variables.md

Variables

Following the steps in the previous section, create a new file and call it variables.py.

Variable Assignment

We can use code like print(3 + 2) to display the result of an operation. However, this value isn't stored anywhere.

Using a variable allows us to place information in a box so that it may be used again later.

In the code below, we can obtain the result of a calculation and then use it again later.

my_val = 10 + 2
print(my_val)

When a variable is given a value, we call this variable assignment. The equals (=) symbol is often referred to as the assignment operator.

The assignment operator is how we let Python know that we want to take a box with a particular label, and place some information in the box. Then if we ever want to see or use what's inside that box again, we can use the variable's name to access the information another time.

Python, unlike some other languages, isn't especially fussed about knowing in advance what type of information you wish to place inside a box.

Name Error

Something we can't do is attempt to access a variable that has never been created. In Python this leads to what is known as a NameError.

Run the code below and examine the output:

my_val = 2
print(my_val)

my_other_val = 3
print(my_other_val)

print(this_does_not_exist)

Variable Reassignment

It's also possible to change the information that's inside a box. This is known as variable reassignment. Try running the code below:

year = 2023
print(year)
year = 2024
print(year)

The year variable starts out as having the value 2023. This is then replaced with the value 2024. This is like taking the 2023 that was in the box to start with then chucking it away, and replacing it with the 2024. The 2023 is now "lost" and can't be retrieved.

Garbage Collection

If some data no longer belongs to any boxes then it can't be accessed. In Python, something called the garbage collector will delete this boxless data for you so that the memory used to store it can be freed for something else. This is in contrast to languages such as C and Fortran, where freeing memory is something that has to be done manually. While this allows programs to be extremely fast, this also has the downside of leading to issues such as memory leaks. Python uses reference counting to determine when some data doesn't belong to any boxes, and is thus safe to delete.

Multiple Boxes

It is possible for the same data to belong to multiple boxes. Try running the code below:

 name = "Sydney"
 city = name
 print(name)
 print(city)

The Python is keyword can be used to check if two variables are referring to the same object (Objects will be covered in greater detail later on in this tutorial). Try seeing what happens when you run this extra line of code below.

city is name

Naming Variables

Python has certain rules about naming variables.

• A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ ).
• A variable name must start with a letter or the underscore character. It cannot start with a number.
• Variable names are case-sensitive (age, Age and AGE are three different variables).
• A variable name cannot be any of the Python keywords (these are also often called reserved words).

Here's an example of some code that violates our final rule:

is = "you shouldn't do this"

This then leads to the following error:

  File "<stdin>", line 1
    is = "you shouldn't do this"
    ^^
SyntaxError: invalid syntax

Summary

• Variables are like labelled boxes for data.
• We are not allowed to attempt to access a variable that has not been created. This leads to the code generating a NameError.
• When we swap what was previously inside a variable with some other data, this is known as reassignment.
• The Python garbage collector will automatically destroy data that doesn't belong to a box.
• It is possible for the same bit of data to belong to multiple boxes.
• Python requires that variables be named in a certain way. Going against these rules leads to the SyntaxError.

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