Learning Python A to Z from Basic

🐍 Python A to Z: Basics Reference Note

A — Assignment & Variables

Variables hold data values. You don't need to specify types manually.

Python

x = 10          # Integer
name = "Dev"    # String
pi = 3.14       # Float
is_ready = True # Boolean

B — Basic Arithmetic Operators

Standard math operations built into Python:

Python

add = 10 + 5       # 15
subtract = 10 - 5  # 5
multiply = 10 * 5  # 50
divide = 10 / 3    # 3.3333...
floor_div = 10 // 3# 3 (integer division)
power = 2 ** 3     # 8 (2 cubed)
modulus = 10 % 3   # 1 (remainder)

C — Comments

Lines ignored by the interpreter—used to explain code.

Python

# This is a single-line comment

"""
This is a multi-line comment
or docstring.
"""

D — Data Types & Conversion (Casting)

Checking and converting between types:

Python

num_str = "100"
num_int = int(num_str)     # Convert String to Integer
num_float = float(num_int) # Convert Integer to Float
text = str(250)            # Convert Integer to String

print(type(num_int))       # <class 'int'>

E — Else & Conditional Logic (if / elif / else)

Controlling execution based on conditions:

Python

score = 85

if score >= 90:
    print("Grade: A")
elif score >= 75:
    print("Grade: B")
else:
    print("Grade: C")

F — For Loops

Iterating over sequence collections or ranges:

Python

# Loop numbers 0 to 4
for i in range(5):
    print(i)

# Loop through list items
items = ["laptop", "mouse", "keyboard"]
for item in items:
    print(item)

G — Getting User Input

Reading input directly from the user in the console:

Python

user_name = input("Enter your name: ")
age = int(input("Enter your age: "))  # input() always returns a string

H — Hello World & Printing

Outputting text and formatting strings:

Python

print("Hello, World!")

# String Formatting (f-strings)
age = 22
print(f"I am {age} years old.")

I — Indentation

Python uses whitespace (usually 4 spaces) instead of curly braces {} to define blocks of code:

Python

if True:
    print("Indented line")  # Correct
# print("Wrong")            # SyntaxError if indented incorrectly!

J — Joining Strings

Concatenating strings together:

Python

first = "Python"
second = "Basics"

full = first + " " + second  # Using '+'
joined = "-".join(["A", "B", "C"])  # "A-B-C"

K — Keys in Dictionaries

Accessing and manipulating dictionary keys:

Python

student = {"name": "Kamal", "age": 20, "city": "Colombo"}

print(student.keys())      # dict_keys(['name', 'age', 'city'])
print(student.get("age"))  # 20

L — Lists

Ordered, mutable (changeable) collections:

Python

numbers = [10, 20, 30, 40]

numbers.append(50)     # Add to end
numbers.remove(20)     # Remove item
print(numbers[0])      # Access first item (10)

M — Methods for Strings

Common built-in string functions:

Python

text = "  python code  "

print(text.upper())     # "  PYTHON CODE  "
print(text.strip())     # "python code" (removes outer spaces)
print(text.replace("code", "notes")) # "  python notes  "

N — None Keyword

Represents the absence of a value (null value in other languages):

Python

result = None

if result is None:
    print("No result calculated yet.")

O — Logical Operators

Combining multiple conditions using and, or, and not:

Python

age = 20
has_license = True

if age >= 18 and has_license:
    print("Allowed to drive")

P — Parameter & Arguments in Functions

Creating reusable blocks of code:

Python

# 'a' and 'b' are parameters
def add_two(a, b):
    return a + b

# 5 and 10 are arguments
sum_result = add_two(5, 10)  # 15

Q — Quick Scope Rules (Local vs Global)

Variables inside functions are local, while outside ones are global:

Python

glob_var = "I am global"

def test_scope():
    loc_var = "I am local"
    print(glob_var) # Accessible

test_scope()
# print(loc_var)   # Error! Cannot access outside function

R — Return Statement

Exiting a function and returning a value:

Python

def multiply(x, y):
    return x * y

ans = multiply(4, 3)  # ans is assigned 12

S — Sets & Tuples

• Tuple: Ordered, immutable (cannot be changed). ()

• Set: Unordered, contains only unique values. {}

Python

# Tuple
point = (10, 20)

# Set
unique_nums = {1, 2, 2, 3, 3}  # Becomes {1, 2, 3}

T — Truthiness & Falsiness

Values evaluated in conditional checks:

• Falsy values: 0, "" (empty string), [] (empty list), None, False

• Truthy values: Everything else

Python

items = []
if not items:
    print("The list is empty!")

U — Unpacking Sequences

Extracting elements into variables directly:

Python

coordinates = (100, 200)
x, y = coordinates  # x = 100, y = 200

a, b, *rest = [1, 2, 3, 4, 5]  # a=1, b=2, rest=[3, 4, 5]

V — Value Errors & Catching Them

Handling basic runtime mistakes using try...except:

Python

try:
    num = int("abc")  # Raises ValueError
except ValueError:
    print("Could not convert text to a number!")

W — While Loops

Executing code repeatedly as long as a condition is True:

Python

count = 1

while count <= 3:
    print(f"Count: {count}")
    count += 1

X — Exit Loops (break & continue)

Controlling loop iteration flow:

Python

for num in range(5):
    if num == 2:
        continue  # Skip 2
    if num == 4:
        break     # Stop loop at 4
    print(num)    # Prints 0, 1, 3

Y — Yielding Lists via Ranges

Creating sequences quickly using range():

Python

numbers_list = list(range(1, 6))  # [1, 2, 3, 4, 5]
even_nums = list(range(2, 11, 2)) # [2, 4, 6, 8, 10]

Z — Zip Function

Pairing elements from multiple lists together:

Python

names = ["Alice", "Bob"]
ages = [22, 25]

for name, age in zip(names, ages):
    print(f"{name} is {age} years old.")