String handling: length, indexing, examining and manipulating

Length, indexing and slicing (spec 8.2.2)

len(s) gives the number of characters. s[i] is one character (from index 0); s[-1] is the last. s[a:b] is a slice from a up to, not including, b; s[::-1] reverses.

word = "Computer"
print(len(word), word[0], word[-1])
print(word[3:6], word[:4], word[4:])
print(word[::-1])

Output:

8 C r
put Comp uter
retupmoC

Strings are immutable: word[0] = "c" raises TypeError. Build a new string instead.

Examining strings

Methods that test a string return True or False; find() and index() locate a substring.

code = "AB12"
print(code.isalnum(), code.isalpha(), code.isdigit())
print("abc".islower(), "ABC".isupper(), "   ".isspace())
print("banana".find("an"), "banana".find("x"))
print("ss" in "mississippi")

Output:

True False False
True True True
1 -1
True

find() returns -1 if not found; index() raises ValueError instead.

Manipulating strings

These methods return a new string:

raw = "  hello WORLD  "
print(raw.strip())
print(raw.strip().lower(), raw.strip().upper())
print("ada lovelace".title(), "python".capitalize())
print("a,b,,c".split(","))
print("one two  three".split())
print("-" * 10)

Output:

hello WORLD
hello world HELLO WORLD
Ada Lovelace Python
['a', 'b', '', 'c']
['one', 'two', 'three']
----------

To build a cleaned string, loop over the characters and concatenate the ones you want.

Building strings in a loop

A common exam task: keep only letters, in lower case, then test for a palindrome.

text = "Never odd, or even!"
clean = ""
for ch in text.lower():
    if ch.isalpha():
        clean = clean + ch
print(clean)
print(clean == clean[::-1])

Output:

neveroddoreven
True

Worked example

Count the vowels in "Programming Language".

1. Loop over text.lower() and test each character with: if ch in "aeiou"
2. Vowels found: o, a, i, a, u, a, e
3. Count: 7

Answer: 7 vowels

Check yourself

1. What is displayed when this code runs? ```python s = "Algorithm" print(s[2:5]) ```
   Show answer

   gor. Indexes 2, 3 and 4 are g, o, r: gor.

2. What is displayed when this code runs? ```python s = "keyboard" print(s[-3:]) ```
   Show answer

   ard. The last three characters: ard.

3. What is displayed when this code runs? ```python s = " data " print(len(s), len(s.strip())) ```
   Show answer

   8 4. The original has 8 characters including 4 spaces; strip() removes them leaving 4: 8 4.

4. What is displayed when this code runs? ```python email = "sam.lee@school.org" at = email.find("@") print(email[at + 1:]) ```
   Show answer

   school.org. find() returns the index of @, so the slice after it is the domain: school.org.

5. What is displayed when this code runs? ```python line = "Kim,17,Maths" parts = line.split(",") print(parts[1] + parts[1]) ```
   • 1717
   • 34
   • 17
   • Kim17
   Show answer

   1717. split gives strings, so parts[1] is "17" and + joins: 1717.

6. What is displayed when this code runs? (The two results are separated by a space.) ```python print("hello world".title(), "hello world".capitalize()) ```
   • HELLO WORLD Hello world
   • Hello world Hello World
   • Hello World Hello world
   • hello World Hello world
   Show answer

   Hello World Hello world. title() capitalises each word; capitalize() only the first character: Hello World Hello world.

7. Which exception is raised when this code runs? ```python name = "jon" name[0] = "J" ```
   • TypeError
   • IndexError
   • ValueError
   • NameError
   Show answer

   TypeError. Strings are immutable, so item assignment raises TypeError. Use name = "J" + name[1:] instead.

8. This code should keep only the digits from a phone number. Which line completes it? ```python raw = "(0161) 496-0000" digits = "" for ch in raw: __________ digits = digits + ch print(digits) ```
   • `if ch.isalpha():`
   • `if ch.isdigit():`
   • `if ch.isalnum() == False:`
   • `if ch != " ":`
   Show answer

   `if ch.isdigit():`. isdigit() is True only for the characters 0 to 9.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification (XCP01/YCP01), first teaching 2026
• Pearson IAL Computer Science Unit 2 Programming Language Subset (PLS), Python 3
• Pearson IAL Computer Science IAS sample assessment materials (Unit 2 paper and mark scheme)

Last updated 8 November 2026. Spotted a mistake? Tell us.