Contributing to python-apt¶

Author:	Julian Andres Klode <jak@debian.org>
Release:	1.4.0~beta3
Date:	January 15, 2019

Let’s say you need a new feature, you can develop it, and you want to get it included in python-apt. Then be sure to follow the following guidelines.

Available branches¶

First of all, let’s talk a bit about the git branches of python-apt. In the following parts, we will assume that you use git to create your changes and submit them.

Repositories¶

https://anonscm.debian.org/cgit/apt/python-apt.git

This is the official Debian repository of python-apt. You can clone it using git by doing:
git clone git://anonscm.debian.org/apt/python-apt.git
All code which will be uploaded to Debian is here. There are also branches for Ubuntu releases, but those may not be up-to-date.

Branch names consist of the distribution vendor, followed by a slash, followed by the release of that distribution, for example: debian/sid.

The current working branch is usually pointed to by HEAD, it is either debian/sid or debian/experimental.

If both sid and experimental are active, bug fixes are either cherry-picked from debian/experimental to debian/sid, or a new release is cut on the sid branch and then merged into experimental.

Updates to stable release branches, such as debian/wheezy, are almost always cherry-picked or backported from the debian/sid branch.

C++ Coding style¶

This document gives coding conventions for the C++ code comprising the C++ extensions of Python APT. Please see the companion informational PEP describing style guidelines for Python code (PEP 8).

Note, rules are there to be broken. Two good reasons to break a particular rule:

When applying the rule would make the code less readable, even for someone who is used to reading code that follows the rules.

To be consistent with surrounding code that also breaks it (maybe for historic reasons) – although this is also an opportunity to clean up someone else’s mess (in true XP style).

This part of the document is derived from PEP 7 which was written by Guido van Rossum.

C++ dialect¶

Use ISO standard C++ (the 2011 version of the standard), headers should also adhere to the 1998 version of the standard.
Use C++ style // one-line comments for single-line comments.
No compiler warnings with gcc -std=c++11 -Wall -Wno-write-strings. There should also be no errors with -pedantic added.

Code lay-out¶

Use 3-space indents, in files that already use them. In new source files, that were created after this rule was introduced, use 4-space indents.

At some point, the whole codebase may be converted to use only 4-space indents.
No line should be longer than 79 characters. If this and the previous rule together don’t give you enough room to code, your code is too complicated – consider using subroutines.
No line should end in whitespace. If you think you need significant trailing whitespace, think again – somebody’s editor might delete it as a matter of routine.

Function definition style: function name in column 2, outermost curly braces in column 1, blank line after local variable declarations:

static int extra_ivars(PyTypeObject *type, PyTypeObject *base)
{
    int t_size = PyType_BASICSIZE(type);
    int b_size = PyType_BASICSIZE(base);

    assert(t_size >= b_size); /* type smaller than base! */
    ...
    return 1;
}

Code structure: one space between keywords like ‘if’, ‘for’ and the following left paren; no spaces inside the paren; braces as shown:
```
if (mro != NULL) {
    ...
}
else {
    ...
}
```

The return statement should not get redundant parentheses:

return Py_None; /* correct */
return(Py_None); /* incorrect */

Function and macro call style: foo(a, b, c) – no space before the open paren, no spaces inside the parens, no spaces before commas, one space after each comma.
Always put spaces around assignment, Boolean and comparison operators. In expressions using a lot of operators, add spaces around the outermost (lowest-priority) operators.
Breaking long lines: if you can, break after commas in the outermost argument list. Always indent continuation lines appropriately, e.g.:
```
PyErr_Format(PyExc_TypeError,
        "cannot create '%.100s' instances",
        type->tp_name);
```

When you break a long expression at a binary operator, the operator goes at the end of the previous line, e.g.:

if (type->tp_dictoffset != 0 && base->tp_dictoffset == 0 &&
    type->tp_dictoffset == b_size &&
    (size_t)t_size == b_size + sizeof(PyObject *))
    return 0; /* "Forgive" adding a __dict__ only */

Put blank lines around functions, structure definitions, and major sections inside functions.
Comments go before the code they describe.
All functions and global variables should be declared static unless they are to be part of a published interface

Naming conventions¶

Use a Py prefix for public functions; never for static functions. The Py_ prefix is reserved for global service routines like Py_FatalError; specific groups of routines (e.g. specific object type APIs) use a longer prefix, e.g. PyString_ for string functions.
Public functions and variables use MixedCase with underscores, like this: PyObject_GetAttr, Py_BuildValue, PyExc_TypeError.
Internal functions and variables use lowercase with underscores, like this: hashes_get_sha1.
Occasionally an “internal” function has to be visible to the loader; we use the _Py prefix for this, e.g.: _PyObject_Dump.
Macros should have a MixedCase prefix and then use upper case, for example: PyString_AS_STRING, Py_PRINT_RAW.

Documentation Strings¶

The first line of each function docstring should be a “signature line” that gives a brief synopsis of the arguments and return value. For example:
```
PyDoc_STRVAR(myfunction__doc__,
"myfunction(name: str, value) -> bool\n\n"
"Determine whether name and value make a valid pair.");
```
The signature line should be formatted using the format for function annotations described in PEP 3107, whereas the annotations shall reflect the name of the type (e.g. str). The leading def and the trailing : as used for function definitions must not be included.

Always include a blank line between the signature line and the text of the description.

If the return value for the function is always None (because there is no meaningful return value), do not include the indication of the return type.

When writing multi-line docstrings, be sure to always use string literal concatenation:

PyDoc_STRVAR(myfunction__doc__,
"myfunction(name, value) -> bool\n\n"
"Determine whether name and value make a valid pair.");

Python Coding Style¶

The coding style for all code written in python is PEP 8. Exceptions from this rule are the documentation, where code is sometimes formatted differently to explain aspects.

When writing code, use tools like pylint, pyflakes, pychecker and pep8.py (all available from Debian/Ubuntu) to verify that your code is OK. Fix all the problems which seem reasonable, and mention the unfixed issues when asking for merge.

All code must work on both Python 2 and Python 3.

Submitting your patch¶

First of all, the patch you create should be based against the most current branch of python-apt (debian/sid or debian/experimental). If it is a bugfix, you should probably use debian/sid. If you choose the wrong branch, we will ask you to rebase your patches against the correct one.

Once you have made your change, check that it:

conforms to PEP 8 (checked with pep8.py). It should, at least not introduce new errors. (and never have whitespace at end of line)

produces no new errors in pychecker, pyflakes and pylint (unless you can’t fix them, but please tell so when requesting the merge, so it can be fixed before hitting one of the main branches).

does not change the behaviour of existing code in a non-compatible way.

works on both Python 2 and Python 3.

If your change follows all points of the checklist, you can commit it to your repository. (You could commit it first, and check later, and then commit the fixes, but commits should be logical and it makes no sense to have to commits for one logical unit).

The changelog message should follow standard git format. At the end of the message, tags understood by gbp-dch and other tags may be added. An example commit message could be:

apt.package: Fix blah blah

Fix a small bug where foo is doing bar, but should be doing baz
instead.

Closes: #bugnumber
LP: #ubuntu-bug-number
Reported-By: Bug Reporter Name <email@example.com>

Once you have made all your changes, you can run git format-patch, specifying the upstream commit or branch you want to create patches against. Then you can either:

report a bug against the python-apt package, attach the patches you created in the previous step, and tag it with ‘patch’. It might also be a good idea to prefix the bug report with ‘[PATCH]’.
send the patches via git send-email.

For larger patch series, you can also publish a git branch on a public repository and request it to be pulled.

If you choose that approach, you may want to base your patches against the latest release, and not against some random commit, for the sake of preserving a sane git history.

Be prepared to rebase such a branch, and close any bugs you fix in the branch by mentioning them in the commit message using a Closes or LP tag.

Documentation updates¶

If you want to update the documentation, please follow the procedure as written above. You can send your content in plain text, but reStructuredText is the preferred format. I (Julian Andres Klode) will review your patch and include it.

Example patch session¶

In the following example, we edit a file, create a patch (an enhanced patch), and report a wishlist bug with this patch against the python-apt package:

user@ pc:~$ git clone git://anonscm.debian.org/apt/python-apt.git
user@pc:~$ cd python-apt
user@pc:~/python-apt$ editor FILES
user@pc:~/python-apt$ pep8 FILES # PEP 8 check, see above.
user@pc:~/python-apt$ pylint -e FILES # Check with pylint
user@pc:~/python-apt$ pyflakes FILES  # Check with pyflakes
user@pc:~/python-apt$ pychecker FILES # Check with pychecker
user@pc:~/python-apt$ git commit -p
user@pc:~/python-apt$ git format-patch origin/HEAD
user@pc:~/python-apt$ reportbug --severity=wishlist --tag=patch --attach=<patch> ... python-apt

You may also send the patches to the mailing list instead of reporting the bug:

user@pc:~/python-apt$ git send-email --to=deity@lists.debian.org <patches created by format-patch>

You can even push your changes to your own repository and request a pull request.