Python Basic Syntax and Variables
Basic Syntax and Variables
Now it's time to start writing real Python code. Let's cover the most fundamental rules: indentation, comments, and variables.
Indentation Rules
Python's most distinctive feature — it uses indentation to define code blocks instead of curly braces {}.
PYTHON
# ✅ Correct: consistent indentation within a block
if 3 > 1:
print("3 is greater than 1")
print("This line is also inside the if block")
# ❌ Error: inconsistent indentation
if 3 > 1:
print("3 is greater than 1")
print("Wrong indentation!") # IndentationError
⚠️ Golden rule: Always use 4 spaces for indentation, never tabs. Most editors (VS Code, PyCharm) will automatically convert a Tab press to 4 spaces.
Indentation applies not just to if statements, but to all code blocks — loops, functions, classes, and more:
PYTHON
# Indentation in a for loop
for i in range(3):
print(i) # This line is inside the loop
print("---") # This line is also inside the loop
print("Done") # This line is NOT inside the loop
Comments
Comments are for human readers — Python ignores them completely.
Single-line Comments: #
PYTHON
# This is a single-line comment
print("Hello") # Comment after code
# Multi-line comments use multiple # symbols
# This is line two
# This is line three
Multi-line String Comments: """ """ or ''' '''
Technically these are strings, but they're commonly used as multi-line comments:
PYTHON
"""
This is a multi-line comment
Useful for writing function descriptions
Or temporarily blocking out large blocks of code
"""
print("The multi-line string above won't execute")
Variables
Python variables don't need type declarations — just assign a value and you're good to go:
PYTHON
name = "Alice" # str
age = 25 # int
height = 1.75 # float
is_student = True # bool
Variable Naming Rules
| Rule | Correct ✅ | Incorrect ❌ |
|---|---|---|
| Letters, digits, underscores only | my_name |
my-name |
| Cannot start with a digit | var1 |
1var |
| Case-sensitive | name and Name are different |
- |
| Cannot be a keyword | - | if, for, class |
Naming Conventions (PEP 8)
PYTHON
# ✅ Variables: lowercase with underscores (snake_case)
user_name = "Alice"
total_price = 99.9
max_value = 100
# ✅ Constants: ALL_CAPS with underscores
PI = 3.14159
MAX_SIZE = 1024
# ❌ Not recommended
userName = "Alice" # CamelCase (Java style)
UserName = "Alice" # Looks like a class name
Multiple Variable Assignment
Python lets you assign multiple variables in one line:
PYTHON
# Assign multiple values at once
a, b, c = 1, 2, 3
print(a, b, c) # 1 2 3
# Swap two variables (other languages need a temporary variable)
x, y = 10, 20
x, y = y, x
print(x, y) # 20 10
# Assign the same value to multiple variables
m = n = p = 0
print(m, n, p) # 0 0 0
The type() Function
Use type() to check a variable's type:
PYTHON
print(type(10)) # <class 'int'>
print(type(3.14)) # <class 'float'>
print(type("Hello")) # <class 'str'>
print(type(True)) # <class 'bool'>
# Variables can change type when reassigned (dynamic typing)
x = 10
print(type(x)) # <class 'int'>
x = "Hello"
print(type(x)) # <class 'str'> ← type changed
Common Errors
PYTHON
# IndentationError: wrong indentation
print("Too much indentation")
print("This line has an extra space") # ❌
# NameError: variable not defined
print(z) # ❌ z was never assigned
# SyntaxError: invalid syntax
print("Hello) # ❌ quotes not closed
📖 Section Summary
- Python uses 4 spaces of indentation to organize code blocks
- Single-line comments use
#, multi-line comments use""" """ - Variables are assigned directly — no type declaration needed (dynamic typing)
- Variable names use
snake_case(lowercase + underscores) - Constants use
ALL_CAPSlikeMAX_VALUE - Use
type()to check a variable's type - You can assign multiple variables at once:
a, b = 1, 2
📝 Homework
- Write code that defines three variables:
city(city name),year(year),score(score 88.5), and print them usingprint() - Swap two variables
a = 5andb = 10so thatabecomes 10 andbbecomes 5 - Use
type()to check the types ofTrue,"123", and3.0 - Intentionally write code with an indentation error, see what error Python throws, and take a screenshot of the error message