python/concepts/functions/introduction.md

Introduction

A function is a way to compartmentalize code so it can be called by name from one or more places in the program. Functions are used to perform specific and repetitive tasks.

More formally: a function is any Python object to which the function call operation can be applied. A function may be used to return one or more values as a result of some operation(s), or it may be used for one or more side effects. If a function does not specify a return value it will still return None.

Following is an example of a function with a side effect:

>>> def hello():
...        print("Hello")
...
>>> hello()
Hello

In the example above, the def keyword is used to signal the beginning of the function definition. hello is the name of the function. The empty parentheses represent that no values are being passed to the function. The body of the function is the single print("Hello") statement. print is also a function. The "Hello" in the print function's parentheses is known as an argument. The argument is used by the print function to know what to print. Note that the body of the function is indented. The indentation is important because Python relies on it to know where that block of code ends. The function body ends at either the end of the program or just before the next line of code that is not indented. Since hello() does not specify a return value, it executes its side effect - which is calling print() -- and then returns None. Finally, we call the function by using its name and the parentheses - which signals to the Python interpreter that this is a callable name.

Following is an example of a function with a return value:

>>> def hello():
...     return "Hello"
... 
print(hello())
Hello

The body of this function has been changed from calling print to returning the string "Hello". In the end, the code still prints "Hello", but the side effect takes place in the calling code instead of in the hello function. If the parentheses were left off from calling the hello function (e.g. print(hello)), then print would output something like

<function hello at 0x0000026E18E93E20>

It's important that, even if a function accepts no arguments (i.e. has no parameters), the empty parentheses must be used to call it correctly. This is different from a language such as Ruby which does not require empty parentheses for calling a function without any arguments.

A function can define zero or more parameters. A parameter defines what argument(s) the function accepts.

Following is an example of a function which accepts an argument:

>>> def hello(name):
...     return f"Hello, {name}"
... 
>>>print(hello("Bob"))
Hello, Bob

The parameter is defined as name. "Bob" is the argument which the program passes to the hello function. The function returns the string "Hello, Bob".

What if someone calls hello without passing an argument? The program throws an error:

print(hello())
Traceback (most recent call last):

  Input In in <line: 1>
    print(hello())

TypeError: hello() missing 1 required positional argument: 'name'
If we don't want the program to error with no argument (_but want to allow the calling code to not supply one_), we can define a [default argument][default arguments].
A default argument defines what value to use if the argument is missing when the function is called.

Following is an example of a function with a default argument:

```python
>>> def hello(name="you"):
...     return f"Hello, {name}"
... 
>>> print(hello())
Hello, you

The name parameter is given the default argument of "you", so the program outputs Hello, you, if not passed a name argument.

For more about function arguments, please see the function arguments concept.