# 3. Using functions and storing code in files
## Functions
Now we know how to make Python show text.
```py
>>> '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.
```py
>>> print('Hello!')
Hello!
>>>
```
Now we are ready for a classic example, which is also the first program
in many tutorials :)
```py
>>> print("Hello World!")
Hello World!
>>>
```
But what exactly is print?
```py
>>> print
>>>
```
In Python 3, print is a function. Functions do something. They are used
by typing their name _[*]_ and parenthesis. Inside the parenthesis, we
can pass some parameters too. In `print("hello")` the function name is
`print` and we give it one parameter, which is `"hello"`.
_[*] Actually, a name of
[a variable that points to the function](https://www.youtube.com/watch?v=_AEJHKGk9ns).
Functions are also variables in Python. This means that
`print_me_a_thingy = print` and then `print_me_a_thingy('hello world')`
works just fine._
Functions are often thoght of as difficult to understand, but they
really are not. They just do something with the parameters they're
given. But if we do `x = print('hello')`, what is x?
```py
>>> x = print('hello')
hello
>>> print(x) # x is now None
None
>>>
```
So doing `x = print('hello')` set `x` to `None`. Here's what happened,
explained in more detail:
- In `x = print('hello')`, the right side is processed first.
- `print('hello')` calls the print function with the parameter
`'hello'`.
- The function runs **immediately** when it's called. It shows the word
hello.
- The print function returns `None`.
- 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
`x = None`.
- x is now `None`.
All functions need to return something. So does the print function, and
that's why it returns None. Of course, `x = print('hello')` is useless
compared to `print('hello')`, because the print function always returns
`None` and we can do `x = None` without any printing.
Calling a function without assigning the return value to anything (e.g.
`print('hello')` instead of `x = print('hello')`) simply throws away
the return value. The interactive `>>>` prompt also echoes the return
value back if it's not None:
```py
>>> str(123)
'123'
>>>
```
You can also pass multiple parameters separated with commas. The print
function will add spaces between them.
```py
>>> print("Hello", "World!")
Hello World!
>>>
```
Unlike with `+`, the parameters don't need to be strings.
```py
>>> 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
>>>
```
Not all functions return None. The input function can be used for
getting a string from the user.
```py
>>> x = input("Enter something:")
Enter something:hello
>>> x
'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 x.
You may want to add a space after the `:`, like this:
```py
>>> x = input("Enter something: ") # now there's space between : and where i type
Enter something: hello
>>>
```
## Storing code in files
Now it's time to write some code into a file for the first time. In
IDLE, go to File at top left and select New File, or just press Ctrl+N.
Type something like this into the window that opens. When your code is
in a file, adding `x` will not show the value of a variable called x to
the user. You need to print it instead.
```py
print("Hello.")
word = input("Enter something: ")
print("You entered", word)
```
Go to File, Save and save the file. Then press F5 to run it. It will run in the window you opened first. Another window like that will be opened if you closed it already. You should see something like this in it:
>>> ================================ RESTART ================================
>>>
Hello.
Enter something: hi
You entered hi
>>>
Awesome, it runs and you can interact with it by typing text into it and pressing Enter! :smiley: Again, when the program ran it waited for me to type something. I typed 'hi' and pressed Enter.
After running the program you can check what the `word` variable contains in the window the program ran in. Again, in the `>>>` prompt you don't need print to show things like you need in files.
```py
>>> word
'hi'
>>>
```
## Exercises
1. Write a program into a file that asks the user to write a word and then prints that word 1000 times. For example, if the user enters `hi` the program would reply `hihihihi...`.
2. Make it to print spaces between the words. It should be like `hi hi hi hi...`.
3. Make something that asks the user to enter two words, and prints 1000 of each with spaces in between. For example, if the user enters `hello` and `hi` the program would print `hello hi hello hi hello hi...`.
***
You may use this tutorial freely at your own risk. See [LICENSE](LICENSE).
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