Strings Are Sequences

word = "Python"
#        P  y  t  h  o  n
#        0  1  2  3  4  5
#       -6 -5 -4 -3 -2 -1

print(word[0])     # First character
print(word[-1])    # Last character
print(len(word))    # Length

The key difference: strings are immutable, so you cannot change individual characters. word[0] = "J" would cause an error.

Basic String Slicing

Use string[start:stop] to extract a substring. Just like lists, stop is exclusive.

text = "Hello, World!"

print(text[0:5])    # First 5 characters
print(text[7:12])   # "World"
print(text[:5])     # From start to index 5
print(text[7:])     # From index 7 to end

# Extract area code from phone number
phone = "(555) 123-4567"
area_code = phone[1:4]
print("Area code:", area_code)

# Extract file extension
filename = "document.pdf"
extension = filename[-3:]
print("Extension:", extension)

# Extract date parts
date = "2024-03-15"
year = date[:4]
month = date[5:7]
day = date[8:]
print(f"Year: {year}, Month: {month}, Day: {day}")

Negative Indexing in Slices

text = "Programming"

# Last 4 characters
print(text[-4:])

# Everything except first and last
print(text[1:-1])

# Everything except last 3
print(text[:-3])

The Step Parameter

The third value [start:stop:step] controls how many characters to skip.

text = "abcdefghij"

# Every other character
print(text[::2])

# Every third character
print(text[::3])

# Every other character from index 1 to 8
print(text[1:8:2])

Reversing Strings

Just like with lists, using a step of -1 reverses the string.

word = "Python"
print(word[::-1])

# Check if a word is a palindrome
def is_palindrome(text):
    text = text.lower()
    return text == text[::-1]

print(is_palindrome("racecar"))
print(is_palindrome("hello"))
print(is_palindrome("Madam"))

Combining Slicing with Methods

You can chain slicing with string methods for powerful text processing.

email = "  USER@EXAMPLE.COM  "

# Clean up and extract the domain
clean = email.strip().lower()
domain = clean[clean.index("@") + 1:]
print(f"Clean email: {clean}")
print(f"Domain: {domain}")

full_name = "john michael smith"

# Get first initial and last name
first_initial = full_name[0].upper()
last_name = full_name.split()[-1].title()
print(f"{first_initial}. {last_name}")