Python: Python Syntax & Variables

Q: What's the difference between = and == ? I keep mixing them up.

= is assignment — it puts a value into a variable ( x = 5 means "x now holds 5"). == is comparison — it asks "are these equal?" ( x == 5 returns True or False ). Think of = as an arrow pointing left ( x ← 5 ) and == as a balance scale checking both sides. This is one of the most common beginner mistakes — writing if x = 5: instead of if x == 5: .

Now it's time to start writing real Python code. Let's cover the most fundamental rules: indentation, comments, and variables.

1. Indentation Rules

Python's most distinctive feature — it uses indentation to define code blocks instead of curly braces {}.

TEXT 📖 Display only

# ✅ 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

2. Comments

Comments are for human readers — Python ignores them completely.

(1) 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

(2) 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")

3. 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

(1) 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`

(2) 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

(3) 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

▶ Example: Variable Types and Swapping (Difficulty ⭐)

PYTHON

# Define variables of different types
name = "Alice"
age = 25
height = 1.68
is_student = True

# Print variable info
print(f"{name} is {age} years old, {height}m tall")
print(f"Student status: {is_student}")

# Swap two variables
a, b = 10, 20
print(f"Before: a={a}, b={b}")
a, b = b, a
print(f"After: a={a}, b={b}")

# Check types
print(f"name type: {type(name)}")
print(f"age type: {type(age)}")
print(f"height type: {type(height)}")

▶ Try it Yourself

Output:

TEXT 📖 Display only

Alice is 25 years old, 1.68m tall
Student status: True
Before: a=10, b=20
After: a=20, b=10
name type: <class 'str'>
age type: <class 'int'>
height type: <class 'float'>

4. The type() Function

▶ Example: Variable Type Exploration (Difficulty ⭐)

PYTHON

# Explore different types and conversions
x = 42
y = 3.14
z = "Hello"
w = True

print(f"x = {x}, type = {type(x).__name__}")
print(f"y = {y}, type = {type(y).__name__}")
print(f"z = {z}, type = {type(z).__name__}")
print(f"w = {w}, type = {type(w).__name__}")

# Type conversion
print(f"\nint(y) = {int(y)}")
print(f"float(x) = {float(x)}")
print(f"str(x) = {str(x)}")
print(f"bool(0) = {bool(0)}, bool(1) = {bool(1)}")

# Dynamic typing: variable can change type
var = 100
print(f"\nvar = {var}, type = {type(var).__name__}")
var = "now a string"
print(f"var = {var}, type = {type(var).__name__}")

▶ Try it Yourself

Output:

TEXT 📖 Display only

x = 42, type = int
y = 3.14, type = float
z = Hello, type = str
w = True, type = bool

int(y) = 3
float(x) = 42.0
str(x) = 42
bool(0) = False, bool(1) = True

var = 100, type = int
var = now a string, type = str

▶ Example: Variable Assignment Tricks (Difficulty ⭐)

PYTHON

# Multiple assignment
a, b, c = 10, 20, 30
print(f"a={a}, b={b}, c={c}")

# Swap without temp variable
x, y = 100, 200
print(f"Before swap: x={x}, y={y}")
x, y = y, x
print(f"After swap: x={x}, y={y}")

# Same value to multiple variables
p = q = r = 0
print(f"p={p}, q={q}, r={r}")

# Unpacking a list
colors = ["red", "green", "blue"]
first, second, third = colors
print(f"First: {first}, Second: {second}, Third: {third}")

▶ Try it Yourself

Output:

TEXT 📖 Display only

a=10, b=20, c=30
Before swap: x=100, y=200
After swap: x=200, y=100
p=0, q=0, r=0
First: red, Second: green, Third: blue

4. 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

5. 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

❓ FAQ

Q Why does Python use indentation instead of braces {} like other languages?

A Python's creator, Guido van Rossum, believed code should be readable by humans first. Indentation visually represents code structure — your eye already sees it, so why require extra symbols? This forces consistent formatting and eliminates entire categories of bugs (missing braces, mismatched brackets). It's a design philosophy: "Beautiful is better than ugly."

Q I mixed tabs and spaces — my code looks fine but throws an IndentationError. Why?

A Python treats tabs and spaces as different characters. A line indented with a tab looks the same as one indented with 4 spaces to your eye, but Python sees them differently. The fix: configure your editor to convert tabs to spaces (VS Code does this by default). Use python -m tabnanny myfile.py to scan for mixed indentation.

Q What's the difference between Python's dynamic typing and static typing in languages like Java? Does it matter?

A In statically typed languages, you declare a variable's type upfront (int x = 5;) and it can't change. In Python, types are determined at runtime and can change (x = 5; x = "hello" works). Dynamic typing is more flexible and faster to write, but type errors only surface at runtime. For beginners, dynamic typing means less boilerplate; for large projects, static typing catches bugs earlier.

Q I keep getting NameError: name 'xxx' is not defined. How do I fix this?

A NameError means Python can't find a variable with that name. Common causes: ① Typo in the variable name (Python is case-sensitive — Name ≠ name); ② Using a variable before assigning it; ③ Variable was defined inside a function or loop but you're trying to access it outside. Quick fix: add print() before the error line to check what variables actually exist at that point.

Q What's the difference between = and ==? I keep mixing them up.

A = is assignment — it puts a value into a variable (x = 5 means "x now holds 5"). == is comparison — it asks "are these equal?" (x == 5 returns True or False). Think of = as an arrow pointing left (x ← 5) and == as a balance scale checking both sides. This is one of the most common beginner mistakes — writing if x = 5: instead of if x == 5:.

📖 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_CAPS like MAX_VALUE
Use type() to check a variable's type
You can assign multiple variables at once: a, b = 1, 2

📝 Exercises

Write code that defines three variables: city (city name), year (year), score (score 88.5), and print them using print()
Swap two variables a = 5 and b = 10 so that a becomes 10 and b becomes 5
Use type() to check the types of True, "123", and 3.0
Intentionally write code with an indentation error, see what error Python throws, and take a screenshot of the error message

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Python: Python Syntax & Variables

1. Indentation Rules

2. Comments

(1) Single-line Comments: #

(2) Multi-line String Comments: """ """ or ''' '''

3. Variables

(1) Variable Naming Rules

(2) Naming Conventions (PEP 8)

(3) Multiple Variable Assignment

▶ Example: Variable Types and Swapping (Difficulty ⭐)

4. The type() Function

▶ Example: Variable Type Exploration (Difficulty ⭐)

▶ Example: Variable Assignment Tricks (Difficulty ⭐)

4. The type() Function

5. Common Errors

❓ FAQ

📖 Summary

📝 Exercises

🙏 帮我们做得更好

(1) Single-line Comments: `#`

(2) Multi-line String Comments: `""" """` or `''' '''`