PSA: Please use new style Qt signals and slots not the old style

Don’t do this:


It’s the old way, the crappy way. It’s prone to error and typing mistakes. And who really wants to be typing strings as functions and arg names in it. Gross.

Do this:


Much nicer. Cleaner. Looks and feels like Python not some mash up between C++ and Python. The int argument is the default so it will use that. If you to pick the signal type you can use [type].

Don’t do this:

self.emit(SIGNAL("changed()", value1, value2))

Do this

class MyType(QObject):
   changed = pyqtSignal(str, int)

   def stuff(self):
       self.changed.emit(value1, value2)

pyqtSignal is a type you can use to define you signal. It will come with type checking, if you don’t want type checking just do pyqtSignal(object).

Please think of the poor kittens before using the old style in your code.

A interactive command bar for QGIS

Something that has been on my mind for a long time is a interactive command interface for QGIS.  Something that you can easily open, run simple commands, and is interactive to ask for arguments when they are needed.

After using the command interface in Emacs for a little bit over the weekend – you can almost hear the Boos! from heavy Vim users :) – I thought this is something I must have in QGIS as well.  I’m sure it can’t be that hard to add.

So here it is.  A interactive command interface for QGIS.



The command bar plugin (find it in the plugin installer) adds a simple interactive command bar to QGIS. Commands are defined as Python code and may take arguments.

Here is an example function:

def load_project(name):
    Load a project from the set project paths
    _name = name
    name += ".qgs"
    for path in project_paths:
        for root, dirs, files in os.walk(path):
            if name in files:
                path = os.path.join(root, name)

All functions are interactive and if not all arguments are given when called it will prompt for each one.

Here is an example of calling the point-at function with no args. It will ask for the x and then the y


Here is calling point-at with all the args


Functions can be called in the command bar like so:

my-function arg1 arg2 arg2

The command bar will split the line based on space and the first argument is always the function name, the rest are arguments passed to the function. You will also note that it will convert _ to - which is easier to type and looks nicer.

The command bar also has auto complete for defined functions – and tooltips once I get that to work correctly.

You can use CTRL + ; (CTRL + Semicolon), or CTRL + ,, to open and close the command bar.

What is a command interface without auto complete


Use Enter to select the item in the list.

How about a function to hide all the dock panels. Sure why not.

def hide_docks():
    docks = iface.mainWindow().findChildren(QDockWidget)
    for dock in docks:

alias command

You can also alias a function by calling the alias function in the command bar.

The alias command format is alias {name} {function} {args}

Here is an example of predefining the x for point-at as mypoint

-> alias mypoint point-at 100

point-at is a built in function that creates a point at x y however we can alias it so that it will be pre-called with the x argument set. Now when we call mypoint we only have to pass the y each time.

-> mypoint
(point-at) What is the Y?: 200

You can even alias the alias command – because why the heck not :)

-> alias a alias
a mypoint 100

a is now the shortcut hand for alias


The Python console is fine and dandy but we are not going for a full programming language here, that isn’t the point. The point is easy to use commands.

You could have a function called point_at in Python that would be


Handling incomplete functions is a lot harder because of the Python parser. In the end it’s easier and better IMO to just make a simple DSL for this and get all the power of a DSL then try and fit into Python.

It should also be noted that the commands defined in the plugin can still be called like normal Python functions because there is no magic there. The command bar is just a DSL wrapper around them.


This is still a bit of an experiment for me so things might change or things might not work as full expected just yet.

Check out the projects readme for more info on things that need to be done, open to suggestions and pull requests.

Also see the docs page for more in depth information

Perth QGIS User Forum/Group

I have a feeling this year is going to be a interesting one for QGIS in Australia. What’s a better way then to start the year of with two great events on the same day in Perth, Australia. A QGIS User Forum/Group, and the regular Perth GeoRabble

Here is the official invitation to the upcoming QGIS User Group/Forum in Perth, Australia.

Inaugural Australian QGIS Forum

Digital Mapping Solutions is pleased to support, what will hopefully be, the first of many inspiring QGIS user forums. The goal for this event is to provide a casual environment where people can see QGIS in action as well as discuss and share their QGIS experiences and thoughts. The event will feature a variety of real-world demonstrations by QGIS users from a cross-section of industries.  There will also be a presentation by Nathan Woodrow, an active member of the open source GIS community, developer on the QGIS project and an OSGeo advocate.  Finally, there will be plenty of time for discussion during the presentations and networking over lunch. Whether you are already a QGIS user who is looking for an opportunity to share your experiences with others or if you are a potential user who is curious as to how QGIS can enhance your GIS toolkit, this is the event for you.

Event Details
Date and Times
Thursday  March 14th 2013

10.00am - Registration
10.20am - Welcome and  introduction
10.30am - Presentations commence
12.00pm - Open discussion
12.30pm - Lunch
1.30pm - Finish
Kailis Bros Fish Market & Cafe
Function Room
101 Oxford Street 
To register please email
on or before Friday March 8th 2013

Please let us know if you have any access issues or specific dietary requirements when you register.
Places are limited so please register your interest today.

and the invitation to the GeoRabble event:

Announcing GeoRabble Perth #5  - Return to Yesteryear -
Thursday, March 14, 2013
Venue: Upstairs at the Leederville Hotel
Time: Door @ 5:30pm for a 6:00pm start of presentations
Mike Bradford
Tom Gardner
More Speakers and the event sponsor to be announced very soon

As you can see from the QGIS Perth User Group invitation I will be attending and presenting – practice makes prefect – so I’m really looking forward to the event. Still not sure what I’m going to cover yet though.

Leaving! Getting a New QGIS Job!

This week is my last week at Southern Downs Regional Council. On Monday I’m starting a new job. A QGIS related job. WIN!

I have worked at Southern Downs Regional Council for the last seven years and have been grateful for every year. The people, the work, the experience, has all be excellent. It has been a great seven years and I never imagined that I would be leaving, maybe at the ten year mark, but here it is. Starting at SDRC right out of high school without any skills or knowledge of what GIS is or was I grew to love it very quick. Learning GIS, evidently, lead me to programming. Nothing to crazy at first, some VBA here, some MAPBASIC there, MapInfo added the ability to call .NET dlls so I got into VB.NET, which lead me to C#, QGIS entered about three years ago which started me down the road of C++ and Python. Throw in some GPS surveying, data collection, database stuff, bushfire mapping, planning scheme mapping, floods, and you have yourself a nice skill set that you never expected to learn – hell some early school teacher even told my parents I would never do anything useful because “I only did computers”.

So enough with the rambling personal history lesson and more about the new stuff. My new job is a Technical Consultant/QGIS Specialist with Digital Mapping Solutions(DMS), a great – of course they are great why else would I work for them – Australian GIS company. DMS were/are the sponsors of the QGIS MS SQL provider and run QGIS training courses around Australia. My new role will be focused around QGIS and QGIS clients in Australia, although it’s not limited to that. I’m really looking forward to promoting, using, and helping other people use QGIS in Australia. I really do think there is a good market for it here, and if the growing interest over the last year is anything to go by I feel it is going to be a really interesting year. Working from home, meeting new people, learning awesome skills, pimping QGIS, what’s not to love!

My blog will continue as normal, if not more. Expect to see more QGIS in Australia, hopefully we can get some regular meetups happening.

I do have to give credit to the QGIS team and community. Without the great team and community around QGIS I very much doubt any of this would have happened

My QGIS git workflow

I thought it might be handy to post the git workflow that I use when working on QGIS, or any project for that matter.

In the following examples upstream = If you have cloned from your github fork of QGIS you can add upstream using:

$ git remote add upstream

The first thing we need to do is pull down the latest changes from the main QGIS repo aka upstream

$ git fetch upstream
remote: Counting objects: 13, done.
remote: Compressing objects: 100% (1/1), done.
remote: Total 7 (delta 6), reused 7 (delta 6)
Unpacking objects: 100% (7/7), done.
   18cd145..89bdb10  master     -> upstream/master

Now that we have the changes in our local repo we need to bring our master branch up to date with the latest changes from upstream. I use rebase here because I don’t want to see merge master into master etc etc each time I want to bring my master branch up to date. In the end I want my local master branch to reflect upstream/master exactly

$ git rebase upstream/master
First, rewinding head to replay your work on top of it...
Fast-forwarded master to upstream/master.

Note: You can combine the two into one call using: git pull upstream master --rebase

In order to do any work in git you should really be using branches. We can check a new one out using:

$ git checkout -b working
Switched to a new branch 'working'

This will checkout a new working branch off my local master branch and switch to it.

Lets do some work.

$ git commit -a -m "Add some feature"
[working 8cd2f4b] Add some feature

$ git commit -a -m "More feature stuff"
[working 72d30ad] More feature stuff

$ git commit -a -m "bug fix"
[working 25b10e5] bug fix

$ git commit -a -m "bug fix"
[working 211e387] bug fix

Note: The -a means add any changed files to the commit. You can also use git add. I’m trusting you already understand how to add files to a commit.

Now at this point I could merge my changes into the master branch and push it up, or if you don’t have commit rights you can issue a pull request. However having heaps of “fix this”, “fix that” commits is pretty ugly. This is where git rebase can come in handy.

We can check which commits we have added that are not in master by doing:

$ git log --oneline master..
211e387 bug fix
25b10e5 bug fix
72d30ad More feature stuff
8cd2f4b Add some feature

Alt Text

There we can see we have four commits that differ and that 8cd2f4b is the first commit we made. I really want to merge all the commits into one to make this a little cleaner.

$ git rebase -i 8cd2f4b^

Note: ^ means go back one commit from the one listed. git rebase doesn’t include the commit that you list so you have to go back one before it.

pick 8cd2f4b Add some feature
f 72d30ad More feature stuff
f 25b10e5 bug fix
f 211e387 bug fix

# Rebase 89bdb10..7d02daf onto 89bdb10
# Commands:
#  p, pick = use commit
#  r, reword = use commit, but edit the commit message
#  e, edit = use commit, but stop for amending
#  s, squash = use commit, but meld into previous commit
#  f, fixup = like "squash", but discard this commit's log message
#  x, exec = run command (the rest of the line) using shell
# If you remove a line here THAT COMMIT WILL BE LOST.
# However, if you remove everything, the rebase will be aborted.

I have changed all but the first commit to f this will merge all the commits into the first one. The latest commit is at the bottom so you should read the rebase screen from bottom up.

[detached HEAD d5620a5] Add some feature
 1 file changed, 3 insertions(+)
 create mode 100644 test.txt
Successfully rebased and updated refs/heads/working.

Alt Text

At this point I normally merge it into master and push it upstream, but if you don’t have commit rights then you can push it up to your github repo and open a pull request.

# Push them up for review
$ git push myrepo working

Important Note:

git rebase -i will change the commit hash for anything that is included in the range of commits. Make sure you only rebase commits that are not public yet. Only rebase commits that in your local repo.

QGIS plugin InaSAFE wins Open Source award

How cool is this! The InaSAFE project, a QGIS plugin, has just won a Open Source Rookie Of the Year award. The award is given by Black Duck who make products such as

The award also got mentioned in Wired.

Talk about some awesome publicity of QGIS and how it can be used for serious tasks.

Quick Tip: Using coalesce to check for NULL or zero

Here is a quick tip that you can use in QGIS expressions, T-SQL, or even PostgresSQL.

Normally if you have a column that you need query on to find all the NULL or zeros you have to do something like this:


Well that isn’t too bad. Sure yeah it’s fine for one column but what if you have three and you need to check them all together


That is pretty long and gets hard to read pretty quick.

To cut this down we can use the coalesce function – T-SQL, PostgresSQL, QGIS Expression. The coalesce function returns the first non-NULL value out of an expression, or list of values. So if you do something like this:

coalesce(NULL, "A", 0)

You will get “A” because the first value is NULL. The function will just evaluate each value/expression until something turns up that isn’t NULL.

Using that logic we can replace the above function with the following:

coalesce(COLA, 0) = 0 AND coalesce(COLB, 0) = 0 AND coalesce(COLC, 0) = 0

To me that is a lot clearer and readable.