# 2. Variables, Booleans and None ## Variables Variables are easy to understand. They simply **contain data**. Actually they [point to data](https://www.youtube.com/watch?v=_AEJHKGk9ns), but think about them as data containers for now. ```py >>> a = 1 # create a variable called a and assign 1 to it >>> a # get the value of a and let Python echo it 1 >>> ``` We can also change the value of a variable after setting it. ``` >>> a = 2 >>> a 2 >>> ``` Trying to access a variable that is not defined is an error. ``` >>> thingy Traceback (most recent call last): File "", line 1, in NameError: name 'thingy' is not defined >>> ``` Variables are an important part of most programming languages, and they allow programmers to write much larger programs than they could write without variables. Variable names can be multiple characters long. They can contain uppercase characters, numbers and some other characters, but most of the time you should use simple, lowercase variable names. You can also use underscores. ```py >>> number_one = 1 >>> number_two = 2 >>> ``` Python also has some words that cannot be used as variable names because they have a special meaning. They are called **keywords**, and you can run `help('keywords')` to see the full list if you want to. We'll learn to use most of them later in this tutorial. Trying to use a keyword as a variable name causes a syntax error: ```py >>> if = 'hello' File "", line 1 if = 'hello' ^ SyntaxError: invalid syntax >>> ``` When assigning something to a variable using a `=`, the right side of the `=` is always executed before the left side. This means that we can do something with a variable on the right side, then assign the result back to the same variable on the left side: ```py >>> a = 1 >>> a = a + 1 >>> a 2 >>> ``` To do something to a variable (for example, to add something to it) we can use `+=`, `-=`, `*=` and `/=` instead of `+`, `-`, `*` and `/`. The "advanced" `%=`, `//=` and `**=` also work. ```py >>> a += 2 # a = a + 2 >>> a -= 2 # a = a - 2 >>> a *= 2 # a = a * 2 >>> a /= 2 # a = a / 2 >>> ``` This is not limited to integers. ```py >>> a = 'hello' >>> a *= 3 >>> a += 'world' >>> a 'hellohellohelloworld' >>> ``` Now you also understand why typing hello to the prompt didn't work in the beginning of this tutorial. But we can assign something to a variable called hello and then type hello: ```py >>> hello = 'hello there' >>> hello 'hello there' >>> ``` ## Booleans There are two Boolean values, True and False. In Python, and in many other programming languages, `=` is assigning and `==` is comparing. `a = 1` sets a to 1, and `a == 1` checks if a equals 1. ```py >>> a = 1 >>> a == 1 True >>> a = 2 >>> a == 1 False >>> ``` `a == 1` is the same as `(a == 1) == True`, but `a == 1` is more readable, so most of the time you shouldn't write `== True` anywhere. ```py >>> a = 1 >>> a == 1 True >>> (a == 1) == True True >>> a = 2 >>> a == 1 False >>> (a == 1) == True False >>> ``` ## None None is Python's "nothing" value. It behaves just like any other value, and it's often used as a default value for different kinds of things. We'll find a bunch of ways to use None later. None's behavior on the interactive prompt might be a bit confusing at first: ``` >>> thingy = None >>> thingy >>> ``` That was weird! We set thingy to None, but typing `thingy` didn't echo back None. This is because the prompt never echoes back None. That is handy, because many things result in None, and it would be annoying to see None coming up all the time. ## Other comparing operators So far we've used `==`, but there are other operators also. At this point, this list probably looks awfully long, but it's actually pretty easy to learn. | `a == b` | a is equal to b | `1 == 1` | | `a != b` | a is not equal to b | `1 == 2` | | `a > b` | a is greater than b | `2 > 1` | | `a >= b` | a is greater than or equal to b | `2 >= 1`, `1 >= 1` | | `a < b` | a is less than b | `1 < 2` | | `a <= b` | a is less than or equal to b | `1 <= 2`, `1 <= 1` | We can also combine multiple comparisons. These are not always correct, because a and b don't need to be Booleans. We'll learn more about that later. | `a and b` | a is True and b is True | `1 == 1 and 2 == 2` | | `a or b` | a is True, b is True or they're both True | `False or 1 == 1`, `True or True` | There is more than one way to do some things. For example, to check if a is not equal to 1 we could do `a != 1` or `not a == 1`. To check if b does not contain a we could do `a not in b` or `not a in b`. However, `!=` and `not in` should be used in cases like this because they're more convinient once you get used to them. There's also `is`, but don't use it instead of `==` unless you know what you are doing. We'll learn more about it later. *** You may use this tutorial freely at your own risk. See [LICENSE](LICENSE). [Back to the list of contents](README.md)