Functions
In programming, a function is code that accepts an input and returns an output. What the function accepts is called a parameter. A function can have as many parameters as needed. Generally, a function takes parameters, does somethings with them, and returns something based on what the parameters were. Running a function is referred to as calling a function or a function call. Based on the function's definition, it will only accepts parameters of specific data types, in a specified order. Furthermore, a function will only return data of the type it is defined to.
Syntax
In Python, and many other programming languages, calling a function has the following syntax: functionName().
The name of the function followed by a parenthetical. Within the parentheses goes the parameters. The parameters are ordered and separated by commas. Let's look at some commonly used functions that are already built into Python.
print()
In the famous "Hello World!" program, we use the print() function.
print("Hello World!")
Here, the print() function accepts a single parameter, a string to be printed.
input()
Another commonly used function is input(). This function also takes a single parameter: the string to be displayed to the user prompting the user to input data.
Pass By Value vs. Pass By Reference
There two ways data can be provided to a function. By value or By reference.
By Value
Some programming languages pass data to functions by value. This means that the only the value and not the object itself is provided to the function. Essentially, the function creates a copy of the variable for use within the function. So if the function changes the variable, the true original variable will not be changed.
By Reference
Python passes objects to functions by reference. Recall that variables in Python are references or pointers to objects. It is not a copy. This means that if a function changes the value of any object that was passed to the function, its value will be truly changed. For example:
>>> listA = [0]
>>> listB = listA
>>> listA = [0,1,2,3,4,5]
>>> listB = listA
>>> listB.reverse()
>>> listB
[5, 4, 3, 2, 1, 0]
listB was reversed and this is demonstrated but knowing that variables in Python are references and that listB was passed to reverse() as a reference to an object; what do you believe listA is now?
>>> listA
[5, 4, 3, 2, 1, 0]
listA was reversed as well! Because listB and listA are references to the same object.
is Operator
The is operator accepts two input variables and returns a boolean. If the variables reference the same object is will return True otherwise, it will return False.
>>> listB is listA
True
listB and listA are references to the same object, so is returns True.