Control Flow - If, Loops, and No Braces!

Learn how if/else statements, for loops, and while loops work in MicroPython without curly braces - indentation is everything

Control Flow: The Python Way

Control flow - if statements, loops, and logic - works similarly in Python and C++, but with one huge difference: no curly braces. Instead, Python uses indentation and colons.

This takes some getting used to, but most programmers find it cleaner once they adjust. Let’s explore each control structure side-by-side.

If Statements

The basic if statement structure changes from braces to colons and indentation:

Arduino C++:

int temperature = 25;

if (temperature > 30) {
    digitalWrite(LED_PIN, HIGH);
    Serial.println("Hot!");
}

MicroPython:

temperature = 25

if temperature > 30:
    led.on()
    print("Hot!")

Key differences:

• Colon : starts the block (instead of opening brace)
• Indentation defines the block (instead of closing brace)
• No parentheses required (but allowed)

If-Else Statements

Arduino C++:

if (temperature > 30) {
    digitalWrite(LED_PIN, HIGH);
} else {
    digitalWrite(LED_PIN, LOW);
}

MicroPython:

if temperature > 30:
    led.on()
else:
    led.off()

Notice the else: also needs a colon, and the code below it is indented.

If-Else If-Else (Elif)

C++ uses else if, Python uses elif:

Arduino C++:

if (temperature > 30) {
    Serial.println("Hot");
} else if (temperature > 20) {
    Serial.println("Warm");
} else if (temperature > 10) {
    Serial.println("Cool");
} else {
    Serial.println("Cold");
}

MicroPython:

if temperature > 30:
    print("Hot")
elif temperature > 20:
    print("Warm")
elif temperature > 10:
    print("Cool")
else:
    print("Cold")

elif is shorter and cleaner than else if. Remember the colon after each condition!

Comparison Operators

These are identical in both languages:

Logical Operators

Here’s where Python differs - it uses words instead of symbols:

Arduino C++:

if (temperature > 20 && humidity < 80) {
    Serial.println("Comfortable");
}

if (temperature > 35 || humidity > 90) {
    Serial.println("Uncomfortable");
}

if (!doorOpen) {
    Serial.println("Door is closed");
}

MicroPython:

if temperature > 20 and humidity < 80:
    print("Comfortable")

if temperature > 35 or humidity > 90:
    print("Uncomfortable")

if not door_open:
    print("Door is closed")

Python’s word-based operators are more readable once you get used to them.

While Loops

While loops are nearly identical, just swap braces for colons:

Arduino C++:

int count = 0;

while (count < 10) {
    Serial.println(count);
    count++;
}

MicroPython:

count = 0

while count < 10:
    print(count)
    count += 1  # Python has +=, but no ++ operator

Important: Python doesn’t have ++ or -- operators. Use += 1 or -= 1 instead.

Infinite Loops

In Arduino, your main code runs in loop() which repeats forever. In MicroPython, you create an infinite loop explicitly:

Arduino C++:

void loop() {
    // This repeats forever automatically
    digitalWrite(LED_PIN, HIGH);
    delay(1000);
    digitalWrite(LED_PIN, LOW);
    delay(1000);
}

MicroPython:

while True:  # Explicit infinite loop
    led.on()
    time.sleep(1)
    led.off()
    time.sleep(1)

while True: is the Python equivalent of Arduino’s loop(). You’ll use this pattern in almost every MicroPython program.

CircuitPython File Naming: boot.py and code.py

If you’re using CircuitPython instead of MicroPython, there’s an important difference in how you structure your main program:

MicroPython:

• boot.py - Runs first on startup (optional, for configuration)
• main.py - Your main program code (runs automatically after boot.py)

CircuitPython:

• boot.py - Runs first on startup (same as MicroPython)
• code.py - Your main program code (instead of main.py)

The key difference: CircuitPython uses code.py as the default filename instead of main.py.

Auto-reload magic:

• When you save code.py on CircuitPython, your board automatically restarts and runs the new code
• This happens because CircuitPython shows up as a USB drive - when you save the file, it detects the change
• MicroPython requires you to manually restart or use Thonny’s “Run” button

Example - Same blink code, different workflow:

MicroPython (main.py):

from machine import Pin
import time

led = Pin(25, Pin.OUT)

while True:
    led.on()
    time.sleep(1)
    led.off()
    time.sleep(1)

CircuitPython (code.py):

import board
import digitalio
import time

led = digitalio.DigitalInOut(board.LED)
led.direction = digitalio.Direction.OUTPUT

while True:
    led.value = True
    time.sleep(1)
    led.value = False
    time.sleep(1)

Practical tip: If you’re switching between platforms:

• You can have both main.py and code.py on your board
• MicroPython will run main.py, CircuitPython will run code.py
• Just remember which platform you’re using!

For Loops

For loops work very differently in Python. There’s no C-style for (int i=0; i<10; i++) syntax.

Arduino C++:

// Count from 0 to 9
for (int i = 0; i < 10; i++) {
    Serial.println(i);
}

// Blink LED 5 times
for (int i = 0; i < 5; i++) {
    digitalWrite(LED_PIN, HIGH);
    delay(500);
    digitalWrite(LED_PIN, LOW);
    delay(500);
}

MicroPython:

# Count from 0 to 9
for i in range(10):
    print(i)

# Blink LED 5 times
for i in range(5):
    led.on()
    time.sleep(0.5)
    led.off()
    time.sleep(0.5)

Python’s range() function generates numbers:

• range(10) → 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
• range(5, 10) → 5, 6, 7, 8, 9
• range(0, 10, 2) → 0, 2, 4, 6, 8 (step by 2)

Iterating Over Lists

Python’s for loops really shine when working with lists:

Arduino C++:

int pins[] = {13, 12, 11, 10};
int numPins = 4;

for (int i = 0; i < numPins; i++) {
    pinMode(pins[i], OUTPUT);
    digitalWrite(pins[i], HIGH);
}

MicroPython:

pins = [13, 12, 11, 10]

for pin_num in pins:
    pin = Pin(pin_num, Pin.OUT)
    pin.on()

No index needed! Python iterates directly over the list items. Much cleaner.

Break and Continue

These work identically in both languages:

Arduino C++:

// Break - exit loop early
for (int i = 0; i < 100; i++) {
    if (digitalRead(BUTTON_PIN)) {
        break;  // Exit loop when button pressed
    }
}

// Continue - skip to next iteration
for (int i = 0; i < 10; i++) {
    if (i % 2 == 0) {
        continue;  // Skip even numbers
    }
    Serial.println(i);
}

MicroPython:

# Break - exit loop early
for i in range(100):
    if button.value():
        break  # Exit loop when button pressed

# Continue - skip to next iteration
for i in range(10):
    if i % 2 == 0:
        continue  # Skip even numbers
    print(i)

Switch Statements: Python Doesn’t Have Them!

Arduino has switch/case. Python doesn’t. Use if/elif instead:

Arduino C++:

int mode = 2;

switch (mode) {
    case 1:
        Serial.println("Mode 1");
        break;
    case 2:
        Serial.println("Mode 2");
        break;
    case 3:
        Serial.println("Mode 3");
        break;
    default:
        Serial.println("Unknown");
}

MicroPython:

mode = 2

if mode == 1:
    print("Mode 1")
elif mode == 2:
    print("Mode 2")
elif mode == 3:
    print("Mode 3")
else:
    print("Unknown")

Alternative: Dictionary dispatch (advanced pattern):

mode = 2

actions = {
    1: lambda: print("Mode 1"),
    2: lambda: print("Mode 2"),
    3: lambda: print("Mode 3")
}

actions.get(mode, lambda: print("Unknown"))()

This is more Pythonic but less beginner-friendly. Stick with if/elif for now.

Nested Control Structures

Indentation makes nesting very clear:

Arduino C++:

if (temperature > 25) {
    if (humidity > 60) {
        digitalWrite(FAN_PIN, HIGH);
        for (int i = 0; i < 3; i++) {
            digitalWrite(LED_PIN, HIGH);
            delay(100);
            digitalWrite(LED_PIN, LOW);
            delay(100);
        }
    }
}

MicroPython:

if temperature > 25:
    if humidity > 60:
        fan.on()
        for i in range(3):
            led.on()
            time.sleep(0.1)
            led.off()
            time.sleep(0.1)

Each nested level adds 4 spaces of indentation. The structure is visually clear.

Complete Example: Traffic Light

Let’s put it all together with a traffic light controller:

Arduino C++:

int redPin = 11;
int yellowPin = 12;
int greenPin = 13;

void setup() {
    pinMode(redPin, OUTPUT);
    pinMode(yellowPin, OUTPUT);
    pinMode(greenPin, OUTPUT);
}

void loop() {
    // Red light
    digitalWrite(redPin, HIGH);
    delay(5000);
    digitalWrite(redPin, LOW);

    // Green light
    digitalWrite(greenPin, HIGH);
    delay(5000);
    digitalWrite(greenPin, LOW);

    // Yellow light
    digitalWrite(yellowPin, HIGH);
    delay(2000);
    digitalWrite(yellowPin, LOW);
}

MicroPython:

from machine import Pin
import time

red = Pin(11, Pin.OUT)
yellow = Pin(12, Pin.OUT)
green = Pin(13, Pin.OUT)

while True:
    # Red light
    red.on()
    time.sleep(5)
    red.off()

    # Green light
    green.on()
    time.sleep(5)
    green.off()

    # Yellow light
    yellow.on()
    time.sleep(2)
    yellow.off()

Try It Yourself

Exercise 1: Temperature Warning System

Write MicroPython code that:

• Reads a temperature variable
• If temp > 30: print “TOO HOT” and turn on red LED
• If temp 20-30: print “COMFORTABLE” and turn on green LED
• If temp < 20: print “TOO COLD” and turn on blue LED

Answer:

from machine import Pin

temperature = 25  # Simulate sensor reading

red_led = Pin(10, Pin.OUT)
green_led = Pin(11, Pin.OUT)
blue_led = Pin(12, Pin.OUT)

if temperature > 30:
    print("TOO HOT")
    red_led.on()
    green_led.off()
    blue_led.off()
elif temperature >= 20:
    print("COMFORTABLE")
    red_led.off()
    green_led.on()
    blue_led.off()
else:
    print("TOO COLD")
    red_led.off()
    green_led.off()
    blue_led.on()

Exercise 2: Button Counter

Create a program that:

• Counts button presses
• Prints the count each time
• If count reaches 10, print “MAX REACHED” and reset to 0
• Blinks an LED for each press

Answer:

from machine import Pin
import time

button = Pin(14, Pin.IN, Pin.PULL_UP)
led = Pin(25, Pin.OUT)

count = 0
last_state = 1

while True:
    current_state = button.value()

    # Detect button press (high to low transition)
    if last_state == 1 and current_state == 0:
        count += 1
        print(f"Count: {count}")

        # Blink LED
        led.on()
        time.sleep(0.1)
        led.off()

        # Check for max
        if count >= 10:
            print("MAX REACHED")
            count = 0

    last_state = current_state
    time.sleep(0.05)  # Debounce delay

Exercise 3: Loop Practice

Write a program that:

• Blinks an LED 10 times
• Each blink should be faster than the last (start at 1 second, decrease by 0.1s)
• Print the delay time before each blink

Answer:

from machine import Pin
import time

led = Pin(25, Pin.OUT)

for i in range(10):
    delay = 1.0 - (i * 0.1)
    print(f"Blink {i+1}, delay: {delay}s")

    led.on()
    time.sleep(delay)
    led.off()
    time.sleep(delay)

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