Python Basics | Variables· Operators

Introduction: What makes Python syntax different?

“Why does Python feel so simple?”

Python prioritizes readability. Indentation defines blocks, and concise syntax helps you learn quickly. This article covers:

• Variables and types
• Operators (arithmetic, comparison, logical)
• Conditionals (if / elif / else)
• Loops (for / while)
• Indentation rules

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Table of contents

1. Variables and types
2. Operators
3. Conditionals
4. Loops
5. Indentation rules
6. Summary

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1. Variables and types

Declaring variables: no type keyword required

Python is dynamically typed. You do not declare types when assigning variables.

# Variables (no type declarations)
name = "Alice"        # str
age = 25              # int
height = 175.5        # float
is_student = True     # bool
print(name, age, height, is_student)
# Output: Alice 25 175.5 True
# Type follows the value
x = 10        # int
x = "Hello"   # now str
x = [1, 2, 3] # now list

Compared to other languages:

# Python: no type declaration
age = 25
# Java: type required
# int age = 25;
# C++: type required (or auto)
# int age = 25;
# auto age = 25;
# TypeScript: optional types
# let age: number = 25;
# let age = 25;  // inferred

Checking and converting types

print(type(name))        # <class 'str'>
print(type(age))         # <class 'int'>
print(type(height))      # <class 'float'>
print(type(is_student))  # <class 'bool'>
print(isinstance(age, int))     # True
print(isinstance(name, str))    # True
print(isinstance(height, int))  # False
x = "123"
print(type(x))        # <class 'str'>
y = int(x)
print(type(y))        # <class 'int'>
print(y)              # 123
z = float(x)
print(z)              # 123.0
# int("abc")  # ValueError
def safe_int(value, default=0):
    try:
        return int(value)
    except ValueError:
        return default
print(safe_int("123"))   # 123
print(safe_int("abc"))   # 0

Multiple assignment and unpacking

x, y, z = 1, 2, 3
print(x, y, z)  # 1 2 3
a = b = c = 0
print(a, b, c)  # 0 0 0
x, y = 10, 20
print(f"before: x={x}, y={y}")
x, y = y, x  # swap in one line
print(f"after: x={x}, y={y}")
numbers = [1, 2, 3]
a, b, c = numbers
print(a, b, c)  # 1 2 3
first, *rest = [1, 2, 3, 4, 5]
print(first)  # 1
print(rest)   # [2, 3, 4, 5]
*beginning, last = [1, 2, 3, 4, 5]
print(beginning)  # [1, 2, 3, 4]
print(last)       # 5

Naming conventions (PEP 8)

# Good (snake_case)
user_name = "Alice"
total_count = 100
MAX_SIZE = 1000
_private_var = 42
# Bad
# userName — camelCase is not idiomatic in Python
# TotalCount — PascalCase is for classes
# 2nd_value — syntax error
# my-var — syntax error
import keyword
print(keyword.kwlist)

Type hints (optional)

name: str = "Alice"
age: int = 25
height: float = 175.5
is_student: bool = True
def greet(name: str) -> str:
    return f"Hello, {name}!"
result = greet("Alice")
print(result)
# Hints are not enforced at runtime
result = greet(123)
print(result)
# pip install mypy
# mypy script.py

---

2. Operators

Arithmetic operators

a, b = 10, 3
print(a + b)   # 13
print(a - b)   # 7
print(a * b)   # 30
print(a / b)   # float division
print(a // b)  # 3 floor division
print(a % b)   # 1
print(a ** b)  # 1000
print(-10 // 3)  # -4
print(-10 % 3)   # 2
x = 10
x += 5
x -= 3
x *= 2
x //= 5
x **= 2
print(x)

Python vs C/Java: / always produces a float in Python 3; use // for integer division. Use ** for exponentiation.

Comparison operators

x, y = 5, 10
print(x == y)
print(x != y)
print(x < y)
print(x > y)
print(x <= y)
print(x >= y)
x = 5
print(1 < x < 10)   # chained comparison
print(x == 5 == 5)

Logical operators

a, b = True, False
print(a and b)
print(a or b)
print(not a)
age = 25
is_student = True
if age >= 18 and is_student:
    print("Adult student")
# Short-circuit
x = 0
y = 10
result = (x != 0) and (y / x > 5)
print(result)
result = (x == 0) or (y / x > 5)
print(result)
name = ""
display_name = name or "Anonymous"
print(display_name)

Membership (in, not in)

fruits = ["apple", "banana", "cherry"]
print("apple" in fruits)
print("grape" not in fruits)
text = "Hello, Python!"
print("Python" in text)
user = {"name": "Alice", "age": 25}
print("name" in user)
print("Alice" in user.values())
print(5 in range(10))

Bitwise operators

a, b = 5, 3
print(a & b)
print(a | b)
print(a ^ b)
print(~a)
print(a << 1)
print(a >> 1)
READ = 1
WRITE = 2
EXECUTE = 4
permission = READ | WRITE
print(permission & READ)
print(permission & EXECUTE)

---

3. Conditionals

if

age = 20
if age >= 18:
    print("Adult")

if / else

age = 15
if age >= 18:
    print("Adult")
else:
    print("Minor")

if / elif / else

score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"
print(f"Grade: {grade}")

Nested if

age = 25
has_license = True
if age >= 18:
    if has_license:
        print("Can drive")
    else:
        print("Need a license")
else:
    print("Too young")

Conditional expression (ternary)

age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)
score = 85
grade = (
    "A" if score >= 90 else
    "B" if score >= 80 else
    "C" if score >= 70 else
    "F"
)
numbers = [1, 2, 3, 4, 5]
doubled = [x * 2 if x % 2 == 0 else x for x in numbers]
print(doubled)

Truthy and falsy

# Falsy: False, None, 0, 0.0, "", [], {}, ()
# Truthy: everything else
if "":
    print("won't run")
if "Hello":
    print("runs")
data = []
if not data:
    print("empty")
value = None
if value is None:
    print("None")
a = [1, 2, 3]
b = [1, 2, 3]
print(a == b)  # True
print(a is b)  # False

---

4. Loops

for over iterables

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")
for i, fruit in enumerate(fruits, start=1):
    print(f"{i}. {fruit}")
for i in range(5):
    print(i, end=" ")
print()
for i in range(1, 11, 2):
    print(i, end=" ")
print()
for i in range(10, 0, -1):
    print(i, end=" ")
print()
for char in "Python":
    print(char, end=" ")
print()
user = {"name": "Alice", "age": 25, "city": "Seoul"}
for key in user:
    print(key, end=" ")
print()
for key, value in user.items():
    print(f"{key}: {value}")

while

count = 0
while count < 5:
    print(count, end=" ")
    count += 1
print()
while True:
    user_input = input("Type q to quit: ")
    if user_input == 'q':
        break
    print(f"You typed: {user_input}")
while True:
    age = input("Age (1-120): ")
    if age.isdigit() and 1 <= int(age) <= 120:
        age = int(age)
        break
    print("Invalid age")
print(f"Age: {age}")

break and continue

for i in range(10):
    if i == 5:
        break
    print(i, end=" ")
print()
for i in range(10):
    if i % 2 == 0:
        continue
    print(i, end=" ")
print()
for num in range(2, 20):
    is_prime = True
    for i in range(2, int(num ** 0.5) + 1):
        if num % i == 0:
            is_prime = False
            break
    if is_prime:
        print(num, end=" ")
print()

for/while + else (runs if no break)

for i in range(5):
    print(i, end=" ")
else:
    print("\nloop finished normally")
for i in range(5):
    if i == 3:
        break
    print(i, end=" ")
else:
    print("\nwon't run")
# 0 1 2
numbers = [1, 3, 5, 7, 9]
target = 4
for num in numbers:
    if num == target:
        print(f"found {target}")
        break
else:
    print(f"{target} not found")

Nested loops

for i in range(2, 10):
    for j in range(1, 10):
        print(f"{i} x {j} = {i*j}")
    print()
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for row in matrix:
    for num in row:
        print(num, end=" ")
    print()

Breaking out of nested loops (no labels)

found = False
for i in range(5):
    for j in range(5):
        if i * j > 10:
            found = True
            break
    if found:
        break
def find_pair():
    for i in range(5):
        for j in range(5):
            if i * j > 10:
                return (i, j)
    return None
print(find_pair())

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5. Indentation rules

Python uses indentation to define blocks:

if True:
    print("Hello")
    print("World")
# Wrong:
# if True:
# print("Hello")  # IndentationError

PEP 8: use 4 spaces per level; do not mix tabs and spaces.

Practice examples

Even / odd

number = int(input("Enter a number: "))
if number % 2 == 0:
    print(f"{number} is even")
else:
    print(f"{number} is odd")

Sum 1..n

n = 10
total = 0
for i in range(1, n + 1):
    total += i
print(f"Sum 1..{n}: {total}")

Multiplication table

dan = int(input("Enter a table (1-9): "))
for i in range(1, 10):
    print(f"{dan} x {i} = {dan * i}")

Find maximum

numbers = [3, 7, 2, 9, 1, 5]
max_num = numbers[0]
for num in numbers:
    if num > max_num:
        max_num = num
print(f"Max: {max_num}")

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Common mistakes

• IndentationError: body must be indented under if/for/def.
• range(10) is 0..9, not 1..10 — use range(1, 11) for 1..10.
• Infinite while True: always provide a break or exit condition.
• Concatenating str + int: use str(age) or an f-string.
• List index: valid indices are 0 ...len-1.

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Exercises

FizzBuzz (1–30)

Show answer

```python for i in range(1, 31): if i % 15 == 0: print("FizzBuzz") elif i % 3 == 0: print("Fizz") elif i % 5 == 0: print("Buzz") else: print(i) ```

### Prime check

Show answer

```python def is_prime(n): if n < 2: return False for i in range(2, int(n ** 0.5) + 1): if n % i == 0: return False return True print(is_prime(7)) print(is_prime(10)) ```

### Reverse a list (no `reversed()`)

Show answer

```python numbers = [1, 2, 3, 4, 5] for i in range(len(numbers) - 1, -1, -1): print(numbers[i], end=" ") for num in numbers[::-1]: print(num, end=" ") ```

--- ## Summary ### Key takeaways 1. **Variables**: assign without types; types are dynamic. 2. **Operators**: arithmetic, comparison, logical, membership, bitwise. 3. **Conditionals**: `if` / `elif` / `else`, ternary expressions. 4. **Loops**: `for` over iterables, `while` for conditions, `break` / `continue`, optional `else`. 5. **Indentation**: four spaces (PEP 8). 6. **Python idioms**: truthy/falsy, chained comparisons, `for`-`else`. ### Next steps - [Python data types](/en/blog/python-series-03-data-types/ — lists, dicts, tuples, sets - [Python functions](/en/blog/python-series-04-functions/ — parameters, lambdas, decorators - [Python modules](/blog/python-series-06-modules/) — imports and packages --- ## Related posts

• Python environment setup
• Python data types
• Arrays and lists (algorithms)

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Frequently Asked Questions (FAQ)

Q. When would I use this in practice?

A. Learn Python syntax: variables, operators, if/elif/else, for/while loops, and indentation. Clear examples for beginners …

Q. What should I read before this?

A. Follow the previous article or related articles links at the bottom of each post to learn in sequence. See the Python series index for the full picture.

Q. Where can I study this more deeply?

A. Check cppreference and the relevant library’s official documentation. The reference links at the end of the article are also worth using.

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Related Articles (Internal Links)

Other articles related to this topic.

• Python Environment Setup
• Python Data Types | Lists· Dictionaries
• Arrays and Lists
• JavaScript Variables and Data Types | let· const

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Keywords Covered in This Article (Related Search Terms)

This article covers Python, Syntax, Variables, Conditionals, Loops, Operators, Beginner.