Reflections on Python, testing, Continuous Integration and other things

It's been a busy few weeks, both in work and out (for out of work, see Shared Illusions). In work, and of a technical nature, I've been focusing on testing and Continuous Integration and I feel it's time to look back and see what has come out of it.

Firstly, what I've been using for this. The list is:

• Python
• Py.test - My first test package. Very nice and easy to use but abandoned for reasons explained later
• Nose - A nice package available for Python for running tests with a variety of options including profiling and code coverage (using coverage.py)
  
• Hudson - A well featured build server package with a plugin architecture and several useful Python related plugins

It should be fairly obvious from my previous posts that I'm using mainly Python for work code at the moment. I've gone on at length about various Python packages and my 'fun' with transforming a complex XML structure into an equally complex and completely disjoint object structure :)

So, onto Py.test. This was my first pick for Python unit testing and I'd written a fair few tests with it. I've now moved away from it, and this is mainly because it was a bit too simple for what I needed for CI. Basically, Py.test is an absolute gem for quickly writing and running unit tests. Simply prefix tests with 'test_' and run py.test in the project's root directory and your tests will be run and results printed out. The approach is great - it's simple, it's transparent and it's fairly stress free. It just doesn't provide some key features from Nose, namely the ability to output the test report in an XUnit XML file and code coverage integration. For a lot of people, these aren't necesarry, but to get the most out of a CI build server, they are very much desirable as then the build server can generate nice graphics over time for test failures and give much more accurate reports on build status. Therefore py.test was abandoned in favour of a more complete tool.

That tool was Nose. This is a more featured test runner for Python and it is pretty easy to transfer from using Py.test to using Nose (and also from using PyUnit to using Nose, if that's what you prefer). For Py.test to nose, you don't need to modify your tests at all, unless you have function/method level fixtures. If you have these then you need to decorate the methods that use them with @setupmethod(setup_func, teardown_func)... hardly a difficult change. Nose will then do the same thing as Py.test and discover all your test functions following a configurable nameing convention (which defaults to 'test_'). For this minimal change, you are now able to run your unit tests with a wider range of report generators (including the aforementiond xunit), profile your code and run code-coverage tools all with Nose plugins and simple command line switches. Well worth it if Py.test just doesn't quite provide the test reports you need.

Finally, I needed an automated build server. While python code isn't built/compiled, the other features of a build server were very much desirable, such as its source code repository monitoring, automated running of unit tests and a nice web interface to view test results and previous builds through. I've previously encountered CruiseControl and was less than impressed with it (not particularly easy to configure, lacking in some key areas, and the interface is a bit clunky). I went looking and found Hudson. This is an 'evil Java app' that runs through tomcat, but is pretty damn good at what it does :) It provides a much cleaner web interface than Cruise Control, it comes with the ability to create new projects through the web interface (I previously had to create my own project creation for Cruise Control) and has the ability to create build scripts within the web interface using Ant targets, Maven targets, shell commands and allows new options to be added with plugins. For all of that, I can stand the minor contamination of java testing my Python code, especially as I don't have to write any java code ;)

So, what was the purpose of all of these tools? Basically, they were to get my projects up to speed with automated unit testing. This is an area I've never managed to get fully on board with (it seemed a waste at uni and I didn't manage to get the build server set up for unit tests at EMCC before they went under) but it's benefits are starting to show themselves. While I don't think the current projects I'm working with will get full unit test suites (not enough time to do so, as they are primarily research rather than production projects), I now have the necesarry legwork out the way to provide much more comprehensive testing in the future and for the testing to be run easily and hassle free every time I commit a change.

Refs:
Hudson: https://hudson.dev.java.net/
Py.test: http://codespeak.net/py/dist/test/test.html
Nose: http://code.google.com/p/python-nose/
Python: http://www.python.org/

XML, Objects and Python

I briefly mentioned that I had created two object related schemas for XML in my previous post and then elaborated on XOMS, the mapping schema. This time I'll elaborate on XOS - XML Object Schema. This is a schema I created to allow the specification of classes in XML (although I've called them objects for the XML syntax). I consider this less interesting than the mapping, although the implementation of the XOS processor in python led to some more interesting problems to overcome.

First though, a quick overview of the XML schema. This is simpler than the XOMS schema. Here is a schema for the same data model as seen previously:

<xos>
<xos:object name="Person">
<xos:attribute name="name" type="xos:string" />
<xos:attribute name="address" type="Address" />
</xos:object>
<xos:object name="Address">
<xos:attribute name="address" />
</xos:object>

Again, this is fairly self explanatory. Given this schema, the XOS processor would produce two classes, one called Person, one called Address, with the attributes specified. In most languages, the XOS processor would really be more of a pre-processor - taking an object model and outputting a set of files that define the classes required. In Python though, I could go a step further and process a model at run-time, creating the classes dynamically and registering them for use the same as any other class.

This is where the more interesting stuff came in - dynamically creating classes in python. It turns out that doing this is remarkably easy, with the following code:


def create_object(object_name):
try:
getattr(sys.modules['__main__'], object_name)
except AttributeError:
class BaseObject(object):
pass
BaseObject.__name__ = object_name
BaseObject.__module__ = '__main__'
setattr(sys.modules['__main__'], object_name, BaseObject)

(apologies for the lack of indentation. It should be clear where the indentation should be though)

This surprisingly simple bit of code creates a 'template' object derived from object, assigns it a name and a module and then registers it in the '__main__' module with the same name. It doesn't create attributes on the object, but with python this isn't required as setattr() can add arbitrary attributes to an object or class. I have plans to add in some ability for this in the future though, as I could then provide chunks of python code to do things such as create SQLAlchemy database objects on the fly.

The next step is then allowing these to be defined in arbitrary modules. For this, I needed another function:

def create_module(module_name):
if sys.modules.has_key(module_name):
return
previous_module = sys.modules['__main__']
full_mod_name = ""
for mod_name in module_name.split('.'):
full_mod_name = ".".join([full_mod_name, mod_name])
try:
previous_module = getattr(previous_module, mod_name)
except AttributeError:
mod = type(previous_module)(name=full_mod_name)
sys.modules[full_mod_name] = mod
setattr(previous_module, mod_name, mod)
previous_mod = mod


This is a fairly interesting function too. It first checks that the module being asked for hasn't already been loaded (the first line of the function). If it hasn't been loaded then the function loops through the module name split down by '.' and builds up the fully qualified name through the loop. For each loop iteration, it first checks that the previous module doesn't already have a module with the expected name. If it doesn't it creates a module with the fully qualified name, registers it in the sys.modules dictionary with that name as well, then uses setattr() on the previous module to set it with the individual name. It then sets the new module as the previous module and iterates.

The next step for XOS would be to have an inheritance mechanism modelled in the XML. Some preliminary experiments in python have shown that I'll need to use a metaclass to correctly set the base class for the objects created but I haven't finished this yet.

In the end, what this experiment has shown is that it's possible to re-implement the python object and module creation using Python and your own syntax. More surprisingly, what has been shown is that the actual creation is easy! Less than 20 lines of code to create objects and modules, something that wouldn't even be possible in something like C++ and would require huge amounts of reflection code in C# or Java.

XML and objects - my approach

So recently, I've been working on a project where I have the two extremes... a very complex tree structure in XML format and a very complex object structure in Python (which is incidentally mapped to an equally complex 56 table database using Elixir). The object structure would make no sense as a tree and XML doesn't work well as objects so I was left to consider alternatives.

My solution was to create a way of specifying the relationship between the XML nodes and the objects and created an XML schema language of sorts to do this (well, I created two, but the second one was more for completeness. I may mention it in a later blog post). I've unimaginatively called this schema the 'XML Object Mapping Schema' (XOMS for short) and a quick sample is as follows:

<xoms>
<person xoms_to="Person" name="Person.name">
<address xoms_to="Address" xoms_link="Person.address" xoms_content="Address.address">
</person>
</xoms>

It should be fairly obvious what the above should do. It says that a 'person' element should be mapped to a 'Person' object and it's 'name' attribute should be shoved into the 'Person.name' property. The 'address' node should similarly be mapped to an 'Address' object and the content of an 'address' element should be shoved into the 'Address.address' property. The xoms_link says that this Address object should then be linked to the Person.address property. So with this XML:

<person name="Joe Bloggs">
<address>
1 Somewhere drive, Someplace, Earth
</address>
</person>

would produce the following objects:

[<person name="Joe Bloggs" address="<Address address="...">" >, <Address address="...">]

with a Python implementation.

I've got a fair few more features than this now implemented in a Python library that allows automated object construction from XML to a fairly arbitrary object structure, including the ability to call functions on the objects with parameters take from subnodes (for when I can't manage what I need just with XOMS). It's not perfect but it's doing well enough to populate the above mentioned data models and it's a lot leaner than my original approach. With about 500 lines of code and about 100 lines of XML I've replaced the same amount of XML processing, except that my new approach is complete where my original code was only mapping about 10 objects and not all their attributes. If I'd gone down that road I'd have gotten a couple of thousand lines of code that would have been messing, horrible to maintain and impossible to see where everything was going. This way I have 500 lines of code that performs some magic and all of the interesting stuff is kept in the XOMS file where I can easily see what's happening and change it.

So I've now become guilty of that heinous crime - solving problems with XML by throwing more XML at it... this time it seems to have worked though!

Elixir and SQLAlchemy

After my initial foray into SQLAlchemy I started the process of mapping my current object model onto database tables and came across issues with my custom, hand-rolled relationship classes. While I expect I could have shoehorned these into SQLAlchemy mappers I decided instead that I'd be better served with an object model that was slightly more integrated with the database structure.

Cue Elixir. Elixir is a thin, ActiveRecord style wrapper on top of SQLAlchemy. It provides some useful features (such as simple to use relationship objects, and polymorphic associations) and allows you to drop down into SQLAlchemy for those features that they haven't wrapped, such as AssociationProxy (for doing a relationship through another relationship). Elixir is the missing link in the stack I was building up, giving me a powerful ORM and database layer with a lovely declarative syntax for building up all the objects I needed.

It took me about a week to go from my old model, through SQLAlchemy and into Elixir but in doing so, I have avoided a whole mess of spaghetti code involved in writing my objects out to a database, obtained database independence, and forced myself to refactor out a lot of code that was proving less than suitable for my requirements. On top of that, I should be able to integrate my system into a pylons or django web application much more easily now than I could have a week ago. Now if only I could get a similar system for XML parsing and allow me to concentrate on the important tasks I still have to do :)

SQLAlchemy

Continuing on with my sort of 'series' on various libraries and languages, I've recently started to use SQLAlchemy as an ORM layer in my current project.

The reason for using an ORM library came from the immortal problem of suddenly realising that I had a complicated object model defined and an equally complicated database schema and needing to get information between the two of them. I had started to hack together a script that would dump the data in using SQL, but I realised after the first few inserts that it would be a long hard slog that would be inflexible and complicated spaggeti code.

I didn't want to have to rewrite my object model significantly though, which meant I had to find an ORM library that would allow me to set up the mapping myself. I also needed it to work in Python, which then led me to SQLAlchemy. This nifty library comes in several libraries and is very flexible in the right ways to make it suitable for my needs (at least so far).

I'm still in the process of setting up the internal data structures. I'm needing to redo some parts of my internal object system but not significantly so, although I'm worried about it becoming a bit of a mess.

I'll comment on a bit more of the technical side of things in a few days when I've gotten more up to speed with the library.

SAX Parsing

After some time playing with minidom in Python, I finally looked at alternatives and discovered SAX. For my usual requirements (pulling data out of XML files into various internal data structures) this is so much of an improvement over minidom that I wish I'd come to it first, rather than deal with the mess that is minidom for reading XML. I'm still learning the ins and outs of SAX, but as a brief summary for those who know it even less than me I'll explain the basics.

SAX is an event-based XML parser. What this means is that it will read in XML and generate events based on what it has read in. Events would be something like the start of an element, the end of an element, etc. Your application then receives these events and can respond to them appropriately. This can entail a small amount of extra book-keeping in your application to know where you are in the XML tree (assuming that's important for your app), but it is so much easier to deal with than the multiple levels of looping through child nodes that is required to pull data out of an entire DOM tree.

My first problem with SAX (at least from what I know of it so far anyway) is that with a complicated XML file you can end up with an extremely large and non-cohesive handler for your file. To get around this I created a SAX content handler that maintains a 'processor stack' that allows you to split down your processing into more flexible, cohesive units. It isn't perfect (it has some ugliness when it comes to transferring data between processors currently, and adding a new processor is something that is hard-coded into each processor) but it makes for a nice system to me as I can split up my processing to any level I like, the processor stack acts as that extra book keeping I mentioned and it does so transparently. It allows for more flexibility in changing the structure and reuse of components.

All in all, I quite like SAX and I'll probably like it even more if I spend some time on my content processing to make it a more generic solution :)

Adobe Flex

I finally got around to trying this out over the last few days, and I have to admit I was surprised by how much I enjoyed the experience. To start explaining, I'll first detail a bit about what Flex is.

Flex is the free, open source SDK for creating Flash applications (or rather swf applications). It does away with that annoying 'timeline' concept thats used in Flash animation, replacing it with a very nice, very functional XML layout format with the ability to embed ActionScript 3 code into the layout. Add onto this the Flex Builder IDE (based on Eclipse, and unfortunately not free, but it does have a free 30 day trial version) with extremely useful code completion and all the nice organisation features I'm used to with IDE projects and you have an environment that makes it, speaking plainly, just makes it easy and fun to do things.

So, to start with, my initial thoughts upon trying Flex were dubious. I've never really liked Flash and have always had the mental model of it being an 'animation package' that shouldn't be used for mainstream web applications. I guess Adobe realised this, which is why they hid animation features in Flex to make it more a mainstream programming language. For reasons I don't want to get into here, I had to put this aside and actually try to create something useful with the language.

After about an hour of downloading(1) and then finding the 'Flex in a week' tutorials on Adobe(2) and I was set up, ready to start producing these hated flash applications. About 30 minutes later, I had a grasp of how to layout the XML applications (using the MXML library), how to link that into ActionScript code to do... whatever... and about 10 minutes later, even how to request data from a website and push that into my application to populate list controls. It was easy, despite having never done any AS coding before and having the barest grasp of the syntax. The tutorials didn't even cover the syntax. It assumed you were clever enough to pick it up by osmosis (a welcome relief I can tell you... I don't mind learning a new syntax, but being told for the 50th time that 'this is a for loop, you use it to iterate over something N times' is not something fun) and concentrated on the stuff that would be less familiar to a programmer first coming to flex - specifying a layout with XML, the conventions used, how to link to services, the event model of Flex and how to specify custom events, etc.

So, I haven't gotten through the complete 'week' course yet, but I rushed through the first two-and-a-half days fairly quickly before deciding to create something 'useful'. I decided that what I would try is a custom Flash reader for my wife's hamster comic strip, HAMIC. I used what I had picked up to request the RSS feeds (that I had created, and modified slightly to throw out some extra stuff I needed) and used my newly found knowledge to create the reader in about a day. I then modified it again to add the ability to create and update comics using a similar interface to the web interface on the actual site. I ran into a couple of small issues that helped me understand what the framework does better (such as when controls get instantiated, how bindable data works a bit better and so forth) but overcame them (apart from one small issue with single comics in a category... but we won't mention that ;)). I may end up doing a lot of web-related stuff as a job soon (fingers crossed, I really need a job right now) and this was the first step towards that goal.

It's certainly a far cry from Symbian, and that's almost certainly a good thing :) It's definitely made me re-evaluate my views on Flash technology.

(1) Flex builder 3 trial - http://www.adobe.com/cfusion/entitlement/index.cfm?e=flex3email

(2) Flex in a week video training course - http://www.adobe.com/devnet/flex/videotraining/

(3) The reader I created, for anyone interested. Please be nice to it, it's only a new app - http://flextestsite.workmad3.com/HAMICFlex/index.html