers. 

def likesHamburgers(name):
    if name == "Alisher":
        likes_hamburgers ="yes"
    return likes_hamburgers
print likesHamburgers("Bill")
print likesHamburgers("Alisher")    
print likesHamburgers("foo")

Another classic example of a function

# this is a simple function

def calculator(number1, number2):
    answer = number1 + number2
    return answer
print calculator(12,43)
print calculator(91,673)
print calculator(1,3)
print calculator(87,1098)

Some decent videos about functions in Python

See Also