Ultimate Python Guide

1.1 Your First Python Program

Learn how to write your very first Python program using print() and variables.

print("Hello, World!")

• print() – a built‑in function that outputs text to the screen.
• "Hello, World!" – a string (text) that will be printed.
• Python executes code line by line, from top to bottom.

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1.2 Variables & Data Types

Variables store data. Python has several built‑in data types.

name = "Alice"
age = 30
height = 5.9
is_student = True

• str – string (text).
• int – integer (whole numbers).
• float – numbers with a decimal point.
• bool – True or False.

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1.3 Lists & Tuples

Lists are mutable collections; tuples are immutable.

fruits = ["apple", "banana", "cherry"]
fruits.append("orange")
fruits.remove("banana")

coordinates = (10, 20)

• list – mutable, created with [].
• tuple – immutable, created with ().
• Common list methods: .append(), .remove(), .pop().

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1.4 Conditional Statements

Use if, elif, and else to make decisions.

score = 85
if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
else:
    print("Grade: C")

• if – starts a conditional block.
• elif – else if (multiple checks).
• else – runs if all conditions are false.
• Indentation is crucial – the code block must be indented.

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1.5 Loops (for/while)

Loops repeat code.

# for loop
for i in range(5):
    print(i)

# while loop
count = 0
while count < 5:
    print(count)
    count += 1

• for – iterates over a sequence (e.g., range(), list).
• while – repeats while a condition is true.
• range(start, stop, step) – generates a sequence of numbers.

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1.6 Functions

Functions are reusable blocks of code.

def greet(name):
    return f"Hello, {name}!"

result = greet("Alice")
print(result)

• def – defines a function.
• Parameters – values passed to the function.
• return – sends a value back to the caller.

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1.7 String Methods & Formatting

Strings have many built‑in methods and can be formatted with f‑strings.

text = "  Hello World!  "
print(text.upper())       # "  HELLO WORLD!  "
print(text.lower())       # "  hello world!  "
print(text.strip())       # "Hello World!"

name = "Alice"
age = 30
print(f"My name is {name} and I'm {age} years old.")

• .upper(), .lower(), .strip(), .replace(), .split(), .join().
• f‑strings – f"Hello {name}".

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1.8 Dictionaries

Dictionaries store key‑value pairs.

person = {
    "name": "Alice",
    "age": 30,
    "city": "Lahore"
}

print(person["name"])
person["age"] = 31
person["country"] = "Pakistan"

• Created with {}.
• Access via key: dict["key"] or .get().
• Keys must be immutable (string, number, tuple).

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1.9 Error Handling

Handle exceptions gracefully with try/except.

try:
    num = int(input("Enter a number: "))
    print(f"You entered: {num}")
except ValueError:
    print("That's not a valid number!")

• try – block that might raise an error.
• except – runs if an error occurs.
• finally – always runs (cleanup).

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1.10 Project: Calculator

Apply everything you've learned to build a functional calculator.

• Functions – add, subtract, multiply, divide.
• User menu – display options and loop until exit.
• Error handling – catch division by zero and invalid input.
• Input validation – ensure valid menu choices.

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