What is a String?

A string is a sequence of characters enclosed in quotes. Strings are immutable in Python, meaning you cannot change them after creation, but you can create new strings from existing ones.

Creating Strings

Create strings using single, double, or triple quotes:

# Single quotes
name = 'Alice'

# Double quotes
message = "Hello, World!"

# Triple quotes for multiline
text = """This is
a multiline
string"""

# Escape characters
quote = "She said, \"Hello\""
newline = "Line 1\nLine 2"
tab = "Column1\tColumn2"

String Operations

Concatenation

first = "Hello"
last = "World"
greeting = first + " " + last        # "Hello World"

# Using join() for multiple strings
words = ["Hello", "World", "Python"]
sentence = " ".join(words)           # "Hello World Python"

Repetition

word = "Hi"
repeated = word * 3                  # "HiHiHi"

Length

text = "Python"
length = len(text)                   # 6

String Indexing and Slicing

Access individual characters or substrings:

text = "Python"

# Indexing (0-based)
first = text[0]                      # "P"
last = text[-1]                      # "n"

# Slicing
sub = text[0:3]                      # "Pyt" (from 0 to 2)
sub = text[:3]                       # "Pyt" (from start)
sub = text[3:]                       # "hon" (from 3 to end)
sub = text[::2]                      # "Pto" (every 2nd character)
sub = text[::-1]                     # "nohtyP" (reversed)

String Methods

Case Conversion

text = "Hello World"

upper_text = text.upper()            # "HELLO WORLD"
lower_text = text.lower()            # "hello world"
title_text = text.title()            # "Hello World"
swap_text = text.swapcase()          # "hELLO wORLD"

Searching and Checking

text = "Hello World"

# Check if starts/ends with
text.startswith("Hello")            # True
text.endswith("World")              # True

# Find substring
index = text.find("World")          # 6 (returns -1 if not found)
index = text.index("World")         # 6 (raises error if not found)

# Count occurrences
count = text.count("l")             # 3

# Check content
text.isalpha()                      # False (has space)
text.isdigit()                      # False
text.isalnum()                      # False

Modifying Strings

text = "  Hello World  "

# Strip whitespace
cleaned = text.strip()               # "Hello World"
left = text.lstrip()                 # "Hello World  "
right = text.rstrip()                # "  Hello World"

# Replace
new_text = text.replace("World", "Python")  # "  Hello Python  "

# Split
words = "apple,banana,orange".split(",")    # ["apple", "banana", "orange"]

String Formatting

f-strings (Python 3.6+)

name = "Alice"
age = 25

# f-string (recommended)
message = f"My name is {name} and I'm {age} years old"

# Expressions
result = f"2 + 3 = {2 + 3}"         # "2 + 3 = 5"

# Formatting numbers
price = 19.99
formatted = f"Price: ${price:.2f}"  # "Price: $19.99"

format() Method

name = "Alice"
age = 25

message = "My name is {} and I'm {} years old".format(name, age)
message = "My name is {0} and I'm {1} years old".format(name, age)
message = "My name is {name} and I'm {age} years old".format(name=name, age=age)

% Formatting (Old Style)

name = "Alice"
age = 25

message = "My name is %s and I'm %d years old" % (name, age)

Common String Operations

Check if substring exists

text = "Hello World"
if "World" in text:
    print("Found!")

Split and Join

# Split string into list
sentence = "apple,banana,orange"
fruits = sentence.split(",")        # ["apple", "banana", "orange"]

# Join list into string
fruits = ["apple", "banana", "orange"]
sentence = ",".join(fruits)          # "apple,banana,orange"

Remove whitespace

text = "  Hello World  "
cleaned = text.strip()               # "Hello World"

String Immutability

Strings cannot be changed in place:

text = "Hello"
# text[0] = "h"  # Error! Strings are immutable

# Create new string instead
new_text = "h" + text[1:]           # "hello"

Raw Strings

Use raw strings to ignore escape characters:

# Regular string
path = "C:\\Users\\Name"             # Need to escape backslashes

# Raw string
path = r"C:\Users\Name"              # No escaping needed

Best Practices

  • Use f-strings: Most readable and efficient (Python 3.6+)
  • Use join() for concatenation: More efficient than + for multiple strings
  • Be careful with string methods: They return new strings, don't modify in place
  • Use strip() for user input: Remove unwanted whitespace

Next Steps

Learn about file handling to read and write strings to files, or explore regular expressions for advanced pattern matching.