Intro to Functions

Functions are reusable pieces of programs. They allow you to give a name to a block of statements, allowing you to run that block using the specified name anywhere in your program and any number of times. This is known as calling the function. We have already used many built-in functions such as len and range.

The function concept is probably the most important building block of any non-trivial software (in any programming language), so we will explore various aspects of functions in this chapter.

Functions are defined using the def keyword. After this keyword comes an identifier name for the function, followed by a pair of parentheses which may enclose some names of variables, and by the final colon that ends the line. Next follows the block of statements that are part of this function. An example will show that this is actually very simple:

Example:

def say_hello():
    # block belonging to the function
    print('hello world')
# End of function

say_hello()  # call the function
say_hello()  # call the function again

Output:

hello world
hello world

How It Works

We define a function called say_hello using the syntax as explained above. This function takes no parameters and hence there are no variables declared in the parentheses. Parameters to functions are just input to the function so that we can pass in different values to it and get back corresponding results.

Notice that we can call the same function twice which means we do not have to write the same code again.

Function Parameters

A function can take parameters, which are values you supply to the function so that the function can do something utilizing those values. These parameters are just like variables except that the values of these variables are defined when we call the function and are already assigned values when the function runs.

Parameters are specified within the pair of parentheses in the function definition, separated by commas. When we call the function, we supply the values in the same way. Note the terminology used - the names given in the function definition are called parameters whereas the values you supply in the function call are called arguments.

Example:

def print_add_10(value):
    x = value + 10
    print(x)
# End of function

print_add_10(2)  # call the function with integer as a parameter
print_add_10(10)  # call the function again with different parameter

Output:

12
20

How It Works

Here, we define a function called print_add_10 that uses one parameters called value. We create a new variable x that hold the evaluation of the value plus 10 and we printn the value in x.

The first time we call the function print_add_10, we directly supply 2 as an argument. In the second case, we call the function with the value 10 as an argument.

Local Variables

When you declare variables inside a function definition, they are not related in any way to other variables with the same names used outside the function - i.e. variable names are local to the function. This is called the scope of the variable. All variables have the scope of the block they are declared in starting from the point of definition of the name.

Example:

x = 50

def func(x):
    print('x is', x)
    x = 2
    print('Changed local x to', x)


func(x)
print('x is still', x)

Output:

x is 50
Changed local x to 2
x is still 50

How It Works

The first time that we print the value of the name x with the first line in the function’s body, Python uses the value of the parameter declared in the main block, above the function definition.

Next, we assign the value 2 to x. The name x is local to our function. So, when we change the value of x in the function, the x defined in the main block remains unaffected.

With the last print statement, we display the value of x as defined in the main block, thereby confirming that it is actually unaffected by the local assignment within the previously called function.

Default Argument Values

For some functions, you may want to make some parameters optional and use default values in case the user does not want to provide values for them. This is done with the help of default argument values. You can specify default argument values for parameters by appending to the parameter name in the function definition the assignment operator (=) followed by the default value.

Note that the default argument value should be a constant. More precisely, the default argument value should be immutable - this is explained in detail in later chapters. For now, just remember this.

Example:

def say(message, times=1):
    print(message * times)

say('Hello')
say('World', 5)

Output:

Hello
WorldWorldWorldWorldWorld

How It Works

The function named say is used to print a string as many times as specified. If we don’t supply a value, then by default, the string is printed just once. We achieve this by specifying a default argument value of 1 to the parameter times.

In the first usage of say, we supply only the string and it prints the string once. In the second usage of say, we supply both the string and an argument 5 stating that we want to say the string message 5 times.

CAUTION

Only those parameters which are at the end of the parameter list can be given default argument values i.e. you cannot have a parameter with a default argument value preceding a parameter without a default argument value in the function’s parameter list.

This is because the values are assigned to the parameters by position. For example,def func(a, b=5) is valid, but def func(a=5, b) is not valid.

The return statement

The return statement is used to return from a function i.e. break out of the function. We can optionally return a value from the function as well.

Example:

def add_10(value):
    x = value + 10
    return x
# End of function

result = add_10(5)
print(result)

Output:

15

How It Works

The add_10 function returns the parameter value added 10.

Note that a return statement without a value is equivalent to return None. None is a special type in Python that represents nothingness. For example, it is used to indicate that a variable has no value if it has a value of None.

Every function implicitly contains a return None statement at the end unless you have written your own return statement. You can see this by running print(some_function()) where the function some_function does not use the return statement such as:

def some_function():
    pass

The pass statement is used in Python to indicate an empty block of statements.

Summary

We have seen so many aspects of functions but note that we still haven’t covered all aspects of them.