Git commit conventions

Each commit message should start with a directory or full file path prefix, so it was clear which part of codebase a commit affects. If a change affects one file, it's better to use path to a file. If it affects few files in a subdirectory, using subdirectory as a prefix is ok. For longish paths, it's acceptable to drop intermediate components, which still should provide good context of a change. It's also ok to drop file extensions.

Besides prefix, first line of a commit message should describe a change clearly and to the point, and be a grammatical sentence with final full stop. First line should fit within 78 characters. Examples of good first line of commit messages:

py/objstr: Add splitlines() method.
py: Rename FOO to BAR.
docs/machine: Fix typo in reset() description.
ports: Switch to use lib/foo instead of duplicated code.

After the first line, add an empty line and in following lines describe a change in a detail, if needed. Any change beyond 5 lines would likely require such detailed description.

To get good practical examples of good commits and their messages, browse the git log of the project.

MicroPython doesn't require explicit sign-off for patches ("Signed-off-by" lines and similar). Instead, the commit message, and your name and email address on it construes your sign-off of the following:

That you wrote the change yourself, or took it from a project with a compatible license (in the latter case the commit message, and possibly source code should provide reference where the implementation was taken from and give credit to the original author, as required by the license).
That you are allowed to release these changes to an open-source project (for example, changes done during paid work for a third party may require explicit approval from that third party).
That you (or your employer) agree to release the changes under MicroPython's license, which is the MIT license. Note that you retain copyright for your changes (for smaller changes, the commit message conveys your copyright; if you make significant changes to a particular source module, you're welcome to add your name to the file header).
Your signature for all of the above, which is the 'Author' line in the commit message, and which should include your full real name and a valid and active email address by which you can be contacted in the foreseeable future.

Python code conventions

Python code follows PEP 8.

Naming conventions:

Module names are short and all lowercase; eg pyb, stm.
Class names are CamelCase, with abreviations all uppercase; eg I2C, not I2c.
Function and method names are all lowercase with words separated by a single underscore as necessary to improve readability; eg mem_read.
Constants are all uppercase with words separated by a single underscore; eg GPIO_IDR.

C code conventions

When writing new C code, please adhere to the following conventions.

White space:

Expand tabs to 4 spaces.
Don't leave trailing whitespace at the end of a line.
For control blocks (if, for, while), put 1 space between the keyword and the opening parenthesis.
Put 1 space after a comma, and 1 space around operators.

Braces:

Use braces for all blocks, even no-line and single-line pieces of code.
Put opening braces on the end of the line it belongs to, not on a new line.
For else-statements, put the else on the same line as the previous closing brace.

Header files:

Header files should be protected from multiple inclusion with #if directives. See an existing header for naming convention.

Names:

Use underscore_case, not camelCase for all names.
Use CAPS_WITH_UNDERSCORE for enums and macros.
When defining a type use underscore_case and put '_t' after it.

Integer types: MicroPython runs on 16, 32, and 64 bit machines, so it's important to use the correctly-sized (and signed) integer types. The general guidelines are:

For most cases use mp_int_t for signed and mp_uint_t for unsigned integer values. These are guaranteed to be machine-word sized and therefore big enough to hold the value from a MicroPython small-int object.
Use size_t for things that count bytes / sizes of objects.
You can use int/uint, but remember that they may be 16-bits wide.
If in doubt, use mp_int_t/mp_uint_t.

Comments:

Be concise and only write comments for things that are not obvious.
Use // prefix, NOT /* ... */. No extra fluff.

Memory allocation:

Use m_new, m_renew, m_del (and friends) to allocate and free heap memory. These macros are defined in py/misc.h.

Examples

Braces, spaces, names and comments:

#define TO_ADD (123)

// This function will always recurse indefinitely and is only used to show
// coding style
int foo_function(int x, int some_value) {
    if (x < some_value) {
        foo(some_value, x);
    } else {
        foo(x + TO_ADD, some_value - 1);
    }

    for (int my_counter = 0; my_counter < x; my_counter++) {
    }
}

Type declarations:

typedef struct _my_struct_t {
    int member;
    void *data;
} my_struct_t;

Documentation conventions

MicroPython generally follows CPython in documentation process and conventions. reStructuredText syntax is used for the documention.

Specific conventions/suggestions:

Use * markup to refer to arguments of a function, e.g.:

.. method:: poll.unregister(obj)

   Unregister *obj* from polling.

Use following syntax for cross-references/cross-links:

:func:`foo` - function foo in current module
:func:`module1.foo` - function foo in module "module1"
    (similarly for other referent types)
:class:`Foo` - class Foo
:meth:`Class.method1` - method1 in Class
:meth:`~Class.method1` - method1 in Class, but rendered just as "method1()",
    not "Class.method1()"
:meth:`title <method1>` - reference method1, but render as "title" (use only
    if really needed)
:mod:`module1` - module module1

`symbol` - generic xref syntax which can replace any of the above in case
    the xref is unambiguous. If there's ambiguity, there will be a warning
    during docs generation, which need to be fixed using one of the syntaxes
    above

Cross-referencing arbitrary locations

.. _xref_target:

Normal non-indented text.

This is :ref:`reference <xref_target>`.

(If xref target is followed by section title, can be just
:ref:`xref_target`).

Linking to external URL:

`link text <http://foo.com/...>`_

Referencing builtin singleton objects:

``None``, ``True``, ``False``

Use following syntax to create common description for more than one element:

.. function:: foo(x)
              bar(y)

   Description common to foo() and bar().

More detailed guides and quickrefs: