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Strings (Deep Dive)

A string is a sequence of characters. In Python, strings are created by enclosing characters in single quotes ' or double quotes ".

1. Indexing and Slicing

Strings are ordered sequences, which means you can access individual characters using index numbers (starting from 0), or select a range of characters using slicing.

  • Positive Indices: Count from the beginning (start at 0).
  • Negative Indices: Count from the end (start at -1).
  • Slicing Syntax: string[start:end:step]
text = "Hello, World!"

# Indexing
print(text[0])   # Output: H
print(text[-1])  # Output: !

# Slicing
print(text[0:5])    # Output: Hello (excludes index 5)
print(text[7:])     # Output: World! (from index 7 to the end)
print(text[::2])    # Output: Hlo ol! (every second character)
print(text[::-1])   # Output: !dlroW ,olleH (reverses the string)

2. String Immutability

Python strings are immutable, meaning that once a string is created, its characters cannot be changed or replaced.

text = "hello"

# The line below will raise a TypeError:
# text[0] = "H"

# To modify a string, you must create a new one:
text = "H" + text[1:]
print(text)  # Output: Hello

3. Escape Characters and Raw Strings

Escape Characters

Escape characters allow you to print special characters that are otherwise difficult or impossible to type in a string:

  • \n: Newline
  • \t: Tab
  • \\: Backslash
  • \": Double Quote
print("Hello\nWorld")  # Prints Hello on one line and World on the next

Raw Strings

If you prefix a string with r or R, Python will ignore all escape characters. This is extremely useful for file paths and regular expressions.

# Normal string (interprets \n as newline)
print("C:\new_folder") 

# Raw string (prints exactly what is typed)
print(r"C:\new_folder")  # Output: C:\new_folder

4. Advanced f-Strings

f-strings do not just embed variable values; they can also evaluate expressions and format numbers.

# Math expressions
print(f"5 + 5 is {5 + 5}")  # Output: 5 + 5 is 10

# Float precision formatting
pi = 3.14159265
print(f"Pi to 2 decimal places: {pi:.2f}")  # Output: Pi to 2 decimal places: 3.14

# Text alignment
print(f"{'right-aligned':>20}")  # Output:        right-aligned

# Nested quotes
user = {"name": "Alice"}
print(f"User's name is {user['name']}")

5. String Methods Reference

Here is a reference list of the most common string methods:

Method Description Example
upper() Converts string to uppercase. "hello".upper() $\rightarrow$ "HELLO"
lower() Converts string to lowercase. "HELLO".lower() $\rightarrow$ "hello"
title() Capitalizes the first letter of each word. "hello world".title() $\rightarrow$ "Hello World"
strip() Removes leading and trailing whitespace. " hello ".strip() $\rightarrow$ "hello"
lstrip() / rstrip() Removes left or right whitespace. " hello ".lstrip() $\rightarrow$ "hello "
split(sep) Splits string into a list using separator. "a,b".split(",") $\rightarrow$ ['a', 'b']
join(iterable) Joins an iterable into a string. "-".join(["a", "b"]) $\rightarrow$ "a-b"
replace(old, new) Replaces occurrences of a substring. "abc".replace("b", "x") $\rightarrow$ "axc"
find(sub) Returns the index of first occurrence. "hello".find("l") $\rightarrow$ 2 (returns -1 if not found)
count(sub) Counts occurrences of a substring. "hello".count("l") $\rightarrow$ 2
startswith(prefix) Checks if string starts with prefix. "hello".startswith("he") $\rightarrow$ True
endswith(suffix) Checks if string ends with suffix. "hello".endswith("lo") $\rightarrow$ True
isdigit() Returns True if all characters are digits. "123".isdigit() $\rightarrow$ True
isalpha() Returns True if all characters are alphabetic. "abc".isalpha() $\rightarrow$ True

Further Reading