Strings

Beginner ~30 min read

Strings are one of Python's most used data types. They represent text and are immutable sequences of characters. This lesson covers creating strings, accessing characters, slicing, and formatting.

Creating Strings

Python strings can be created using single quotes, double quotes, or triple quotes for multiline strings. There's no difference between single and double quotes - use whichever is more convenient.

Output

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String Immutability: Once created, strings cannot be changed. Operations like concatenation or slicing create new strings rather than modifying the original.

String Indexing

Every character in a string has a position called an index. Python uses zero-based indexing - the first character is at index 0. You can also use negative indices to count from the end.

# String Indexing in Python
# Each character has a position (index) starting from 0

text = "Python"
print(f"String: '{text}'")
print(f"Length: {len(text)}")

print("\n" + "=" * 40)
print("Positive Indexing (left to right, starts at 0):")
print(f"Index 0: '{text[0]}'  (first character)")
print(f"Index 1: '{text[1]}'")
print(f"Index 2: '{text[2]}'")
print(f"Index 3: '{text[3]}'")
print(f"Index 4: '{text[4]}'")
print(f"Index 5: '{text[5]}'  (last character)")

print("\n" + "=" * 40)
print("Negative Indexing (right to left, starts at -1):")
print(f"Index -1: '{text[-1]}'  (last character)")
print(f"Index -2: '{text[-2]}'")
print(f"Index -3: '{text[-3]}'")
print(f"Index -4: '{text[-4]}'")
print(f"Index -5: '{text[-5]}'")
print(f"Index -6: '{text[-6]}'  (first character)")

print("\n" + "=" * 40)
print("Index Visualization:")
print("  P   y   t   h   o   n")
print("  0   1   2   3   4   5   (positive)")
print(" -6  -5  -4  -3  -2  -1   (negative)")

# Practical example
email = "user@example.com"
print(f"\nEmail: '{email}'")
print(f"First character: '{email[0]}'")
print(f"Last character: '{email[-1]}'")

Output

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Negative Indexing Tip: Use -1 to get the last character without knowing the string length. This is one of Python's most beloved features!

String Slicing

Slicing extracts a portion of a string using the syntax string[start:end:step]. The start index is included, but end is excluded.

# String Slicing in Python
# Syntax: string[start:end:step]
# - start: beginning index (inclusive)
# - end: ending index (exclusive)
# - step: increment (optional, default=1)

text = "Python Programming"
print(f"Original: '{text}'")
print(f"Length: {len(text)}")

print("\n" + "=" * 40)
print("Basic Slicing [start:end]:")
print(f"text[0:6]  = '{text[0:6]}'")   # 'Python'
print(f"text[7:18] = '{text[7:18]}'")  # 'Programming'
print(f"text[7:11] = '{text[7:11]}'")  # 'Prog'

print("\n" + "=" * 40)
print("Omitting start or end:")
print(f"text[:6]   = '{text[:6]}'")    # First 6 chars
print(f"text[7:]   = '{text[7:]}'")    # From index 7 to end
print(f"text[:]    = '{text[:]}'")     # Copy entire string

print("\n" + "=" * 40)
print("Negative indices in slicing:")
print(f"text[-11:]  = '{text[-11:]}'")  # Last 11 chars
print(f"text[:-12] = '{text[:-12]}'")   # All except last 12
print(f"text[-11:-4] = '{text[-11:-4]}'")

print("\n" + "=" * 40)
print("Using step [start:end:step]:")
print(f"text[::2]   = '{text[::2]}'")     # Every 2nd char
print(f"text[::3]   = '{text[::3]}'")     # Every 3rd char
print(f"text[0:6:2] = '{text[0:6:2]}'")   # 'Pto'

print("\n" + "=" * 40)
print("Reverse a string (step = -1):")
print(f"text[::-1] = '{text[::-1]}'")

# Practical examples
print("\n" + "=" * 40)
print("Practical Examples:")
url = "https://example.com/page"
print(f"URL: {url}")
print(f"Protocol: {url[:5]}")        # 'https'
print(f"Domain: {url[8:19]}")        # 'example.com'

filename = "document.pdf"
print(f"\nFilename: {filename}")
print(f"Name: {filename[:-4]}")      # 'document'
print(f"Extension: {filename[-3:]}") # 'pdf'

Output

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Remember: The slice [start:end] includes start but excludes end. Think of indices as pointing between characters.

Escape Characters

Escape characters let you include special characters in strings. The backslash \ is the escape character.

Escape	Description	Example
`\n`	Newline	`"Line1\nLine2"`
`\t`	Tab	`"Col1\tCol2"`
`\\`	Backslash	`"C:\\path"`
`\'`	Single quote	`'It\'s'`
`\"`	Double quote	`"Say \"Hi\""`
`\r`	Carriage return	`"Before\rAfter"`

# Escape Characters in Python
# Backslash (\) is used to include special characters

print("Common Escape Characters:")
print("=" * 40)

# Newline
print("Line 1\nLine 2\nLine 3")

print("\n" + "=" * 40)
# Tab
print("Tab Example:")
print("Name\tAge\tCity")
print("Alice\t25\tNew York")
print("Bob\t30\tLondon")

print("\n" + "=" * 40)
# Backslash itself
print("Backslash: C:\\Users\\Documents\\file.txt")

print("\n" + "=" * 40)
# Quotes inside strings
print("Single quote: It\'s working!")
print("Double quote: She said \"Hello!\"")

print("\n" + "=" * 40)
# Carriage return (overwrites from beginning)
print("Before\rAfter")  # Shows: After

print("\n" + "=" * 40)
print("Raw Strings (prefix with r):")
# Raw strings - backslash is literal
path = r"C:\Users\Documents\file.txt"
print(f"Raw string: {path}")

# Useful for regex patterns
pattern = r"\d{3}-\d{4}"
print(f"Regex pattern: {pattern}")

print("\n" + "=" * 40)
print("Escape Character Reference:")
print(r"\n  - Newline")
print(r"\t  - Tab")
print(r"\\  - Backslash")
print(r"\'  - Single quote")
print(r'\"  - Double quote')
print(r"\r  - Carriage return")

Output

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Raw Strings: Prefix a string with r to create a raw string where backslashes are treated literally. Perfect for file paths and regex patterns!

String Operations

Python provides operators for string concatenation, repetition, and membership testing.

# String Operations in Python
# Concatenation, repetition, and membership

print("=" * 40)
print("1. Concatenation (joining strings)")
print("=" * 40)

first_name = "John"
last_name = "Doe"

# Using + operator
full_name = first_name + " " + last_name
print(f"Full name: {full_name}")

# Concatenating multiple strings
greeting = "Hello" + ", " + "World" + "!"
print(f"Greeting: {greeting}")

# Note: Can only concatenate strings
# age = 25
# print("Age: " + age)  # ERROR! Must convert: str(age)
age = 25
print("Age: " + str(age))

print("\n" + "=" * 40)
print("2. Repetition (* operator)")
print("=" * 40)

# Repeat a string
line = "-" * 20
print(f"Line: {line}")

echo = "Hello! " * 3
print(f"Echo: {echo}")

# Creating patterns
pattern = "=-" * 10 + "="
print(f"Pattern: {pattern}")

print("\n" + "=" * 40)
print("3. Membership (in / not in)")
print("=" * 40)

sentence = "Python is a powerful programming language"

# Check if substring exists
print(f"'Python' in sentence: {'Python' in sentence}")
print(f"'Java' in sentence: {'Java' in sentence}")
print(f"'powerful' in sentence: {'powerful' in sentence}")
print(f"'PYTHON' in sentence: {'PYTHON' in sentence}")  # Case sensitive!

# not in
print(f"'Ruby' not in sentence: {'Ruby' not in sentence}")

print("\n" + "=" * 40)
print("4. Practical Example")
print("=" * 40)

# Email validation (basic)
email = "user@example.com"
if "@" in email and "." in email:
    print(f"'{email}' looks like a valid email")
else:
    print(f"'{email}' is not a valid email")

# Building a box
width = 30
print("+" + "-" * (width - 2) + "+")
print("|" + " Welcome! ".center(width - 2) + "|")
print("+" + "-" * (width - 2) + "+")

Output

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String Formatting

Python offers several ways to format strings. f-strings (Python 3.6+) are the most modern and recommended approach.

# String Formatting in Python
# Multiple ways to format strings

name = "Alice"
age = 25
price = 49.99

print("=" * 50)
print("1. f-strings (Python 3.6+) - RECOMMENDED")
print("=" * 50)
print(f"Name: {name}, Age: {age}")
print(f"Price: ${price}")
print(f"Math: 2 + 3 = {2 + 3}")
print(f"Function: {name.upper()}")

print("\n" + "=" * 50)
print("2. f-string Formatting Options")
print("=" * 50)
# Width and alignment
print(f"Right align: '{name:>10}'")
print(f"Left align:  '{name:<10}'")
print(f"Center:      '{name:^10}'")

# Number formatting
pi = 3.14159265359
print(f"2 decimals: {pi:.2f}")
print(f"5 decimals: {pi:.5f}")

# Thousands separator
big_num = 1234567
print(f"With commas: {big_num:,}")

# Percentage
ratio = 0.856
print(f"Percentage: {ratio:.1%}")

print("\n" + "=" * 50)
print("3. .format() Method")
print("=" * 50)
print("Name: {}, Age: {}".format(name, age))
print("Name: {0}, Age: {1}".format(name, age))
print("Name: {n}, Age: {a}".format(n=name, a=age))

print("\n" + "=" * 50)
print("4. % Operator (Old Style)")
print("=" * 50)
print("Name: %s, Age: %d" % (name, age))
print("Price: %.2f" % price)

print("\n" + "=" * 50)
print("5. String Concatenation")
print("=" * 50)
print("Hello, " + name + "! You are " + str(age) + " years old.")

print("\n" + "=" * 50)
print("Summary: Use f-strings for most cases!")

Output

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f-string Requirement: f-strings require Python 3.6 or later. For older Python versions, use .format() or the % operator.

Common Mistakes

1. Index out of range

text = "Hello"
print(text[5])  # IndexError! Valid indices are 0-4
print(text[4])  # Correct - gets 'o'

2. Trying to modify strings

text = "Hello"
text[0] = "J"  # TypeError! Strings are immutable

# Do this instead:
text = "J" + text[1:]  # Creates new string "Jello"

3. Concatenating strings with numbers

age = 25
# print("Age: " + age)  # TypeError!

# Solutions:
print("Age: " + str(age))   # Convert to string
print(f"Age: {age}")        # Use f-string (recommended)

4. Forgetting case sensitivity

text = "Python"
print("python" in text)  # False! Case matters
print("python" in text.lower())  # True

Exercise: String Manipulation

Task: Practice string indexing, slicing, and operations with the given string.

Requirements:

Extract specific characters using indexing
Extract words using slicing
Reverse the string
Check for substring membership
Create a pattern using repetition

Output

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Show Solution

text = "Learning Python is Fun!"

# 1. First character
print(text[0])              # 'L'

# 2. Last character
print(text[-1])             # '!'

# 3. Extract "Python"
print(text[9:15])           # 'Python'

# 4. Reverse the string
print(text[::-1])           # '!nuF si nohtyP gninraeL'

# 5. Check if "Python" is in text
print("Python" in text)     # True

# 6. Length of string
print(len(text))            # 23

# 7. Every other character
print(text[::2])            # 'Lann yhn sFn'

# 8. Create pattern
print("=-" * 6 + "=")       # '=-=-=-=-=-=-='

Summary

Creation: Use single '...', double "...", or triple """...""" quotes
Indexing: Access characters with string[index], starting at 0
Negative indices: -1 is last character, -2 is second-to-last
Slicing: string[start:end:step] extracts substrings
Escape characters: \n (newline), \t (tab), \\ (backslash)
Raw strings: Prefix with r to disable escape processing
f-strings: Use f"..." for modern, readable formatting
Immutable: Strings cannot be changed - operations create new strings

What's Next?

In the next lesson, we'll explore string methods - powerful built-in functions for searching, transforming, and manipulating strings.

Previous Numbers

Next String Methods

Creating Strings

String Indexing

String Slicing

Escape Characters

String Operations

String Formatting

Common Mistakes

1. Index out of range

2. Trying to modify strings

3. Concatenating strings with numbers

4. Forgetting case sensitivity

Exercise: String Manipulation

Summary

What's Next?

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