Accessing composer item properties via custom expressions in QGIS


So here is a neat trick. Lets say you wanted to access the scale of a composer map to make it part of a label. The scale bar can already be set to numeric to show the number value but what if it needs to be part of an existing label with other text. Not to fear, expression functions are here.

  • Create a new composer. Add the map frame and a label.
  • Set the item ID of the map frame to something you can remember, lets just use themap
  • Select the label and add some text
  • Click Insert Expression

Now for the cool part

  • Select Function Editor
  • Click New File. Give the file a new name and hit save. I called it composer functions.

In the code editor paste this code:

from qgis.utils import iface
from qgis.core import *
from qgis.gui import *

@qgsfunction(args="auto", group='Composer')
def composeritemattr(composername, mapname, attrname, feature, parent):
    composers = iface.activeComposers()
    # Find the composer with the given name
    comp = [composer.composition() for composer in composers 
                if composer.composerWindow().windowTitle() == composername][0]
    # Find the item
    item = comp.getComposerItemById(mapname)
    # Get the attr by name and call 
    return getattr(item, attrname)()
  • Click Run Script

run

Now in your label use this text:

Scale: [% composeritemattr('Composer 1', 'themap', 'scale')%]

Update the Composer 1 to match your composer name, and the themap to match your item ID.

and like magic here is the scale from the map item in a label:

2015-05-21 22_00_09-Composer 1

Check the expression error section if the label doesn’t render

error

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


Don’t do this:

self.connect(self.widget, 
             SIGNAL("valueChanged(int)"), 
             self.valuechanged)

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:

self.widget.valueChanged.connect(self.valuechanged)
self.widget.valueChanged[str].connect(self.valuechanged)

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.

commandbar

commandbar2

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:

@command.command("Name")
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)
                iface.addProject(path)
                return
    iface.addProject(_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

pointat

Here is calling point-at with all the args

pointatfunc

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

autocomplete

Use Enter to select the item in the list.

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

@command.command()
def hide_docks():
    docks = iface.mainWindow().findChildren(QDockWidget)
    for dock in docks:
        dock.setVisible(False)

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

WHY U NO USE PYTHON CONSOLE

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

point_at(123,1331)

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.

Notes

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

In memory of Tim York


The other weekend the world lost a great guy, someone so nice that you struggle to find anything bad to say about him at all. Tim was only 27 but died tragically on the weekend, to the complete shock of everyone that knew him. I don’t think anyone really knew what to feel at that point, and even now it is still really hard to take it.

Tim was very well known in the GIS space here in Australia, and extremely well respected by everyone that knew him. I attended Locate15, where I was going catch up for a beer and chat with him, and almost everyone I talked to knew who he was and how great he really was. I would not doubt at all he could have become CEO or leader at a large company, no doubt at all.

It was mentioned at the funeral that we should try to think of how Tim made our lives better. For that I can fully thank Tim for my job at DMS. It was at a SSSI event after talking with my now manager about if they would like me at DMS that I had caught up with Tim and he encouraged me to move into the private sector. For that I thank him a lot.

I have started to read “How To Win Friends & Influence People” by Dale Carnegie, because one can always do with more skills in life, and some important points from the book:

  • Become genuinely interested in other people.
  • Smile.
  • Remember a person’s name.
  • Be a good listener. Encourage others to talk about themselves.
  • Talk in terms of the other person’s interests

This. All of this is what Tim did. This is the reason it always felt great to talk to him and be around him, even when I only saw him a few times.

I don’t know if Tim’s life was better for knowing me, given his large group of friends and great partner I suspect not, but I know sure as shit that mine was better for having known him.

319187_10150351976087518_3388507_n

New addition


Stace and I would like to welcome Matilda Anne Woodrow, born 5/1/15 at 10:36am.

tilly

She is a nice way to start 2015.

After the pretty crappy year that was 2014 it was finally nice to see Matilda in the flesh. The relief of seeing her alive and well is hard to put in words after losing Ellie in 2013.

People always ask about the lack of sleep, but it feels like we have more energy now then we did before. The emotional and physical toll that the pregancy took on our family had pretty much run us into the ground and it felt like it was never ending.

The toll of the pregancy had lead me to massive lack of motivation towards QGIS and pretty much everything else in life, which isn’t healthy when you have a family to look after. Pro Tip: Get help if you find yourself in this spot, it be hard to recover if you go over the edge.

Anyway. Here is to a new year. A better year.

Function editor for QGIS expressions


A new feature for QGIS 2.8 is a function editor for expressions. Being able to define your own custom functions has be possible for a while now, over a year, I even have a plugin (Expressions+) that adds some extra ones, however it wasn’t easy for new users and required you to create files that lived on python path while also editing the startup.py file for QGIS so that functions got registed at start up. Way too much effort for my liking.

This feature is now exposed via the expression builder on the Function Editor tab. The function editor will create new Python files in qgis2\python\expressions and will auto load all functions defined when starting QGIS. I remember Sean Gillies saying that Python support in a field calculator should be a gateway to full Python programming, can’t get any more Python then full Python modules.

The best way to show this feature is a quick video so here it is

The play button in the editor will run the current editor file in the QGIS session and register any functions it finds, it will also save the file in the expressions folder.

You can make a new file by pressing the new file button which will add the basic function template, changing the name in the combobox and hitting save will save the file.

The function editor allows you have to define reuseable functions for your QGIS. If you want to share a function you simply share the .py file from the expressions folder and give it to another user. In future versions of QGIS we might have a way to download other users functions.

Auto expanding values

You can also use the args="auto" keyword in place of the number of args which will let QGIS work it out and expand the arguments for you.

Here is an example

@qgsfunction(args="auto", group='Custom')
def myfunc(value1, value2 feature, parent):
    pass

and can be used like this:

myfunc('test1', 'test2')

QGIS will workout how many arguments your function takes based on what you have defined. Note: The last arguments should always be feature, parent, these are not included in the count.

Raising errors

If you need to report a error from a function simply use the raise method like normal in QGIS and raise an exception.

@qgsfunction(args="auto", group='Custom')
def myfunc(value1, value2 feature, parent):
    raise Expection("Hahah Nope!")

The function wrapper will catch the exception and raise it though the expression engine.

A couple of things to note

  • New functions are only saved in the expressions folder and not in the project file. If you have a project that uses one of your custom functions you will need to also share the .py file in the expressions folder.
  • The editor doesn’t unregister any functions. If you define a function called test, hit play, change the name to test_2, hit play, both functions will exist until you reload QGIS.

Shortcuts to hide/show panels in QGIS


Just a quick one to show how to assign shortcuts to hide and show for panels in QGIS. Add this to your .qgis2\python\startup.py

from functools import partial
from qgis.utils import iface

from PyQt4.QtCore import *
from PyQt4.QtGui import *

mappings = {"Layers": Qt.ALT + Qt.Key_1,
            "Browser": Qt.ALT + Qt.Key_2,
            "PythonConsole": Qt.ALT + Qt.Key_3}
shortcuts = []

def activated(dock):
    dock = iface.mainWindow().findChild(QDockWidget, dock)
    visible = dock.isVisible()
    dock.setVisible(not visible)

def bind():
    for dock, keys in mappings.iteritems():
        short = QShortcut(QKeySequence(keys), iface.mainWindow())
        short.setContext(Qt.ApplicationShortcut)
        short.activated.connect(partial(activated, dock))
        shortcuts.append(short)

bind()

and now you can hide and show using ALT + number