Variables

We have already seen in the previous sections that we can print out values and expressions of values that are evaluated by Python. However, it is often necceasry to remember a given value beyond the scope of a singular print statement or line of code. This is what motivates the need for a variable, which stores elements in computer memory.

You can think of a variable as a "storage box" that has some kind of "datatype" assigned to them. Each of these boxes has a value associated with it.

Here is how you assign a variable, or fill that "box." This is known as an assignment statement, or an initialization statement if this is the first time you are introducing a variable.

a = 1

lets break it down character by charater:

• a: the variable name
• =: this keyword is how we assign to variables. Most notably, is that it is explicitly assignment. You will learn about == in the next section, conditionals, but it always sets the left side equal to the right side, never the other way
• 1 the value of which we are assigning the variable a to. For now, we will assgin a to be an integer, or type int (a nonfractional number). We will go over the different datatypes that can be taken on in the next section.

note that python doesn't care about the spacing for assignment statements, so all examples below yield equivelent results

a=1
a =1
a= 1
a = 1
a          =                    1
a=                       1

Unlike other languages, python will dynamically infer the datatype (i.e. you don't have to predeclare a type like in Java or C/C++) for a given variable. Ever since python 3.6 you can also explicitly mark the type, but at runtime it will ignore these markers. For programming best practices it is recommended that you assign types in the following manner:

a: int = 1

this will tell your IDE (a place where you edit code, such as VS Code) that it should expect the type of a to be an int or integer. Despite explicitly being told, Python will not explicitly throw any error if you set a variable to a different type at runtime

a: int = 1  # set the variable a to be an int with value 1
print(a)    # => 1
a = "hi"    # change the value and of `a` at runtime
print(a)    # => hi

It is good code practice to have types be static as it helps significantly with the debugging process

As mentioned before, the equals sign is explicitly assignment

a = 2      # create variable a with value 2
a = a + 1  # lets set a to 1 + itself
print(a)   # => 3

for most basic arithmetic operations +, -, * and /, we can do the following substitution:

a = 5
a = a + 2
a = a - 2
a = a * 5
a = a / 2.5

is the same as

a = 5
a += 2
a -= 2
a *= 5
a /= 2.5

it even works for the modulo (%) operation!

a = 49
b = 49
print(a,b)  # => 49 49
a %= 5
b %= 5
print(a,b)  # => 4 4

We can assign multiple variables at once

a, b = 1, 2 # multi assignment
print(a)    # => 1
print(b)    # => 2

# now lets swap the variables!
a, b = b, a 
print(a) # => 2
print(b) # => 1

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