doit is an automation tool. It is kind of build-tool but more generic…

My motivation was to demonstrate how to create a specialized interface for defining tasks. doit can be used for many different purposes, so its default interface can be quite verbose if compared to tools created to solve one specific problem.

Instead of creating an interface myself I decided to use an existing interface. I picked SCons. So the goal was to be able to build C/C++ project using an existing SConstruct file without any modification. And of course it should be as good as SCons on dependency tracking, ensuring always a correct result.

A secondary goal was to make it fast.

Note that this implementation is very far from complete. I only implemented the bare minimum to get some benchmarks running.

Implementation

I wont go into the gory details… Just a few notes. You can check the code here.

In docons.py there is an implementation of the API available in SConstrcut files. When a “Builder Method” (like Program or Object) is executed a reference to the builder is saved in a global variable. These “Builder Methods” are actually implemented as class that can generate dictionaries representing doit tasks.

In doit the configuration file that define your tasks is called dodo.py. In this case the end user wont edit this file directly. dodo.py will import the SCons API namespace from docons, than it will execfile the SConstruct file and collect the tasks from the “Builder Methods”.

Creating tasks for compile/link is straightforward. The hard part is automatically finding out the dependencies in the source code and mapping it into account on your tasks. To find out the dependencies (the #include for C code) in the source I am using the same C preprocessor as used by SCons.

SCons uses the concept of a “Scanner” function associated with a Builder. In doit the implicit dependencies are “calculated” in a separate task. The dependencies are than put into the build (compile/link) tasks through calc_dep (calculated dependencies).

A faster implementation

It seems SCons creates the whole dependency graph before starting to execute the tasks/builders. Because of this it doesn’t scale so well when the number of files increase.

doit creates the dependency graph dynamically during task execution. But even on a no-op build it will end-up with a complete graph built because it will check all tasks dependencies.

tup is a build-tool that saves the dependency graph in a SQLite database. I decided to give it a try to its approach.  So I created “dup” – sorry for the name :) . It will still read the build configuration from SConstrcut files but it will keep a SQLite database mapping the targets to all of its dependencies. This enables a much faster no-op and incremental builds. The  underlying graph dependency of a target will only be built if required.

Benchmarks

I did some very basic benchmarks from SCons and my two SCons implementations. The benchmarks were created using the gen-bench from wonderbuild benchmarks.

I used gen-bench script was run with the arguments “50 100 15 5″.  This generates 10000 tiny interdependent C++ source and header files, to be built into 50 static libs.

All benchmarks were run on a intel i3 550 quad-core (3.20GHz), running Ubuntu 10.10, python2.6.6, doit 0.13.0, SCons 2.0.0. All benchmarks were run using a single process.

The graph doesn’t include full build time. It was SCons=266 seconds, docons=249 seconds, dup=253 seconds.

• no-op 1 lib -> no-operation build when all files are up-to-date and only one of the 50 libraries were selected to be built (scons build-scons/lib_0/lib0.a)
• no-op -> no-operation build
• partial build – cpp file -> only the file lib_1/class_0.cpp was modified. rebuilt 1 object file and 1 lib.
• partial build – hpp file -> only the file lib_0/class_0.hpp was modified. rebuild 30 object files and 14 libs.

Analysis

1. comparing SCons and docons on no-op build you can see how doit is considerably faster than SCons on creating the dependency graph and checking what to build.
2. comparing “no-op 1 lib” with “no-op” you can see how both SCons and docons have a performance degradation from creating the dependency graph (5.2 and 3.6 times slower respectivelly). And how dup shows little influence on no-op build relative to the size of the dependency graph.
3. all 3 solutions have almost no difference between a no-op build and a partial/incremental build where a single source file is modified.
4. as the number of built objects increase the advantage of dup over docons is reduced because it requires the dependency graph from the affected tasks to be built.

Should you stop using SCons?

Probably not. The implementation is very incomplete and probably buggy in many ways. This code was written just as proof-of-concept (and for fun) to check how powerful, flexible and fast doit can be.

I have personally no interest in developing a C/C++ build tool and if I would build one I would create a different interface from the one used by SCons.

UPDATE2: includes fix for appengine 1.6.0

This article will explain how to setup Google AppEngine (GAE) with virtualenv.

GAE does not provide a “setup.py” to make the SDK “installable”, it is supposed to be used from a folder without being “installed”. GAE actually forbids the use of any python library in the site-packages folder. All included libraries must be in the same folder as your application, this allows GAE to automatically find and upload third-party libraries together with your application code when you upload the code to GAE servers.

So what would be the advantages of using of using virtualenv with GAE? The main reason is to have an environment to run unit-tests and functional tests. It will allow us to use the interactive shell to make operations on DB. And it also enforce you are using the correct python version.

Step 0 – install App Engine SDK

As described on official docs

Step 1 – create and activate a virtualenv

Same as usual…


$ virtualenv --python python2.5 --no-site-packages gae-env
$ source gae-env/bin/activate


Step 2 – add google_appengine path

Add a file name “gae.pth” to the virtualenv site-packages with the path to google_appengine. This way google_appengine will be in sys.path enabling it to be imported by other modules.

You will need to adjust the content of the file according to where you created your virtualenv and google_appengine location. Mine looks like this:


$ cat gae-env/lib/python2.5/site-packages/gae.pth
../../../../google_appengine


Simple test to make sure your gae.pth is correct:

(gae-env)$ python
>>> from google import appengine


If you did not get any exception you are good to go on.

Step 3 – fix path for third-party libs

The AppEngine SDK comes with a few third-party libraries. They are not in the same path as google’s libraries. If you look at dev_appserver.py you will see a function called fix_sys_path, this function adds the path of the third-party libraries to python’s sys.path. One option would be to add these paths to gae.pth… But I prefer to use the function fix_sys_path so we have less chances of having problems with future releases of the SDK.

Python automatically imports the module site.py when a Python interpreter is executed. It can be used to add some site specific code to executed. So we need to add the following lines at the bottom of the main function.

site.py is located at:


gae-env/lib/python2.5/site.py



from dev_appserver import fix_sys_path
fix_sys_path()


Check if it is working fine:


(gae-env)$ python
>>> import yaml


Again. You should not any exceptions on this…

Step 4 – add lib folder to FakeFile.ALLOWED_DIRS

This is only required if you are using virtualenv 1.5.2.

GAE imports the os module and add its path to ALLOWED_DIRS. It will also add the original file folder if the module is a link. virtualenv adds a link from your venv lib/python2.5/os.py to the system lib/python2.5 (i.e. /usr/lib/python2.5). When GAE imports os it is actually using the file os.pyc. On virtualenv 1.5.2 pyc files are not linked anymore so you will probably get an error like “ImportError: No module named cgi” when you try to access a page from your GAE application.

To fix this will also use the “cgi” module file locations, since it is not linked by virtualenv. (For appengine < 1.6.0) Edit google_appengine/google/appengine/tools/dev_appserver.py , edit the class FakeFile class attribute ALLOWED_DIRS. It should look like this:

UPDATE2: appengine 1.6.0 the edit google_appengine/google/appengine/tools/dev_appserver_import_hook.py . You alse need to “import cgi” in the top of the file.


ALLOWED_DIRS = set([
os.path.normcase(os.path.realpath(os.path.dirname(os.__file__))),
os.path.normcase(os.path.abspath(os.path.dirname(os.__file__))),
os.path.normcase(os.path.dirname(os.path.realpath(os.__file__))),
os.path.normcase(os.path.dirname(os.path.abspath(os.__file__))),
# required when using virtualenv
# cgi is on system python folder /usr/lib/python2.5/cgi.py
# os is on virtualenv python folder '/path/to/virtualenv/lib/python2.5'])
os.path.normcase(os.path.realpath(os.path.dirname(cgi.__file__))),
os.path.normcase(os.path.abspath(os.path.dirname(cgi.__file__))),
os.path.normcase(os.path.dirname(os.path.realpath(cgi.__file__))),
os.path.normcase(os.path.dirname(os.path.abspath(cgi.__file__))),
])

UPDATE: I guess this wont be necessary when they fix the issue 4339

Step 5 – add dev_appserver.py to bin

Not really required but handy.


gae-env/bin $ ln -s ../../google_appengine/dev_appserver.py .


Conclusion

Now you have an isolated environment running AppEngine! But pay attention libraries used your production code should not be installed in your virtualenv, you should do “GAE way” and link them from your application folder. You should install on virtualenv only stuff used on your tests.

On my next post I will cover how to access the datastore from the interactive prompt and how to write tests using py.test and webtest.

So see you in singapure ;)

Let’s say you want to create a slide presentation and you are not very much into presentation software.

S5 is a good enough HTML based alternative for a slideshow presentation. reStructuredText and rst2s5 can free you from writing HTML yourself…

S5 can generate a "printer-friendly" version of your slides. But I was really missing a way to create a PDF version of my slides to ease its distribution. I finally found a tool that could handle that Prince.

Example (slides.rst):

.. include:: <s5defs.txt>

======================================================================
reStructuredText to PDF
======================================================================

in 2 easy steps

:Author: Eduardo Schettino

(1) rst2s5
=======================

rst => s5

::

  rst2s5 --theme=small-white slides.rst slides.html

(2) prince
=======================

s5 => PDF

::

  prince --media projection -s page.css slides.html -o slides.pdf

Where page.css controls the PDF page size:

@page { size: 1280px 800px }