Extending geoalchemy through monkeypatching

I’ve been working on the data collection part of my cycle route modelling. I’m hoping that I can, as a first output, put together a map of where people are cycling in Melbourne. A crowd-sourced view of the best places to cycle, if you will. Given I will probably be running this in the cloud1, I thought it was best to actually store the data in a GIS database, rather than lots and lots of flat files.

A quick Google turned up GeoAlchemy, which are GIS extensions for SQLAlchemy. Provides lots of the standard things you want to do as methods on fields, but this is only a limited set of what you can do with PostGIS. Since I’m going to be wanting to do things like binning data, I thought it was worth figuring out how hard it was to call other PostGIS methods.

GeoAlchemy supports subclassing to create new dialects, but you have to subclass 3 classes, and it’s basically a pain in the neck when you just want to extend the functionality of the PostGIS dialect. Probably what I should do is submit a pull request with the rest of the PostGIS API as extensions, but I’m lazy. Henceforth, for the second time this week I am employing monkey patching to get the job done (and for the second time this week, kittens cry).

Functions in GeoAlchemy require two things, a method stub saying how we collect the arguments and the return (look at geoalchemy.postgis.pg_functions) and a mapping from this to the SQL function. Since we only care about one dialect, we can make this easier on ourselves by combining these two things. Firstly we monkeypatch in the method stubs:

from geoalchemy.functions import BaseFunction
from geoalchemy.postgis import pg_functions

class more_pg_functions:
    Additional functions to support for PostGIS

    class length_spheroid(BaseFunction):
        _method = 'ST_Length_Spheroid'

Note the _method attribute which isn’t something used anywhere else. We can then patch in support for this:

from geoalchemy.dialect import SpatialDialect

def get_function(self, function_cls):
    Add support for the _method attribute

        return function_cls._method
    except AttributeError:
        return self.__super__get_function(function_cls)

The monkeypatching functions look like this:

def monkeypatch(*args):
    Decorator to monkeypatch a function into class as a method

    def inner(func):
        name = func.__name__

        for cls in args:
            old = getattr(cls, name)
            setattr(cls, '__super__{}'.format(name), old)

            setattr(cls, name, func)

    return inner

def monkeypatchclass(cls):
    Decorator to monkeypatch a class as a baseclass of @cls

    def inner(basecls):
        cls.__bases__ += (basecls,)

        return basecls

    return inner

Finally we can do queries like this:

>>> track = session.query(Track).get(1)
>>> session.scalar(track.points.length_spheroid('SPHEROID["WGS 84",6378137,298.257223563]'))

Code on GitHub.

  1. your recommendations for cloud-based services please, must be able to run Flask and PostGIS and be super cheap []

scratching my own itch [or: why I still love open source]

I’m visiting my parents for the long weekend. Sitting in the airport I decided I should use my spare time to write some documentation, so sitting in the airport was the first time I’d tried to connect to work’s OpenVPN server. While it’s awesome that Network Manager can now import OpenVPN configs, it didn’t work because NM doesn’t support the crucial keysize parameter.

Rather than work around the problem, which some people have done, but would annoyingly break my other OpenVPNs, I used the fact that it’s open source to fix the problem properly.

My Dad asked if I was working. No, well, not really. I’m fixing the interface to my VPN client so I can connect to work’s VPN, I replied. Unglaublich! my father remarked. Not unbelievable, because it’s open source!

Review: GNOME 3 Application Development: Beginner’s Guide

GNOME 3 Application DevelopmentThe folk at Packt Publishing sent me an e-copy of GNOME 3 Application Development Beginners Guide a month or so ago.

I’ve been putting off this review because I don’t think this is an very good book and it’s hard to write bad reviews.

First off, the book’s Javascript sections use Seed. I think this is an unconventional choice given that the shell and most of GNOME uses gjs. It had been my experience with the Javascript bindings that gjs was significantly more mature, a view which is confirmed by the fact that Seed has had very little development in the last 18 months.

The book does not seem to use GTK+ best practice, like using Gtk.Grid or Gtk.Application and not using c_new constructor. It is full of things like use of Vala’s [CCode] pragma, but I don’t see why. I felt important and powerful facilities in GLib like properties were not properly explained, especially property binding. There was also a lack of understanding, for example, referring to Timeout objects, which don’t exist (the structure you’re looking for is a Source).

I do like that it uses Anjuta. It’s a shame that it requires unexplained hacks to get things building.

The Clutter section was very poor. Comparing Clutter to GTK+ is simply not reasonable. Clutter is a scene graph API, which doesn’t really have a comparison in the GTK+ stack, which goes from drawing layer to widget layer with no intermediate layer. I immediately noticed the Clutter examples hardcoded layout instead of using a layout manager.

The multimedia section had the user installing non-free codecs. Then it uses alsasink and not auto*sink. It spends a lot of time setting up GStreamer pipelines, rather than using decodebin and playbin, maybe this improves understanding, but I think it mostly will lead to the creation of very rigid apps.

I stopped reading and started skimming at this point. I did again notice weird things like creating JSON using append methods and not the handy JSON-GLib. The examples of HTML5 applications with WebKit perhaps would explain why Seed, except the wrapper is written in Vala, so there’s no problem of conflicting JS engines (I think it works fine anyway, right?). Similarly the application accesses applications by looking in /usr/share/applications rather than libgnome-menus. Again, this will lead to very rigid apps that don’t work very well and doesn’t teach beginners the best practice for GNOME development.

There’s stuff that’s just weird, the system requirements are significantly more powerful than my last computer, on which I was doing GNOME development just fine. There is discussion of how to switch to GNOME Shell, as if it’s required, whereas you can develop GNOME apps in Unity and XFCE just fine.

The typesetting of the book is poor. The source code is weirdly indented and I feel like it lacked readability (there’s great syntax highlighting available for printed text). There are grammatical mistakes that really should have been picked up in editing. The screenshots are blurry in the PDF (looks like some kind of busted bilinear filtering?). Also I can see the resize indicator on the mouse. Generally these serve to make the book look unprofessional.

Finally I don’t think the book really leads the new developer into the community as the best source to get help, which they will undoubtedly need. Of course, the community has already produced some excellent tutorials, which I think new developers would be much better off with.

All up I’m giving it 1 star.

GNOME 3 Application Development: Beginner’s Guide: ★☆☆☆☆

IdentitiesOnly + ssh-agent

I’m really hoping that someone can provide me with some enlightenment.

I have a lot of ssh keys. 6 by today’s count. On my desktop I have my ssh configured with IdentitiesOnly yes and an IdentityFile for each host. This works great.

I then forward my agent to my dev VM. I can see the keys with ssh-add -l. So far so good. If I then ssh into a host, I can see it trying every key from the agent in sequence, which is sometimes going to fail with too many keys tried. However, if I try IdentitiesOnly yes in my dev VM config, it doesn’t offer any keys, if I add IdentityFile it doesn’t work because I don’t have those key files on my VM.

So what’s the solution? What I want is to specify identities by their identifier in the agent, e.g. danni@github, however I can’t see config to do that. Anyone got a nifty solution?

generic lettuce steps for Django models

After I left the Bureau approximately a month ago I’ve taken up a new role with Infoxchange Australia. My first project here is working on a rewrite of an application using Django.

People here are really into behaviour driven testing, and we’re using Lettuce to do it (using a branch with better Django integration).

I sort of dislike this sort of testing, because it creates an annoying abstraction layer on top of the code, with a poorly defined, quasi-real language. It’s like a bad knock off of Applescript. Anyway, I got sick of defining steps per model, so I put together some generic steps for manipulating Django models (that I’ll have to contribute back).

Anyway they look like this (examples of the step in the docstrings):

# build a hash of model verbose names to models
# this is used by get_model()
def _models_generator():
    for model in get_models():
        yield (model._meta.verbose_name, model)
        yield (model._meta.verbose_name_plural, model)

MODELS = dict(_models_generator())

def get_model(model):
    Convert a model's verbose name to the model class. This allows us to
    use the models verbose name in steps.

    name = model.lower()
    model = MODELS.get(model, None)

    assert model, "Could not locate model by name '%s'" % name

    return model

def create_models(model, hashes):
    for hash_ in hashes:

def models_exist(model, hashes):
    for hash_ in hashes:
        assert \
            model.objects.filter(**hash_).exists(), \
            "Object does not exist"

@step(r'I have ([a-z][a-z0-9_ ]*) in the database:')
def create_models_generic(step, model):
    And I have admin field values in the database:
    | name         | value   |
    | project_type | Twine   |

    The generic method can be overridden for a specific model by defining a
    function create_badgers(step), which creates the Badger model.

        globals()['create_%s' % model](step)
    except KeyError:
        model = get_model(model)

        create_models(model, step.hashes)

@step(r'(?:Given|And|Then) ([A-Z][a-z0-9_ ]*) with ([a-z]+) "([^"]*)" has ([A-Z][a-z0-9_ ]*) in the database:')  # noqa
def create_models_for_relation(step, rel_model_name,
                               rel_key, rel_value, model):
    And project with name "Ball Project" has goals in the database:
    | description                             |
    | To have fun playing with balls of twine |

    lookup = {rel_key: rel_value}
    rel_model = get_model(rel_model_name).objects.get(**lookup)

    for hash_ in step.hashes:
        hash_['%s_id' % rel_model_name] = rel_model.id

    create_models_generic(step, model)

@step('(?:Given|And|Then) ([A-Z][a-z0-9_ ]*) should be present in the database')
def step_models_exist(step, model):
    And objectives should be present in the database:
    | description      |
    | Make a mess      |

    model = get_model(model)

    models_exist(model, step.hashes)

@step(r'There should be (\d+) ([a-z][a-z0-9_ ]*) in the database')
def model_count(step, count, model):
    Then there should be 0 goals in the database

    model = get_model(model)

    assert_equals(model.objects.count(), int(count))