Conditions and loops

Conditions (if) and loops (while, for) steer the way a program unfolds, either acting on a value or repeating an action. In MicroPython one feature shapes everything else: blocks are marked out by indentation (the left indent), not by braces. It is clean and readable, but inconsistent indentation raises an error. This page covers conditions, loops, and that central role of indentation.

Indentation marks out the blocks

Where C++ wraps a block in braces { }, MicroPython marks it with indentation. The header line ends with :, and everything indented below it belongs to the block:

if temperature > 25:
    print("It is hot")         # inside the block (indented)
    ventilateur.on()           # inside the block
print("Rest of the program")   # outside the block (back to the left)

The convention is 4 spaces per level (Thonny does it for you). No braces, and no semicolon at the end of a line.

Conditions: if / elif / else

valeur = capteur.read_u16()    # an ADC reading, 0..65535
 
if valeur < 20000:
    etat = "dark"
elif valeur < 45000:
    etat = "medium"
else:
    etat = "bright"

The comparisons: ==, !=, <, >, <=, >=. The logical operators are spelled out as words — and, or, not — rather than && / || / !:

if bouton.value() == 0 and not alarme_active:
    declencher()

Loops: while and for

The while loop repeats as long as a condition holds true. while True: is the main loop of an embedded program:

while True:
    led.toggle()
    sleep(0.5)

The for loop walks through a sequence, often range(n) to repeat n times:

for i in range(5):      # i takes 0, 1, 2, 3, 4
    led.on()
    sleep(0.1)
    led.off()
    sleep(0.1)

You can also walk directly through a list: for broche in [14, 15, 16]:. break leaves the loop, continue moves on to the next pass.

Pitfalls

Indentation error. An inconsistent indent (or a mix of tabs and spaces) raises an IndentationError. Stick to 4 spaces everywhere, and let Thonny handle the indentation.

Forgetting the :. The header line of an if, a while or a for ends with a colon. Leaving it out raises a SyntaxError.

and/or/not, not &&/||/!. A reflex to unlearn if you come from C: the logical operators are words.

= is not ==. if x = 5 is an error. The comparison is written if x == 5. (Unlike C, assignment inside a condition is not allowed, which rules out the classic bug.)

Exercises

Exercise 1 — Three brightness bands

From an ADC reading valeur (0 to 65535), display "dark", "medium" or "bright" according to three bands of your choosing.

Solution

valeur = capteur.read_u16()
if valeur < 20000:
    print("dark")
elif valeur < 45000:
    print("medium")
else:
    print("bright")

elif chains the cases: as soon as one condition is true, the following ones are skipped. Order matters, so test the thresholds from the lowest to the highest.

Exercise 2 — Blink N times

Make the LED blink exactly 10 times (0.1 s on / 0.1 s off), then stop. Which loop, and why not while True?

Solution

from machine import Pin
from time import sleep
led = Pin("LED", Pin.OUT)
 
for i in range(10):
    led.on()
    sleep(0.1)
    led.off()
    sleep(0.1)

for ... in range(10) repeats a known number of times, which is what is needed here. while True would repeat endlessly. range(10) produces the values 0 to 9, so 10 passes.

See also

• The MicroPython language — hub of the language path
• Variables and types — the values the conditions are testing
• Functions — grouping a reusable block
• Conditions · Loops in C++ — worth comparing (braces, operators)