Syntax, Types, and Control Flow

What You'll Learn

Python's core syntax rules, the five built-in types every program uses, and how to control program flow with if, for, and while.

Python Syntax Rules

1. Indentation defines blocks

# Correct: 4 spaces
if x > 0:
    print("positive")
    x = x - 1

# Wrong: no indentation
if x > 0:
print("this causes IndentationError")

2. Colons start blocks

Every if, for, while, def, class ends with ::

if x > 0:       # colon required
    ...

for i in range(3):  # colon required
    ...

def greet():    # colon required
    ...

3. One statement per line (usually)

x = 1
y = 2
z = x + y       # keep it readable

Semicolons work but are not Pythonic:

x = 1; y = 2    # ❌ avoid this style

The Five Core Types

str — Text

name = "Alice"
greeting = 'Hello, World!'
multiline = """Line 1
Line 2"""

print(type(name))  # <class 'str'>
print(len(name))   # 5
print(name[0])     # A  (indexing)
print(name.upper()) # ALICE

int — Whole Numbers

age = 25
year = 2024
negative = -10

print(type(age))   # <class 'int'>
print(age + 5)     # 30
print(age ** 2)    # 625  (power)
print(age // 7)    # 3    (floor division)
print(age % 7)     # 4    (remainder)

float — Decimal Numbers

price = 19.99
pi = 3.14159

print(type(price))   # <class 'float'>
print(round(price, 1))  # 20.0

bool — True or False

is_active = True
is_admin = False

print(type(is_active))   # <class 'bool'>
print(True + True)       # 2 (True == 1, False == 0)

NoneType — The Absence of a Value

result = None    # "nothing here yet"

print(type(result))  # <class 'NoneType'>
print(result is None)  # True (use `is`, not `==`)

Control Flow: if / elif / else

score = 75

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

print(f"Score {score} → Grade {grade}")
# Score 75 → Grade C

Comparison operators:

Operator	Meaning	Example
==	equals	x == 5
!=	not equals	x != 5
>	greater than	x > 5
<	less than	x < 5
>=	greater or equal	x >= 5
<=	less or equal	x <= 5

Logical operators:

age = 25
has_ticket = True

if age >= 18 and has_ticket:
    print("Allowed")

if age < 18 or not has_ticket:
    print("Not allowed")

Control Flow: for Loops

Loop over any sequence:

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

# apple
# banana
# cherry

Loop a fixed number of times:

for i in range(5):
    print(i)
# 0 1 2 3 4

for i in range(1, 6):
    print(i)
# 1 2 3 4 5

for i in range(0, 10, 2):
    print(i)
# 0 2 4 6 8

Control Flow: while Loops

Repeat while a condition is True:

count = 0
while count < 5:
    print(count)
    count += 1
# 0 1 2 3 4

break exits the loop early:

while True:
    user = input("Enter 'quit' to stop: ")
    if user == "quit":
        break
    print(f"You typed: {user}")

continue skips to the next iteration:

for i in range(10):
    if i % 2 == 0:
        continue        # skip even numbers
    print(i)            # prints 1 3 5 7 9

Combining Types and Control Flow

temperatures = [22, 35, 18, 41, 29]
hot_days = 0

for temp in temperatures:
    if temp > 30:
        hot_days += 1
        print(f"Hot day: {temp}°C")

print(f"\nTotal hot days: {hot_days}")

Output:

Hot day: 35°C
Hot day: 41°C

Total hot days: 2

Common Mistakes

Mistake	Example	Fix
Using = instead of ==	if x = 5:	Use == for comparison
Infinite while loop	while True: with no break	Always have a break condition
Off-by-one in range	range(5) gives 0–4, not 1–5	Use range(1, 6) for 1–5
Comparing None with ==	if x == None:	Use if x is None:

Quick Reference

# Types
str, int, float, bool, None

# if/elif/else
if condition:
    ...
elif other:
    ...
else:
    ...

# for
for item in iterable:
    ...

for i in range(n):
    ...

# while
while condition:
    ...
    break     # exit loop
    continue  # next iteration

What's Next

→ Lesson 2: Strings, Numbers, and Dates