Select by location in Python

Question

I have a polygon layer and a point layer (with coordinates for each point). The polygon layer got created using the MMQGIS plugin (Hexagonal Polygons). I want to select all the points inside each polygon.

I worked with ArcGIS before but I want to change to open source. Furthermore, I want to approach that through a Python script since I want to analyse the extracted data. I have QGIS 2.8 installed. I am a bit overwhelmed by all the GIS libraries there are for Python.

Can I approach that sort of selection with a GIS library that is already installed with QGIS? If so, could anyone pin point me into the right direction?

• Comrade Che: Related question and answer: gis.stackexchange.com/questions/256569/…

Answer 1

Assuming you want to run your script within QGIS (from a script file or Python console), you can use the following:

import processing
processing.runalg("qgis:selectbylocation", INPUT, INTERSECT, METHOD, OUTPUT)

Here is the help description provided by the Python console which defines each parameter:

processing.alghelp("qgis:selectbylocation")
ALGORITHM: Select by location
 INPUT <ParameterVector>
 INTERSECT <ParameterVector>
 METHOD <ParameterSelection>
 OUTPUT <OutputVector>
METHOD(Modify current selection by)
 0 - creating new selection
 1 - adding to current selection
 2 - removing from current selection

To create a script entirely from scratch which doesn't call on QGIS functions, the following links might help in developing similar functionality:

• From Processing plugin

• From fTools plugin

---

EDIT:

There are a couple of posts which might be of some use with writing scripts outside QGIS for Mac:

• How to run a simple python script for QGIS from outside (e.g. Sublime Text)?

• Fixing Python path for QGIS scripting on Mac OSX

• four-eyes: Hello Joseph. Thanks for your reply. I want to use QGis functions! But I think I do not want to make that in the QGis console but call a script from the console of my computer (OS X 10.10). Which library are you using there? Or which library is QGis providing and how can I access that API? Is there anything to read on that?
• Joseph: No problem Stophface! I've edited my post to include links which describe how to use scripts outside QGIS for Mac. I use Windows :) In terms of scripting, I don't think there's much difference, the biggest being setting the PATHS I think (I'm also a beginner!).
• four-eyes: Yes, that path setting is something which wont work over here at all. But once thats fixed, it will save a lot of work later on :) Is there a documentation on the library QGis is using? That way I do not have to come back here but just look things there up. Is it PyQGis?
• Joseph: Yes, it uses PyQGIS :)
• Joseph: Most welcome buddy!

Answer 2

Joseph's answer is correct, although there needs to be another parameter included: "PREDICATE".

ie:

processing.run("qgis:selectbylocation", {
 "INPUT":lyr_input,\
 "PREDICATE":0,\
 "INTERSECT":lyr_intersect,\
 "METHOD":0,\
 "OUTPUT":path}
)

From processing.algorithmHelp("qgis:selecybylocation"):

PREDICATE: Where the features (geometric predicate)
 Parameter type: QgsProcessingParameterEnum
 Available values:
 - 0: intersect
 - 1: contain
 - 2: disjoint
 - 3: equal
 - 4: touch
 - 5: overlap
 - 6: are within
 - 7: cross
 Accepted data types:
 - int
 - str: as string representation of int, e.g. '1'
 - QgsProperty