PISI Python Coding Standards

Written by Eray Özkural 2005-2006, 2017

These suggestions are generally applicable to Python applications, although they were written during the evaluation of development plans for PISI, which is a neat fast package manager written in Python.


0. Before reading any further please observe PEP 8: Style Guide for Python Code http://www.python.org/peps/pep-0008.html

In particular, this means no lameCaps.

1. When using dirnames, don't expect the dir to end with a trailing slash, and please use the dirnames in pisiconfig. Use util.join_path instead of os.path.join.

2. Python indentation is usually 4 spaces.

3. Follow python philosophy of 'batteries included'.

4. Use exceptions, don't return error codes.

5. Don't make the PISI code have runtime dependencies on a particular distribution (as much as possible).

6. Don't assume narrow use cases. Allow for a mediocre amount of generalization in your code, for pieces that will be required later.

7. If you are changing something, check if that change breaks anything and fix breakage. For instance a name. Running the tests is not always enough!

8. A good design ensures separation of concerns. Every module has a specific documented responsibility. Don't make the horse clean your windows.

9. To ensure readability avoid nesting python constructs more than 3 levels deep. Python is a good language (unlike C), so you can define inner functions in a convenient way, use such decomposition techniques to break down your code into manageable chunks. The worst code you can write is one huge procedure that goes on for 1000 (or more) lines.

10. Use a particular abstraction like a class or function only if it makes sense. Don't just define things because they can be defined. Define only things that will/may be used.

11. If you are doing an expensive task like searching through 10000 text chunks, please use an efficient data structure and algorithm. We are not MS engineers who know no data structure beyond doubly linked lists and no algorithm beyond quicksort.

12. Resist the temptation to develop kludges and workarounds in response to pressure. Take your time to solve the problems by the book. The payoff comes later.

13. Same thing goes for premature optimizations. Knuth and Dijkstra are watching over your shoulder. 🙂

Branches and SVN

There are two branches of pisi, one is called pisi-devel and new features that are large enough to cause instability go into that branch. The trunk version is supposed to be stable at all times. This means that you *must* run unit tests and other test scripts after committing any change that cannot be tested in isolation. Run the unit tests periodically to catch unseen bugs. A release from the stable branch *must not* break any tests whatsoever, so extensive use of the test suite must precede any release.

Unit testing

Unit tests are located in unittests directory. Running the tests is trivial. But you must synchronize your code and data with the test code, which can be a tedious work if you lose discipline.

Sample data files are located in the same directory with test modules.

For running the entire test suite, use the following command:

$ ./tests/run.py

The following command will run tests in specfiletests and archivetests in unittests dir:

$ ./tests/run.py specfile archive

Do not depend on the output of unittests. Instead of producing an output message/data in your tests, check the data internally. By definition, unittest should just report succeeding and failing cases.

If you didn't, take a look at the links below for having an idea of unit testing. http://www.extremeprogramming.org/rules/unittests.html http://www.extremeprogramming.org/rules/unittests2.html

Other tests

There are a couple of nice test bash scripts for testing the basic capabilities of the command line interface such as building and upgrading. Unlike unit tests, you have to take a look at the output to understand that the scripts are doing well 🙂

Misc. Suggestions

  1. Demeter's Law: In OO programming, try to invoke Demeter's law. One of the "rules" there is not directly accessing any objects that are further than, 2/3 refs, away. So the following code is OK destroy_system(a.system().name()) but the following isn't as robust:
    As you can tell, this introduces too many implementation dependencies. The rule of thumb is that, in these cases this statement must have been elsewhere... It may be a good idea to not count the object scope in this case, so in Python self.a means only one level of reference, not two.
    One quibble with this: it may be preferable not to insist on this where it would be inefficient. So if everything is neatly packed into one object contained in another object, why replicate everything in the upper level? If the semantics prevents dependency changes, then chains of 3 or even 4 could be acceptable.
    OTOH, in Python and C++, it's not always good to implement accessor/modifier pairs for every property of an object. It would be much simpler if you are not doing any special processing on the property (e.g. if what the type system does is sufficient). The main rule of thumb in Demeter's Law is avoiding putting more than, say, 10 methods in a class. That works really well in practice, forcing refactoring every now and then.
  2. If you are interested in "Playstation 2 Linux Games Programming" or "How to extend C programs with Guile", please do not exercise your valuable skills in this project. Only, half joking. 😛
  3. Please do not suggest replacements of used libraries and auxiliary software and formats unless you have a very good rationale for it.
  4. Please do not remove existing features without a very good reason.
  5. Please try to maintain maximum backwards compatibility, though some minor changes might be made for major revisions.
  6. Best coding practices result from writing software that has a clearly stated purpose, pre-conditions, and post-conditions. PISI is written in a library-like manner, for this very reason, the parts of it that are modular are generally useful, like the XML metaclass autoxml, the persistency backend that uses Berkeley DB, and the command metaclass. It is advisable to develop new feature sets following a similar, re-usable, generic programming style.
  7. Test-driven development is a most preferable rapid development methodology. The sooner you may write the tests for your application, the quicker you can start writing code that will not break it. One of PISI's strengths is the comprehensive test suite. If you start writing the code with some test cases, it should diminish your coding time.
  8. Functional programming style reduces coding effort. Try to use functional programming like list comprehensions, and higher order functions, if there are complex relations you are dealing with.
PISI Python Coding Standards

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