Lesson 3 — Working with Strings

Estimated time: 60 minutes

• Learning Objectives
• Concepts
  • What is a String?
  • Concatenation
  • f-Strings (Formatted Strings)
  • String Methods
  • String Indexing
  • String Slicing
  • The input() Function
• Guided Walkthrough
  • Step 1: Concatenation and f-Strings
  • Step 2: String Methods
  • Step 3: Indexing and Slicing
  • Step 4: Using input() — Colour Selector
  • Step 5: String Analysis
• Challenges
  • ⭐ Core
  • ⭐⭐ Extension
  • ⭐⭐⭐ Stretch
• Common Mistakes & Debugging
• Key Vocabulary

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Learning Objectives

By the end of this lesson you will be able to:

1. Join strings together using + and f-strings
2. Use string methods: .upper(), .lower(), .find(), len()
3. Access individual characters and slices of a string
4. Use input() in the Thonny console to get text from the user

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Concepts

What is a String?

A string is an ordered sequence of characters. Every character has a position (called an index) starting at 0.

"MicroPython"
 0123456789...

So "MicroPython"[0] is 'M', and "MicroPython"[5] is 'P'.

Concatenation

Concatenation means joining strings together. Use +:

first = "Hello"
second = "World"
combined = first + " " + second   # "Hello World"

Think of it like linking train carriages together.

f-Strings (Formatted Strings)

f-strings are a cleaner way to embed variable values inside strings. Put f before the opening quote, then use {} to insert variables:

name = "Alex"
age = 15
print(f"My name is {name} and I am {age} years old.")
# Output: My name is Alex and I am 15 years old.

f-strings are almost always easier to read than concatenation. Use them!

String Methods

A method is a function that belongs to an object, called with a dot. Strings come with several built-in tools:

text = "Hello MicroPython"

text.upper()    # "HELLO MICROPYTHON" — all uppercase
text.lower()    # "hello micropython" — all lowercase
len(text)       # 17 — number of characters
text.find("Micro")   # 6 — position where "Micro" starts
text.find("xyz")     # -1 — not found (always -1 for "not found")

len() is not a method — it’s a standalone function. You call it with the string as an argument: len(text), not text.len().

String Indexing

Access individual characters using square brackets and the index:

word = "Python"
#       012345

print(word[0])   # 'P'  (first character — index 0)
print(word[2])   # 't'
print(word[-1])  # 'n'  (last character — negative index counts from the end)
print(word[-2])  # 'o'

String Slicing

Extract a slice (portion) of a string using [start:end]. The result includes start but not end:

word = "MicroPython"
#       01234567890

print(word[0:5])   # "Micro"  (indices 0,1,2,3,4)
print(word[5:])    # "Python" (from index 5 to the end)
print(word[:5])    # "Micro"  (from start up to but not including 5)
print(word[::-1])  # "nohtyPorciM" (reversed — step of -1)

Slicing never causes an error even if the indices are out of range — it just gives you as much as it can.

The input() Function

input() asks the user to type something and returns their text as a string:

name = input("What is your name? ")
print(f"Hello, {name}!")

Important — how input() works in this course: When your ESP32-S3 is connected to Thonny, input() works fine — the text you type in the Thonny Shell is sent to the ESP32. However, if the ESP32 is running standalone (not connected to Thonny), there’s no keyboard to type on! That’s why later lessons use physical buttons as the hardware equivalent of input. For now, always use input() with Thonny connected.

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Guided Walkthrough

Step 1: Concatenation and f-Strings

Type this in the Script Editor:

first_name = "Alex"
last_name = "Smith"

# Method 1: concatenation with +
full_name = first_name + " " + last_name
print("Hello, " + full_name + "!")

# Method 2: f-string (cleaner!)
print(f"Hello, {full_name}!")
print(f"First: {first_name} | Last: {last_name}")

# f-strings can contain expressions, not just variables
age = 15
print(f"In 5 years you will be {age + 5} years old.")

Step 2: String Methods

message = "Hello MicroPython World"

print(len(message))             # Number of characters: 23
print(message.upper())          # HELLO MICROPYTHON WORLD
print(message.lower())          # hello micropython world

position = message.find("Python")
print(f"'Python' starts at index: {position}")  # 11

not_found = message.find("xyz")
print(f"'xyz' found at: {not_found}")           # -1

Step 3: Indexing and Slicing

word = "MicroPython"

print(f"First character: {word[0]}")      # M
print(f"Sixth character: {word[5]}")      # P
print(f"Last character: {word[-1]}")      # n
print(f"First 5: {word[0:5]}")            # Micro
print(f"From index 5: {word[5:]}")        # Python
print(f"Last 6: {word[-6:]}")             # Python
print(f"Reversed: {word[::-1]}")          # nohtyPorciM

Step 4: Using input() — Colour Selector

This program asks the user to type a colour name, then sets the NeoPixel to that colour:

import machine, neopixel, time

pin = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin, 1)

# Get colour choice from user (via Thonny console)
colour_name = input("Enter a colour (red/green/blue/yellow/purple): ")
colour_name = colour_name.lower()   # Convert to lowercase so "RED" and "red" both work

if colour_name == "red":
    np[0] = (255, 0, 0)
elif colour_name == "green":
    np[0] = (0, 255, 0)
elif colour_name == "blue":
    np[0] = (0, 0, 255)
elif colour_name == "yellow":
    np[0] = (255, 200, 0)
elif colour_name == "purple":
    np[0] = (128, 0, 255)
else:
    np[0] = (255, 255, 255)   # White for unknown
    colour_name = "unknown (showing white)"

np.write()
print(f"LED set to: {colour_name}")
time.sleep(3)
np[0] = (0, 0, 0)
np.write()

Run this and type a colour name in the Thonny Shell when prompted. Notice we use .lower() so the comparison works regardless of capitalisation.

You’re previewing if/elif/else here — you’ll learn exactly how this works in Unit 2. For now, just notice that it checks which colour was typed and sets the LED accordingly.

Step 5: String Analysis

sentence = input("Type a sentence: ")

print(f"Your sentence: '{sentence}'")
print(f"Length: {len(sentence)} characters")
print(f"Uppercase version: {sentence.upper()}")
print(f"First character: '{sentence[0]}'")
print(f"Last character: '{sentence[-1]}'")

# Find a word
search = input("Search for a word: ")
pos = sentence.lower().find(search.lower())
if pos == -1:
    print(f"'{search}' was not found.")
else:
    print(f"'{search}' found at position {pos}.")

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Challenges

⭐ Core

Write a program that asks for the user’s name. Print:

• Their name in uppercase
• Their name in lowercase
• How many characters their name has
• The first and last characters of their name

⭐⭐ Extension

Ask the user to enter a sentence. Then print:

• The total length of the sentence
• The first word (hint: use .find(" ") to find the first space, then slice)
• Whether the word "python" appears in the sentence (case-insensitive)
• The sentence reversed

⭐⭐⭐ Stretch

Create a “word analyser” program. Ask the user for a word. Print:

• The word reversed
• The number of vowels (a, e, i, o, u) — use a loop to count them
• The word in “alternating case” — first character upper, second lower, third upper, etc.
• Whether the word is a palindrome (reads the same forwards and backwards)

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Common Mistakes & Debugging

TypeError: can only concatenate str (not "int") to str You tried to join a string and a number with +. Use an f-string instead: f"Score: {score}", or convert the number: "Score: " + str(score).

IndexError: string index out of range You tried to access a character at an index beyond the string’s length. Check with len() first.

.find() returning -1 unexpectedly Check capitalisation! "hello".find("Hello") returns -1. Use .lower() on both strings before comparing.

input() prompt not appearing Make sure you’re running on the ESP32 through Thonny, not “Local Python 3”. Check the bottom-right corner of Thonny.

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Key Vocabulary

Term	Definition
string	A sequence of characters enclosed in quotes
concatenation	Joining two or more strings together using +
f-string	A string prefixed with f that allows embedding variable values using {}
index	The position of a character in a string — starts at 0
negative index	Counting from the end: -1 is the last character, -2 is second-to-last
slicing	Extracting a portion of a string using [start:end:step]
len()	A built-in function returning the number of characters in a string
method	A function attached to an object, called with a dot (e.g., .upper())
input()	A function that prompts the user to type something and returns their answer as a string