Strings in Python — Indexing, Slicing and String Methods

A string is text: a sequence of characters inside quotes. Strings are immutable — you can't change a character in place; methods return a new string.

1. Indexing and slicing

s = "PYTHON"
print(s[0], s[-1])     # first and last
print(s[1:4])          # index 1, 2, 3
print(s[:2], s[2:])    # start to 2, 2 to end
print(s[::2])          # every second character
print(s[::-1])         # reversed
print(len(s))

P N
YTH
PY THON
PTO
NOHTYP
6

Slice rule: s[start:stop:step] — start included, stop excluded. Missing start = beginning; missing stop = end.

2. Useful string methods

text = "  Learn Python at Skills101  "
clean = text.strip()
print(clean)
print(clean.lower())
print(clean.upper())
print(clean.replace("Python", "Java"))
print(clean.split())
print("-".join(["2026", "11", "08"]))
print(clean.startswith("Learn"), clean.endswith("101"))
print(clean.find("Python"), clean.count("n"))

Learn Python at Skills101
learn python at skills101
LEARN PYTHON AT SKILLS101
Learn Java at Skills101
['Learn', 'Python', 'at', 'Skills101']
2026-11-08
True True
6 2

3. Checking and looping

pin = "302029"
print(pin.isdigit(), len(pin) == 6)

word = "programming"
vowels = 0
for ch in word:
    if ch in "aeiou":
        vowels += 1
print(vowels)

True True
3

4. Strings are immutable

name = "ravi"
# name[0] = "R"   ← TypeError: strings can't be changed in place
name = "R" + name[1:]   # build a new string instead
print(name)

Ravi

5. Interview programs

Palindrome check

def is_palindrome(s):
    s = s.lower().replace(" ", "")
    return s == s[::-1]

print(is_palindrome("Madam"), is_palindrome("Never odd or even"), is_palindrome("Skills"))

True True False

Count each character

from collections import Counter
print(Counter("banana").most_common())

[('a', 3), ('n', 2), ('b', 1)]

Reverse each word

sentence = "learn to code"
print(" ".join(word[::-1] for word in sentence.split()))

nrael ot edoc

Common mistakes

• Expecting s.upper() to change s — it returns a new string; write s = s.upper().
• Off-by-one in slices (stop is excluded).
• Using + in a loop to build big strings — collect parts in a list and join them.

Quick recap

• Index from 0; negative indexes count from the end.
• s[start:stop:step]; s[::-1] reverses.
• Methods return new strings: strip, lower, upper, replace, split, join, find, count.
• Strings are immutable.