String Functions and Methods

What Are String Methods?

String methods are built-in functions that work specifically with text (strings). They let you transform, check, or manipulate strings easily.

Syntax:

result = text.method_name()

The method comes after the string variable with a dot (.) in between.

Example:

name = "Alice"
name_upper = name.upper()  # Convert to uppercase
print(name_upper)  # Output: ALICE

Important: Most string methods don’t change the original string - they return a new string with the changes.

greeting = "hello"
greeting.upper()  # Returns "HELLO" but doesn't change greeting
print(greeting)    # Still prints: hello

# To keep the change, assign it to a variable:
greeting_upper = greeting.upper()
print(greeting_upper)  # Prints: HELLO

Common String Methods

Case Conversion Methods

Method	What It Does	Example	Result
`.upper()`	Convert all letters to UPPERCASE	`"hello".upper()`	`"HELLO"`
`.lower()`	Convert all letters to lowercase	`"HELLO".lower()`	`"hello"`
`.capitalize()`	Capitalize only the first letter	`"hello world".capitalize()`	`"Hello world"`
`.title()`	Capitalize the first letter of each word	`"hello world".title()`	`"Hello World"`

Examples:

city = "new york"
print(city.upper())        # NEW YORK
print(city.capitalize())   # New york
print(city.title())        # New York

# Original string unchanged
print(city)                # new york

Common use case: Making comparisons case-insensitive

user_answer = input("Continue? (yes/no): ")

# This works regardless of how user types it: "YES", "Yes", "yes"
if user_answer.lower() == "yes":
    print("Continuing...")

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Whitespace Methods

Method	What It Does	Example	Result
`.strip()`	Remove spaces from both ends	`" hello ".strip()`	`"hello"`
`.lstrip()`	Remove spaces from left end only	`" hello ".lstrip()`	`"hello "`
`.rstrip()`	Remove spaces from right end only	`" hello ".rstrip()`	`" hello"`

Examples:

messy_name = "  Alice  "
clean_name = messy_name.strip()
print(f"'{clean_name}'")  # 'Alice'

Common use case: Cleaning user input

name = input("Enter your name: ").strip()
# User types: "  Bob  " (with spaces)
# name becomes: "Bob" (spaces removed)

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Checking String Content

Method	What It Does	Returns	Example
`.isdigit()`	Check if all characters are digits	`True` or `False`	`"123".isdigit()` → `True`
`.isalpha()`	Check if all characters are letters	`True` or `False`	`"abc".isalpha()` → `True`
`.isalnum()`	Check if all characters are letters or digits	`True` or `False`	`"abc123".isalnum()` → `True`
`.isspace()`	Check if all characters are whitespace	`True` or `False`	`" ".isspace()` → `True`

Examples:

"123".isdigit()      # True
"12.5".isdigit()     # False (decimal point)
"abc".isdigit()      # False (letters)
"-5".isdigit()       # False (minus sign)

"hello".isalpha()    # True
"hello123".isalpha() # False (has numbers)
"hello ".isalpha()   # False (has space)

Common use case: Validating input

age_input = input("Enter your age: ")

if age_input.isdigit():
    age = int(age_input)
    print(f"Your age: {age}")
else:
    print("Please enter only numbers!")

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Finding and Replacing

Method	What It Does	Example	Result
`.startswith(text)`	Check if string starts with text	`"hello".startswith("he")`	`True`
`.endswith(text)`	Check if string ends with text	`"hello".endswith("lo")`	`True`
`.count(text)`	Count how many times text appears	`"hello".count("l")`	`2`
`.replace(old, new)`	Replace old text with new text	`"hello".replace("l", "r")`	`"herro"`

Examples:

filename = "report.pdf"
print(filename.endswith(".pdf"))  # True
print(filename.endswith(".doc"))  # False

sentence = "I love Python and Python is great"
print(sentence.count("Python"))  # 2

message = "Hello World"
new_message = message.replace("World", "Python")
print(new_message)  # Hello Python

Common use case: File extension checking

filename = input("Enter filename: ")

if filename.endswith(".py"):
    print("This is a Python file!")
elif filename.endswith(".txt"):
    print("This is a text file!")
else:
    print("Unknown file type")

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Splitting and Joining

Method	What It Does	Example	Result
`.split()`	Split string into a list	`"a b c".split()`	`["a", "b", "c"]`
`.split(separator)`	Split by specific separator	`"a,b,c".split(",")`	`["a", "b", "c"]`
`separator.join(list)`	Join list items with separator	`"-".join(["a", "b"])`	`"a-b"`

Examples:

# Split by spaces (default)
sentence = "Hello my friend"
words = sentence.split()
print(words)  # ["Hello", "my", "friend"]

# Split by comma
data = "apple,banana,orange"
fruits = data.split(",")
print(fruits)  # ["apple", "banana", "orange"]

# Join list into string
words = ["Python", "is", "fun"]
sentence = " ".join(words)
print(sentence)  # Python is fun

# Join with different separator
words = ["apple", "banana", "orange"]
result = ", ".join(words)
print(result)  # apple, banana, orange

Common use case: Processing comma-separated input

colors_input = input("Enter your favorite colors (comma-separated): ")
# User types: "red, blue, green"

colors = colors_input.split(",")
# Result: ["red", " blue", " green"]

# Clean up spaces
colors = [color.strip() for color in colors]
# Result: ["red", "blue", "green"]

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Chaining Methods

You can chain multiple methods together:

messy_text = "  HELLO WORLD  "
clean_text = messy_text.strip().lower().capitalize()
print(clean_text)  # Hello world

# Step by step:
# 1. strip()      → "HELLO WORLD"
# 2. lower()      → "hello world"
# 3. capitalize() → "Hello world"

Common use case: Clean and normalize user input

answer = input("Continue? (yes/no): ").strip().lower()

if answer == "yes":
    print("Continuing...")
elif answer == "no":
    print("Stopping...")
else:
    print("Please enter yes or no")

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Practical Examples

Example 1: Username Validation

username = input("Create username: ").strip()

if not username:
    print("Username cannot be empty!")
elif not username.isalnum():
    print("Username must be letters and numbers only!")
elif len(username) < 3:
    print("Username must be at least 3 characters!")
else:
    print(f"Username '{username}' created successfully!")

Example 2: Clean and Format Name

name = input("Enter your full name: ").strip().title()
print(f"Hello, {name}!")

# User types: "  alice SMITH  "
# Output: Hello, Alice Smith!

Example 3: Parse Email

email = input("Enter email: ").strip().lower()

if "@" not in email:
    print("Invalid email - missing @")
elif not email.endswith((".com", ".org", ".edu")):
    print("Invalid email - must end with .com, .org, or .edu")
else:
    username = email.split("@")[0]
    domain = email.split("@")[1]
    print(f"Username: {username}")
    print(f"Domain: {domain}")

Example 4: Count Words

sentence = input("Enter a sentence: ").strip()
words = sentence.split()
word_count = len(words)
print(f"Your sentence has {word_count} words")

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Quick Reference Table

Category	Methods
Case	`.upper()`, `.lower()`, `.capitalize()`, `.title()`
Whitespace	`.strip()`, `.lstrip()`, `.rstrip()`
Check Type	`.isdigit()`, `.isalpha()`, `.isalnum()`, `.isspace()`
Find	`.startswith()`, `.endswith()`, `.count()`
Transform	`.replace()`, `.split()`, `.join()`

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Remember

Strings are immutable:

name = "alice"
name.upper()      # Returns "ALICE" but doesn't change name
print(name)       # Still "alice"

# To save the change:
name = name.upper()
print(name)       # Now "ALICE"

Methods return new strings:

original = "hello"
modified = original.upper()

print(original)   # hello (unchanged)
print(modified)   # HELLO (new string)

Empty strings are falsy:

text = "  ".strip()  # Results in ""

if text:
    print("Has content")
else:
    print("Empty!")  # This runs

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Summary

Key takeaways:

String methods transform or check strings
Methods don’t change the original string - they return a new one
Save the result if you want to keep changes: text = text.upper()
Chain methods for multiple operations: .strip().lower()
Use for validation - check user input before processing

Useful functions

.lower() - Compare text case-insensitively
.strip() - Remove unwanted spaces
.isdigit() - Check if input is a number
.split() - Break text into pieces
.replace() - Swap text

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