docs/library/index: Update built-in extension docs.

- Make the docs match the new behavior which only allows certain modules
  to be extended.
- List the modules that currently have the u-prefix.
- Add a note about the sys.path method for forcing a built-in import.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
This commit is contained in:
Jim Mussared 2023-05-31 15:59:07 +10:00 committed by Damien George
parent 952a78f88a
commit 5159304ca1

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@ -8,15 +8,17 @@ MicroPython libraries
Important summary of this section
* MicroPython provides built-in modules that mirror the functionality of the
Python standard library (e.g. :mod:`os`, :mod:`time`), as well as
MicroPython-specific modules (e.g. :mod:`bluetooth`, :mod:`machine`).
* Most standard library modules implement a subset of the functionality of
the equivalent Python module, and in a few cases provide some
MicroPython-specific extensions (e.g. :mod:`array`, :mod:`os`)
:ref:`Python standard library <micropython_lib_python>` (e.g. :mod:`os`,
:mod:`time`), as well as :ref:`MicroPython-specific modules <micropython_lib_micropython>`
(e.g. :mod:`bluetooth`, :mod:`machine`).
* Most Python standard library modules implement a subset of the
functionality of the equivalent Python module, and in a few cases provide
some MicroPython-specific extensions (e.g. :mod:`array`, :mod:`os`)
* Due to resource constraints or other limitations, some ports or firmware
versions may not include all the functionality documented here.
* To allow for extensibility, the built-in modules can be extended from
Python code loaded onto the device.
* To allow for extensibility, some built-in modules can be
:ref:`extended from Python code <micropython_lib_extending>` loaded onto
the device filesystem.
This chapter describes modules (function and class libraries) which are built
into MicroPython. This documentation in general aspires to describe all modules
@ -41,6 +43,8 @@ Beyond the built-in libraries described in this documentation, many more
modules from the Python standard library, as well as further MicroPython
extensions to it, can be found in :term:`micropython-lib`.
.. _micropython_lib_python:
Python standard libraries and micro-libraries
---------------------------------------------
@ -77,6 +81,7 @@ library.
zlib.rst
_thread.rst
.. _micropython_lib_micropython:
MicroPython-specific libraries
------------------------------
@ -181,23 +186,48 @@ The following libraries are specific to the Zephyr port.
zephyr.rst
.. _micropython_lib_extending:
Extending built-in libraries from Python
----------------------------------------
In most cases, the above modules are actually named ``umodule`` rather than
``module``, but MicroPython will alias any module prefixed with a ``u`` to the
non-``u`` version. However a file (or :term:`frozen module`) named
``module.py`` will take precedence over this alias.
Many built-in modules are actually named ``umodule`` rather than ``module``, but
MicroPython will alias any module prefixed with a ``u`` to the non-``u``
version. This means that, for example, ``import time`` will first attempt to
resolve from the filesystem, and then failing that will fall back to the
built-in ``utime``. On the other hand, ``import utime`` will always go directly
to the built-in.
This allows the user to provide an extended implementation of a built-in library
(perhaps to provide additional CPython compatibility). The user-provided module
(in ``module.py``) can still use the built-in functionality by importing
``umodule`` directly. This is used extensively in :term:`micropython-lib`. See
:ref:`packages` for more information.
(perhaps to provide additional CPython compatibility or missing functionality).
The user-provided module (in ``module.py``) can still use the built-in
functionality by importing ``umodule`` directly (e.g. typically an extension
module ``time.py`` will do ``from utime import *``). This is used extensively
in :term:`micropython-lib`. See :ref:`packages` for more information.
This applies to both the Python standard libraries (e.g. ``os``, ``time``, etc),
but also the MicroPython libraries too (e.g. ``machine``, ``bluetooth``, etc).
The main exception is the port-specific libraries (``pyb``, ``esp``, etc).
This extensibility applies to the following Python standard library modules
which are built-in to the firmware: ``array``, ``binascii``, ``collections``,
``errno``, ``hashlib``, ``heapq``, ``io``, ``json``, ``os``, ``platform``,
``random``, ``re``, ``select``, ``socket``, ``ssl``, ``struct``, ``sys``,
``time``, ``zlib``, as well as the MicroPython-specific libraries: ``bluepy``,
``bluetooth``, ``machine``, ``timeq``, ``websocket``. All other built-in
modules cannot be extended from the filesystem.
*Other than when you specifically want to force the use of the built-in module,
we recommend always using* ``import module`` *rather than* ``import umodule``.
**Note:** In MicroPython v1.21.0 and higher, it is now possible to force an
import of the built-in module by clearing ``sys.path`` during the import. For
example, in ``time.py``, you can write::
_path = sys.path
sys.path = ()
try:
from time import *
finally:
sys.path = _path
del _path
This is now the preferred way (instead of ``from utime import *``), as the
``u``-prefix will be removed from the names of built-in modules in a future
version of MicroPython.