Variables and Data Types

Before we talk about different data types, we need to talk about variables.

Variables

What is a variable?

I like to think of a variable as a jar with a label on it.

In your math class, when you want to use x, you just write it down, and whoever reads it, understands that x is a variable.

Most programming languages require the programmer to declare a variable and assign a type.

This isn't so in python:

>>> x = 5

I made a variable called x, and I assigned the value 5 into it. The weird >>> is the Python shell ... just ignore that!

Variables

So, what is a variable? We've covered this before, but let's think about it again.

A variable is a container with a label that holds a value.

Making a variable in Python is easy:

variable_name = some_value

What's in my variable?

To see the value my variable is holding, use the print() function.

>>> x = 5

>>> print(x)

5

>>>

Variables can be considered either as 'constant' or as 'variable' (as in changing). Python doesn't distinguish between the two as some languages do.

Making Constants (no such thing in Python)

When we think of the freezing point of water, we know it's 0 degrees. It's not going to change unless we add something to the water. Therefore the freezing point of water is constant.

How does this work in Python?

Build a variable called freezing_point and set its value to 0:

freezing_point = 0

print(freezing_point)

In case you missed that: Python doesn't care if variables are constants or variable!

Python does have constant values you can use. These 2 examples come from the math module (or library):

math.pi - The mathematical constant π = 3.141592...

math.e - The mathematical constant e, aka Euler's Number = 2.718281...

Making Variables

What about variables that change, like your age?

Build a variable called my_age and set its value to 62:

my_age = 62

print(my_age)

What two things did we just learn?

1. Building variables in Python is easy:

variable_name = value

2. Python doesn't care if variables are constants or variable, they are both built the same way:

variable_name = value

Left side of '=' VS. right side of '='

name = 'Bob'

age = 16

temperature = 22.5 

I just built a variable to hold a name, an age and the air temperature.

Two things happen on each line:

1. On the left hand side of the '=' is the Initialization (making) of the variable.
2. On the right side of the '=' is the Assignment of a value to that variable.

The '=' is called the assignment operator.

age = 12 # Variable built properly

12 = age # Variable NOT built properly. This will not work

Like I mentioned earlier, in most other languages, you must include the variable's type. Because Python supports dynamic typing, so this isn't a concern for you. Because Python supports dynamic typing, it can alter a variable's type depending on how it's being used. This is a a powerful feature of Python, but you should know how to check the type.

Data Types

So, what about Data Types? What are the different types of data that Python can hold?

Let's build some variables to see what we can get:

snacks_packed = 2

distance_to_school = 3.5

water_bottle_full = False

cell_phone_brand = 'Android'

Let's use the print() function to see what's in those variables:

print(snacks_packed)

print(distance_to_school)

print(water_bottle_full)

print(cell_phone_brand)

Looks like all of our values are there. But what TYPE are they?

We can use the type() function to let us know.

Build a variable called my_name and set its value to Bobo, then check the type:

my_name = 'Bobo'

print(my_name)

print(type(my_name))

What are some types that you will need to know?

Let's check on the type of those previously made variables:

print(type(snacks_packed))         # Should be an int

print(type(distance_to_school))    # Should be a float 

print(type(water_bottle_full))     # Should be a boolean

print(type(cell_phone_brand))      # Should be a string

What do the different types mean?

• int holds whole numbers only
• float holds floating point numbers. Numbers with a decimal
• boolean holds the logical values True or False
• string holds words, single letters, symbols, and numbers that aren't numbers (more about that in a bit)

NOTE: If you call the type() from the Python interpreter, it will output the type. If you call type() from with in source code, you must wrap it in a print() to see the output.

Naming variables

I've said that variables are containers with labels. We can refer to that label as a name. There are a few rules for naming variables:

1. lower case only. This is a coding / programming convention. A convention is a way something is usually done. A programming convention is a way of doing things across many languages. Python is case sensitive, so be aware that the variable cats is different than the variable caTs, or Cats.
2. Variable names tend to be noun based, that is 'things'. We may want to hold a player_name, or the player_score.
3. Variable names must begin with a letter, not a number.
4. If multiple words are used, separate them with an underscore character '_'.
5. It's a generally good idea to keep symbols out of variable names. Python should catch these and provide you with a error message like this: SyntaxError: invalid syntax
6. Variable names cannot be reserved keywords.

Challenge

Can you get Python to hold the string Hello in a variable called message? Can you ask Python what type the variable message is? You're going to need the print() and type() functions.

message = 'Hello'

print(message)

print(type(message))