Introduction

Python is one of the most popular programming languages. It’s simple to use, packed with features and supported by a wide range of libraries and frameworks. Its clean syntax makes it beginner-friendly.

• A high-level language used in data science, automation, AI, web development and more.
• Known for its readability, which means code is easier to write, understand and maintain.
• Backed by strong library support, we don’t have to build everything from scratch.

# This is a comment. It will not be executed.
print("Hello, World!")

• print() is a built-in Python function that instructs the computer to display text on the screen.
• "Hello, World!" is a string enclosed within quotes (either single ' or double ").
• Python uses indentation (spaces or tabs) to define code blocks instead of braces {} in C, C++ and Java .

# is used to write Python Comments. Comments are ignored during execution.

Input and Output in Python

The print() function is used for output in various formats and the input() function enables interaction with users.

name = input("Enter your name: ")
print("Hello,", name, "! Welcome!")

input().split() reads a single line of input and splits it into multiple values based on spaces.

Python Variables

Variables are used to store data that can be referenced and manipulated during program execution. A variable is essentially a name that is assigned to a value.

• Unlike Java and many other languages, Python variables do not require explicit declaration of type.
• Type of the variable is inferred based on the value assigned.

x = 5
name = "Alex"  
print(x)
print(name)

Rules for Naming Variables

To use variables correctly, the following naming rules should be followed:

1. Names can contain letters, digits and underscores (_).
2. The first character cannot be a digit.
3. Names are case-sensitive, so myVar and myvar are treated differently.
4. Keywords such as if, else and for cannot be used as variable names.

Assigning Values to Variables

1. Basic Assignment: Variables are assigned values using the = operator.

x = 5
y = 3.14
z = "Hi"

2. Dynamic Typing: Python is dynamically typed, so the same variable can store different data types during execution.

x = 10
x = "Now a string"

3. Assigning Same Value: Same value can be assigned to multiple variables in a single line.

a = b = c = 100
print(a, b, c)

4. Assigning Different Values: Multiple variables can also be assigned different values in a single line.

x, y, z = 1, 2.5, "Python"
print(x, y, z)

In Python, the type() function returns the type of the specified variable. It is used to determine the data type of an object. When you pass a variable to the type() function, it returns the type of the object that the variable references.

Deleting a Variable

del keyword is used to delete a variable from memory. After deletion, the variable can no longer be accessed.

x = 10
del x
print(x)

Python Data Structures

Python String

Strings are sequence of characters written inside quotes. It can include letters, numbers, symbols and spaces. Python does not have a separate character type.

• A single character is treated as a string of length one.
• Strings are commonly used for text handling and manipulation.

Accessing Characters in String

Strings are indexed sequences. Positive indices start at 0 from the left, negative indices start at -1 from the right

String Slicing

Slicing is a way to extract a portion of a string by specifying the start and end indexes. The syntax for slicing is string[start:end], where start starting index and end is stopping index (excluded).

s = "ABCDEF"
print(s[1:4])    
print(s[:3])     
print(s[3:])    
print(s[::-1])

#OUTPUT
BCD
ABC
DEF
FEDCBA

String Immutability

Strings are immutable, meaning their values cannot be changed after creation. Any modification to a string creates a new string instead of altering the original one.

Deleting a String

Individual characters of a string cannot be deleted because strings are immutable. However, an entire string variable can be removed using del keyword.

s = "ABC"
del s

Updating a String

Strings cannot be changed directly after creation. So any modification results in a new string being created using slicing or methods like replace().

s = "ABCD EF"
s1 = "H" + s[1:]                  
s2 = s.replace("ABC", "abc")  

print(s1)
print(s2)

#OUTPUT
HBCD EF
abcD EF

Common String Methods

Python provides various built-in methods to manipulate strings. Below are some of the most useful methods:

1. len(): returns the total number of characters in a string (including spaces and punctuation).

s = "GeeksforGeeks"
print(len(s))
#OUTPUT
13

2. upper() and lower(): upper() method converts all characters to uppercase whereas, lower() method converts all characters to lowercase.

s = "Hello World"
print(s.upper())
print(s.lower())

#OUTPUT
HELLO WORLD
hello world

3. strip() and replace(): strip() removes leading and trailing whitespace from the string and replace() replaces all occurrences of a specified substring with another.

s = "   ABC   "
print(s.strip())    

s = "Python is fun"
print(s.replace("fun", "awesome"))

#OUTPUT
ABC
Python is awesome

4. Concatenation: Strings can be combined by using + operator.

s1 = "Hello"
s2 = "World"
print(s1 + " " + s2)

5. Repetition: A string can be repeated multiple times using *.

s = "Hello "
print(s * 3)

6. Using f-strings: f-strings allows to directly insert variables and expressions inside a string using {} brackets.

name = "Jake"
age = 22
print(f"Name: {name}, Age: {age}")

7. in keyword is used to check whether a substring exists inside a string. It returns True if the substring is found, otherwise it returns False.

s = "GeeksforGeeks"
print("Geeks" in s)
print("GfG" in s)

8. Count() Returns the number of times a substring appears in the string.

text = "apple apple mango"
print(text.count("apple"))
#output
2