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Strings

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Strings are sequences of characters. Python treats strings as first-class objects with rich functionality.

Creating Strings

# Single or double quotes — both work
single = 'Hello'
double = "World"

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

# No char type — single chars are strings of length 1
letter = "A"
print(len(letter))  # 1

String Indexing and Slicing

text = "Python"

# Indexing — 0-based
text[0]    # 'P'
text[-1]   # 'n' — negative indexes from the end

# Slicing — [start:stop:step]
text[0:3]     # 'Pyt' — indexes 0, 1, 2
text[:3]      # 'Pyt' — start defaults to 0
text[3:]      # 'hon' — stop defaults to end
text[::2]     # 'Pto' — every 2nd character
text[::-1]    # 'nohtyP' — reverse

F-Strings (Python 3.6+)

The modern way to format strings:

name = "Alice"
age = 25

# Basic f-string
print(f"{name} is {age} years old")
# Alice is 25 years old

# Expressions inside
print(f"Next year, {name} will be {age + 1}")

# Formatting numbers
pi = 3.14159
print(f"Pi to 2 decimals: {pi:.2f}")      # 3.14
print(f"Percentage: {0.25:.1%}")           # 25.0%

# Padding and alignment
print(f"|{'left':<10}|")    # |left      |
print(f"|{'right':>10}|")   # |     right|
print(f"|{'center':^10}|")  # |  center  |

Tip

F-strings are preferred over .format() and % formatting for readability and performance.

Common String Methods

text = "  Hello, Python World!  "

# Case manipulation
text.upper()            # '  HELLO, PYTHON WORLD!  '
text.lower()            # '  hello, python world!  '
text.title()            # '  Hello, Python World!  '
text.swapcase()         # '  hELLO, pYTHON wORLD!  '

# Searching
text.find("Python")     # 9 — index of first occurrence
text.find("Java")       # -1 — not found
text.count("o")         # 3
"Python" in text        # True

# Validation
"hello".isalpha()      # True
"123".isdigit()        # True
"hello123".isalnum()   # True
"   ".isspace()        # True

# Manipulation
text.strip()           # 'Hello, Python World!' — removes leading/trailing whitespace
text.replace("Python", "Java")  # '  Hello, Java World!  '
"a,b,c".split(",")     # ['a', 'b', 'c']
"-".join(["a", "b", "c"])  # 'a-b-c'

Escape Sequences

# Common escapes
print("Line 1\nLine 2")     # newline
print("Tab\tseparated")     # tab
print("Quote: \"")          # literal quote
print("Backslash: \\")      # literal backslash

# Raw strings — ignore escapes (great for paths/regex)
path = r"C:\Users\name"
regex = r"\d+\.\d+"

String Immutability

# Strings are immutable — cannot change in place
text = "hello"
# text[0] = "H"  # TypeError!

# Must create a new string
text = "H" + text[1:]  # 'Hello'

Key Takeaways

Strings use f-strings for formatting — f"{variable}"
Slicing: string[start:stop:step] — negative indexes count from the end
Common methods: .strip(), .split(), .join(), .replace(), .find()
Strings are immutable — operations return new strings
Use raw strings (r"...") for paths and regex patterns