Python Variables and Comments: Comprehensive Theory

Understanding how to store data and document your code is the first step toward writing clean and maintainable Python scripts.

1. Python Comments

Comments are used to explain Python code and make it more readable. They can also be used to prevent execution when testing code.

Single-Line Comments

Single-line comments start with a #. Python will ignore the rest of the line.

# This is a comment
print("Hello, World!") # This is also a comment

Multi-Line Comments

Python does not have a specific syntax for multi-line comments. You can use # on each line, or use a multi-line string (triple quotes) that is not assigned to a variable.

"""
This is a multiline comment
written in more than just one line.
"""
print("Hello, World!")

2. Python Variables

Variables are containers for storing data values.

Creating Variables

Python has no command for declaring a variable. A variable is created the moment you first assign a value to it.

x = 5
y = "John"
print(x)
print(y)

Dynamic Typing

In Python, variables do not need to be declared with any particular type, and can even change type after they have been set.

x = 4       # x is of type int
x = "Sally" # x is now of type str
print(x)

Casting

If you want to specify the data type of a variable, this can be done with casting:

x = str(3)    # x will be '3'
y = int(3)    # y will be 3
z = float(3)  # z will be 3.0

3. Variable Naming Rules

A variable can have a short name (like x and y) or a more descriptive name (age, carname, total_volume).

Rules for Python variables:

• Must start with a letter or the underscore character.
• Cannot start with a number.
• Can only contain alpha-numeric characters and underscores (A-z, 0-9, and _).
• Are case-sensitive (age, Age, and AGE are three different variables).
• Cannot be any of the Python keywords (like if, while, class).

4. Outputting Variables

The Python print() function is often used to output variables.

x = "Python is "
y = "awesome"
z = x + y
print(z) # Output: Python is awesome

# You can also output multiple variables separated by a comma
a = 5
b = 10
print(a, b) # Output: 5 10

Practice Problems

1. Variable Swap: Create two variables a = 10 and b = 20. Write code to swap their values so that a becomes 20 and b becomes 10.
2. Naming Check: Which of the following are valid variable names? 2myvar, my-var, _my_var, my var, MYVAR.
3. Basic Arithmetic: Create a variable base = 10 and height = 20. Calculate the area of a triangle (0.5 * base * height) and print the result.
4. Comment Exercise: Take any code you've written and add at least three comments explaining what different parts of the code do.