The REPL

The REPL (Read-Eval-Print Loop) is the interactive shell of MicroPython, an interpreter that runs on the board and executes each line as soon as you type it. It is the Shell panel of Thonny, with its >>> prompt. This is the great advantage of a scripted approach: before writing a whole program, you test one line and see the effect straight away, which is ideal for exploring a sensor, checking a connection, or inspecting a value.

Read–evaluate–print

You type, MicroPython executes, and prints the result of an expression:

>>> 2 + 3
5
>>> "Pico" * 2
'PicoPico'

An assignment returns nothing (so nothing is displayed), but the variable exists afterwards:

>>> x = 10
>>> x * 2
20

And above all, you act on the hardware live:

>>> from machine import Pin
>>> led = Pin("LED", Pin.OUT)
>>> led.on()          # the LED lights up right away
>>> led.value()       # read back its state
1

Typing several lines

When a line opens a block (it ends with :), the REPL switches to the continuation prompt ... and waits for the indented body. An empty line closes the block:

>>> for i in range(3):
...     print("pass", i)
...
pass 0
pass 1
pass 2

Exploring and taking back control

The REPL is also a tool for discovery and for regaining control:

• dir(machine) lists what a module offers, and help(Pin) gives its help text;
• Ctrl-C interrupts a running program (a while True: loop that is spinning, for instance) and hands control back;
• Ctrl-D performs a soft reboot: it restarts the interpreter (and re-runs main.py) — handy for starting clean, but the variables typed at the REPL are wiped in the process;
• Ctrl-E opens paste mode, to paste a block of several lines without the automatic indentation getting in the way.

The typical workflow: you tinker at the REPL until a sequence works, then you copy it into a .py file (see getting started, main.py).

Plotting a value in Thonny

Thonny ships with a plotter (the Plotter view, View → Plotter) that charts the numbers printed to the Shell over time, the equivalent of the Arduino IDE’s serial plotter. One numeric value per line, and the curve draws itself, which is handy for watching a sensor evolve or for tuning a threshold without wiring anything extra.

from machine import ADC, Pin
import time
 
capteur = ADC(Pin(26))           # analogue input (Pico: GP26 = ADC0)
 
while True:
    valeur = capteur.read_u16()  # 0..65535
    print(valeur)                # one value per line -> one curve
    time.sleep_ms(50)            # ~20 Hz, readable by eye

Take a screenshot of the Thonny Plotter view tracing a value that varies, the curve rippling along as the prints come in.

To overlay several values, for instance to compare a setpoint with a measurement, print them on the same line, separated by a space:

print(consigne, mesure)          # two numbers -> two curves

The plotter expects numbers, so a line that mixes text and value (print("sensor", valeur)) plots badly. To plot, print the number or numbers on their own and keep the annotated print() calls for reading in the Shell. Since the plotter reads the Shell, a loop that prints too fast will saturate it, so keep the pace reasonable (sleep_ms).

Pitfalls

An assignment displays nothing. x = 10 returns no value. To see the contents, type x. That is normal, not a bug.

An infinite loop “blocks” the REPL. As long as a while True: is running, the prompt does not come back: Ctrl-C to regain control.

The soft reboot wipes the variables. After Ctrl-D (or a disconnect and reconnect), the variables defined at the REPL no longer exist: they have to be redefined. What does persist is the files saved on the board.

Exercises

Exercise 1 — Drive and read at the REPL

At the REPL, light up the built-in LED, read its state, then switch it off and read again. Which method flips the state in one go?

Solution

>>> from machine import Pin
>>> led = Pin("LED", Pin.OUT)
>>> led.on()
>>> led.value()
1
>>> led.off()
>>> led.value()
0
>>> led.toggle()   # flip the state

toggle() inverts the current state, which is handy for a blink written on one line.

Exercise 2 — Explore a module

With no documentation in front of you, find out at the REPL what the time module contains. Which command? Spot a pause function.

Solution

>>> import time
>>> dir(time)
[..., 'sleep', 'sleep_ms', 'sleep_us', 'ticks_ms', 'ticks_us', ...]
>>> help(time.sleep)

dir() lists the names in a module. sleep (in seconds) and sleep_ms (in milliseconds) are the pauses. dir() and help() are the two reflexes for exploring an unfamiliar library directly on the board.

See also

• The MicroPython language — hub of the language path
• Getting started — where the REPL first appears
• Modules and import — dir()/import to explore and load code
• Debugging a program — the REPL as a diagnostic tool (cross-cutting)