using-functions.md

Using functions

Now we know how to make Python show text.

>>> 'Hello!'
'Hello!'
>>>

But that includes ''. One way to show text to the user without '' is with the print function. In Python, printing doesn't have anything to do with physical printers, it just means showing text on the screen.

>>> print('Hello!')
Hello!
>>>

Now we are ready for a classic example, which is also the first program in many tutorials :)

>>> print("Hello World!")
Hello World!
>>>

But what exactly is print?

What are functions?

Let's see what happens if we type print without the ('Hello') part.

>>> print
<built-in function print>
>>>

We could also type print(print), it would do the same thing. Python replied to us with some information about print wrapped in <>.

As we can see, print is a function. Functions do something when they are called by typing their name and parentheses. Inside the parentheses, we can pass some arguments too. In print("hello") the function is print and we give it one argument, which is "hello".

Functions are easy to understand, They simply do something when they are called. Functions run immediately when we call them, so the text appears on the screen right away when we run print(something).

Sometimes people think that doing thingy = print('hello') means that Python is going to print hello every time we type thingy. But this is not correct! print('hello') runs print right away, and if we type thingy later it's not going to run print('hello') again.

Return values

Now we know that thingy = print('hello') doesn't store the print('hello') call in a variable. But what does it do then?

>>> thingy = print('hello')
hello
>>> print(thingy)       # thingy is now None
None
>>>

So doing thingy = print('hello') set thingy to None.

Here's what happened, explained in more detail:

• When we do thingy = print('hello'), the right side is processed first.
• print('hello') calls the print function with the argument 'hello'.
• The function runs. It shows the word hello.
• The print function returns None. All functions need to return something, and print returns None because there's no need to return anything else.
• Now the right side has been processed. print('hello') returned None, so we can imagine we have None instead of print('hello') there, and the assignment now looks like thingy = None.
• thingy is now None.

Now we understand what a return value is. When we call the function, Python "replaces" function(arguments) with whatever the function returns.

Calling a function without assigning the return value to anything (e.g. print('hello') instead of thingy = print('hello')) simply throws away the return value. The interactive >>> prompt doesn't echo the return value back because it's None.

Of course, thingy = print('hello') is useless compared to print('hello') because the print function always returns None and we can do thingy = None without any printing.

Not all functions return None. The input function can be used for getting a string from the user.

>>> stuff = input("Enter something:")
Enter something:hello
>>> stuff
'hello'
>>>

input("Enter something:") showed the text Enter something: on the screen and waited for me to type something. I typed hello and pressed Enter. Then input returned the hello I typed as a string and it was assigned to stuff.

Usually we want to add a space after the :, like this:

>>> stuff = input("Enter something: ")  # now there's space between : and where i type
Enter something: hello
>>>

Handy things about print

We can also print an empty line by calling print without any arguments.

>>> print()

>>>

In Python, \n is a newline character. Printing a string that contains a newline character also prints a newline:

>>> print('hello\nworld')
hello
world
>>>

If we want to print a real backslash, we need to escape it by typing two backslashes.

>>> print('hello\\nworld')
hello\nworld
>>>

We can also pass multiple arguments to the print function. We need to separate them with commas and print will add spaces between them.

>>> print("Hello", "World!")
Hello World!
>>>

Unlike with +, the arguments don't need to be strings.

>>> print(42, "is an integer, and the value of pi is", 3.14)
42 is an integer, and the value of pi is 3.14
>>>

Variables names and built-in things

In the previous chapter we learned that if is not a valid variable name because it's a keyword.

>>> if = 123
  File "<stdin>", line 1
    if = 123
       ^
SyntaxError: invalid syntax
>>>

But print and input are not keywords, so can we use them as variable names?

>>> print = "hello"
>>> print
'hello'
>>>

We can, but there's a problem. Now we can't even do our hello world!

>>> print("Hello World!")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object is not callable
>>>

The error message complains that strings aren't callable because we just set print to the string 'hello' and now we're trying to call it like a function. As you can see, this is not a good idea at all. Most editors display built-in functions with a special color, so you don't need to worry about doing this accidentally.

Exit out of Python and start it again, and print("Hello World!") should work normally.

Summary

• function() calls a function without any arguments, and function(1, 2, 3) calls a function with 1, 2 and 3 as arguments.
• When a function is called, it does something and returns something.
• function(arguments) is "replaced" with the return value in the code that called it. For example, stuff = function() calls a function, and then does stuff = the_return_value and the return value ends up in stuff.
• Python comes with print and input. They are built-in functions.
• Avoid variable names that conflict with built-in functions.

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