Python: working with strings

Names, messages and passwords are all strings: text in a program. Python has lots of tools for measuring, cutting up and joining strings.

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Length and indexing

word = "computer"
print(len(word))
print(word[0])
print(word[-1])
Output:
8
c
r
len counts the characters. Each character has an index, starting at 0.
A negative index counts from the end: word[-1] is the last character.

Slicing and changing case

word = "computer"
print(word[0:3])
print(word.upper())
Output:
com
COMPUTER
word[0:3] is a slice: it starts at index 0 and stops before index 3.
.upper() makes capitals and .lower() makes small letters. The original string is not changed.

Joining strings

Concatenation joins strings with +:
first = "Ada"
last = "Lovelace"
print(first + " " + last)
prints Ada Lovelace.
You cannot join a string and a number directly. Convert the number with str() first:
age = 13
print("I am " + str(age))
prints I am 13.
A for loop can visit each character in turn: for letter in name:
Worked example

What does this print?
code = "PYTHON"
print(code[2:5])
print(len(code))

  1. Indexes: P=0, Y=1, T=2, H=3, O=4, N=5.
  2. code[2:5] takes indexes 2, 3 and 4: THO.
  3. len counts 6 characters.

Answer: THO then 6

Key idea

len() counts characters, indexes start at 0, a slice [a:b] stops before b, + joins strings and str() turns a number into a string so it can be joined.

Key words
AND
A Boolean operator that gives True (1) only when both inputs are True.
Program
A set of instructions written in a programming language that a computer can run.
String
A data type for text, written in quotation marks, such as "hello".
Output
Information sent out from a computer, for example to a screen, speaker or printer.
NOT
A Boolean operator that flips its input: True becomes False and False becomes True.
For loop
A count-controlled loop that repeats a set number of times.

All KS3 Computing key terms and definitions →

Common mistakes
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Extra practice

2 more questions

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  1. What does this print?
    print(len("Good morning"))

    Show the answer

    Answer: 12

    "Good morning" has 11 letters plus 1 space: 12.

  2. What does this print?
    animal = "tiger"
    print(animal[1])

    Show the answer

    Answer: i

    Index 0 is t, index 1 is i.

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Sources and references

Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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