Python strings and f-strings

What is a string?

A string (str) is text inside quotes. Single, double and triple quotes all make strings.

city = "Ranchi"
grade = 'A'
address = """House 12,
Main Road"""            # triple quotes keep line breaks

len() gives the number of characters, spaces included: len("Hi there") is 8.

Indexing: every character has a position

Positions start at 0. Negative positions count from the end.

word = "Python"
print(word[0])     # P
print(word[-1])    # n  (last character)
print(word[10])    # IndexError: string index out of range

Slicing: cut out part of a string

text[start:stop:step] takes characters from start up to, but not including, stop.

word = "Python"
print(word[0:3])    # Pyt
print(word[2:])     # thon   (to the end)
print(word[:4])     # Pyth   (from the start)
print(word[::2])    # Pto    (every second character)
print(word[::-1])   # nohtyP (reversed)

Slicing never gives an IndexError; out-of-range limits are simply cut down to fit.

Strings cannot be changed

Strings are immutable: you cannot change one character in place.

name = "riya"
name[0] = "R"               # TypeError
name = "R" + name[1:]       # make a new string instead: "Riya"

Useful string methods

Every method returns a new string; the original stays the same.

A common pattern is to chain methods to clean what a user typed:

email = "  Asha.Roy@Gmail.COM ".strip().lower()
print(email)    # asha.roy@gmail.com

split() and join()

split() breaks a string into a list; join() glues a list back into a string.

line = "Asha,91,Ranchi"
parts = line.split(",")          # ['Asha', '91', 'Ranchi']
words = "learn python today".split()   # splits on spaces

print("-".join(["10", "05", "2026"]))  # 10-05-2026

f-strings: mix text and values

Put f before the quotes and write variables inside {}.

name = "Asha"
marks = 91.456
print(f"{name} scored {marks:.1f} marks")   # Asha scored 91.5 marks
print(f"Next year: {2026 + 1}")             # any expression works

Formatting inside f-strings

Alignment is how you print neat bills and tables:

print(f"{'Item':<10}{'Price':>8}")
print(f"{'Tea':<10}{10:>8}")
print(f"{'Samosa':<10}{15:>8}")

Escape characters and the in operator

in checks whether one string is inside another:

print("py" in "python")       # True
print("@" not in "abc.com")   # True

Example: mask a mobile number

mobile = "9876543210"
masked = "X" * 6 + mobile[-4:]
print(masked)        # XXXXXX3210

Common mistakes

• Starting to count at 1: the first character is text[0].
• Expecting text.upper() to change text. Store the result: text = text.upper().
• Forgetting the f in an f-string: "{name}" prints the braces as they are.
• Joining a string and a number with +: "Age " + 15 is a TypeError; use an f-string.

Key points

• Indexes start at 0; negative indexes count from the end.
• text[a:b] stops just before b; text[::-1] reverses.
• Strings are immutable, so methods return new strings.
• f-strings with format codes like :.2f and :<10 give clean output.

Practice questions

word = input("Word: ").lower()
print(word == word[::-1])

sentence = input("Sentence: ")
print(len(sentence.split()))

y, m, d = "2026-10-29".split("-")
print(f"{d}/{m}/{y}")