The MicroPython Interactive Interpreter Mode (aka REPL)

This section covers some characteristics of the MicroPython Interactive Interpreter Mode. A commonly used term for this is REPL (read-eval-print-loop) which will be used to refer to this interactive prompt.


When typing python statements which end in a colon (for example if, for, while) then the prompt will change to three dots (…) and the cursor will be indented by 4 spaces. When you press return, the next line will continue at the same level of indentation for regular statements or an additional level of indentation where appropriate. If you press the backspace key then it will undo one level of indentation.

If your cursor is all the way back at the beginning, pressing RETURN will then execute the code that you’ve entered. The following shows what you’d see after entering a for statement (the underscore shows where the cursor winds up):

>>> for i in range(30):
...     _

If you then enter an if statement, an additional level of indentation will be provided:

>>> for i in range(30):
...     if i > 3:
...         _

Now enter break followed by RETURN and press BACKSPACE:

>>> for i in range(30):
...     if i > 3:
...         break
...     _

Finally type print(i), press RETURN, press BACKSPACE and press RETURN again:

>>> for i in range(30):
...     if i > 3:
...         break
...     print(i)

Auto-indent won’t be applied if the previous two lines were all spaces. This means that you can finish entering a compound statement by pressing RETURN twice, and then a third press will finish and execute.


While typing a command at the REPL, if the line typed so far corresponds to the beginning of the name of something, then pressing TAB will show possible things that could be entered. For example, first import the machine module by entering import machine and pressing RETURN. Then type m and press TAB and it should expand to machine. Enter a dot . and press TAB again. You should see something like:

>>> machine.
__name__        info            unique_id       reset
bootloader      freq            rng             idle
sleep           deepsleep       disable_irq     enable_irq

The word will be expanded as much as possible until multiple possibilities exist. For example, type machine.Pin.AF3 and press TAB and it will expand to machine.Pin.AF3_TIM. Pressing TAB a second time will show the possible expansions:

>>> machine.Pin.AF3_TIM
AF3_TIM10       AF3_TIM11       AF3_TIM8        AF3_TIM9
>>> machine.Pin.AF3_TIM

Interrupting a running program

You can interrupt a running program by pressing Ctrl-C. This will raise a KeyboardInterrupt which will bring you back to the REPL, providing your program doesn’t intercept the KeyboardInterrupt exception.

For example:

>>> for i in range(1000000):
...     print(i)
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>

Paste mode

If you want to paste some code into your terminal window, the auto-indent feature will mess things up. For example, if you had the following python code:

def foo():
    print('This is a test to show paste mode')
    print('Here is a second line')

and you try to paste this into the normal REPL, then you will see something like this:

>>> def foo():
...         print('This is a test to show paste mode')
...             print('Here is a second line')
...             foo()
  File "<stdin>", line 3
IndentationError: unexpected indent

If you press Ctrl-E, then you will enter paste mode, which essentially turns off the auto-indent feature, and changes the prompt from >>> to ===. For example:

paste mode; Ctrl-C to cancel, Ctrl-D to finish
=== def foo():
===     print('This is a test to show paste mode')
===     print('Here is a second line')
=== foo()
This is a test to show paste mode
Here is a second line

Paste Mode allows blank lines to be pasted. The pasted text is compiled as if it were a file. Pressing Ctrl-D exits paste mode and initiates the compilation.

Soft reset

A soft reset will reset the python interpreter, but tries not to reset the method by which you’re connected to the MicroPython board (USB-serial, or Wifi).

You can perform a soft reset from the REPL by pressing Ctrl-D, or from your python code by executing:


For example, if you reset your MicroPython board, and you execute a dir() command, you’d see something like this:

>>> dir()
['__name__', 'pyb']

Now create some variables and repeat the dir() command:

>>> i = 1
>>> j = 23
>>> x = 'abc'
>>> dir()
['j', 'x', '__name__', 'pyb', 'i']

Now if you enter Ctrl-D, and repeat the dir() command, you’ll see that your variables no longer exist:

MPY: sync filesystems
MPY: soft reboot
MicroPython v1.5-51-g6f70283-dirty on 2015-10-30; PYBv1.0 with STM32F405RG
Type "help()" for more information.
>>> dir()
['__name__', 'pyb']

The special variable _ (underscore)

When you use the REPL, you may perform computations and see the results. MicroPython stores the results of the previous statement in the variable _ (underscore). So you can use the underscore to save the result in a variable. For example:

>>> 1 + 2 + 3 + 4 + 5
>>> x = _
>>> x

Raw mode and raw-paste mode

Raw mode (also called raw REPL) is not something that a person would normally use. It is intended for programmatic use and essentially behaves like paste mode with echo turned off, and with optional flow control.

Raw mode is entered using Ctrl-A. You then send your python code, followed by a Ctrl-D. The Ctrl-D will be acknowledged by ‘OK’ and then the python code will be compiled and executed. Any output (or errors) will be sent back. Entering Ctrl-B will leave raw mode and return the the regular (aka friendly) REPL.

Raw-paste mode is an additional mode within the raw REPL that includes flow control, and which compiles code as it receives it. This makes it more robust for high-speed transfer of code into the device, and it also uses less RAM when receiving because it does not need to store a verbatim copy of the code before compiling (unlike standard raw mode).

Raw-paste mode uses the following protocol:

  1. Enter raw REPL as usual via ctrl-A.

  2. Write 3 bytes: b"\x05A\x01" (ie ctrl-E then “A” then ctrl-A).

  3. Read 2 bytes to determine if the device entered raw-paste mode:

    • If the result is b"R\x00" then the device understands the command but doesn’t support raw paste.

    • If the result is b"R\x01" then the device does support raw paste and has entered this mode.

    • Otherwise the result should be b"ra" and the device doesn’t support raw paste and the string b"w REPL; CTRL-B to exit\r\n>" should be read and discarded.

  4. If the device is in raw-paste mode then continue, otherwise fallback to standard raw mode.

  5. Read 2 bytes, this is the flow control window-size-increment (in bytes) stored as a 16-bit unsigned little endian integer. The initial value for the remaining-window-size variable should be set to this number.

  6. Write out the code to the device:

    • While there are bytes to send, write up to the remaining-window-size worth of bytes, and decrease the remaining-window-size by the number of bytes written.

    • If the remaining-window-size is 0, or there is a byte waiting to read, read 1 byte. If this byte is b"\x01" then increase the remaining-window-size by the window-size-increment from step 5. If this byte is b"\x04" then the device wants to end the data reception, and b"\x04" should be written to the device and no more code sent after that. (Note: if there is a byte waiting to be read from the device then it does not need to be read and acted upon immediately, the device will continue to consume incoming bytes as long as reamining-window-size is greater than 0.)

  7. When all code has been written to the device, write b"\x04" to indicate end-of-data.

  8. Read from the device until b"\x04" is received. At this point the device has received and compiled all of the code that was sent and is executing it.

  9. The device outputs any characters produced by the executing code. When (if) the code finishes b"\x04" will be output, followed by any exception that was uncaught, followed again by b"\x04". It then goes back to the standard raw REPL and outputs b">".

For example, starting at a new line at the normal (friendly) REPL, if you write:


Then the device will respond with something like:

b"\r\nraw REPL; CTRL-B to exit\r\n>R\x01\x80\x00\x01\x04123\r\n\x04\x04>"

Broken down over time this looks like:

# Step 1: enter raw REPL
write: b"\x01"
read: b"\r\nraw REPL; CTRL-B to exit\r\n>"

# Step 2-5: enter raw-paste mode
write: b"\x05A\x01"
read: b"R\x01\x80\x00\x01"

# Step 6-8: write out code
write: b"print(123)\x04"
read: b"\x04"

# Step 9: code executes and result is read
read: b"123\r\n\x04\x04>"

In this case the flow control window-size-increment is 128 and there are two windows worth of data immediately available at the start, one from the initial window-size-increment value and one from the explicit b"\x01" value that is sent. So this means up to 256 bytes can be written to begin with before waiting or checking for more incoming flow-control characters.

The tools/ program uses the raw REPL, including raw-paste mode, to execute Python code on a MicroPython-enabled board.