My second PyQGIS code snippet – looping the layer features
by GISP Dr. John Van Hoesen, Dr. Luigi Pirelli, GISP Dr. Richard Smith Jr., GISP Ku
Mastering QGIS - Second Edition
- Mastering QGIS - Second Edition
- Mastering QGIS - Second Edition
- Credits
- Foreword
- About the Authors
- About the Reviewer
- www.PacktPub.com
- Preface
- 1. A Refreshing Look at QGIS
- 2. Creating Spatial Databases
- 3. Styling Raster and Vector Data
- Choosing and managing colors
- Managing color ramps
- Styling singleband rasters
- Styling multiband rasters
- Creating a raster composite
- Raster color rendering
- Raster resampling
- Styling vectors
- Vector layer rendering
- Using diagrams to display thematic data
- Saving, loading, and setting default styles
- Summary
- 4. Preparing Vector Data for Processing
- Merging shapefiles
- Creating spatial indices
- Checking for geometry errors
- Converting vector geometries
- Creating polygon centroids
- Converting polygons to lines and lines to polygons
- Creating polygons surrounding individual points
- Extracting nodes from lines and polygons
- Simplifying and densifying features
- Converting between multipart and singlepart features
- Adding geometry columns to an attribute table
- Using basic vector geoprocessing tools
- Defining coordinate reference systems
- Viewing a statistical summary of vector layers
- Advanced field calculations
- Conditional formatting for attribute table cells
- Complex spatial and aspatial queries
- Summary
- 5. Preparing Raster Data for Processing
- 6. Advanced Data Creation and Editing
- 7. Advanced Data Visualization
- 8. The Processing Toolbox
- About the Processing Toolbox
- Performing raster analyses with GRASS
- SAGA
- Evaluating a habitat
- Calculating elevation ranges using the SAGA Raster calculator
- Clipping land use to the park boundary using Clip grid with polygon
- Querying land use for only surface water using SAGA Raster calculator
- Finding proximity to surface water using GDAL Proximity
- Querying the proximity for 1,000 meters of water using GDAL Raster calculator
- Reclassifying land use using the Reclassify grid values tool
- Combining raster layers using SAGA Raster calculator
- Evaluating a habitat
- Exploring hydrologic analyses with TauDEM
- R
- LAStools and Fusion
- Summary
- 9. Automating Workflows with the Graphical Modeler
- An introduction to the graphical modeler
- Opening the graphical modeler
- Configuring the modeler and naming a model
- Adding data inputs to your model
- Adding algorithms to your model
- Running a model
- Editing a model
- Documenting a model
- Saving, loading, and exporting models
- Executing model algorithms iteratively
- Nesting models
- Using batch processing with models
- Converting a model into a Python script
- Summary
- 10. Creating QGIS Plugins with PyQGIS and Problem Solving
- Webography – where to get API information and PyQGIS help
- The Python Console
- My second PyQGIS code snippet – looping the layer features
- Exploring iface and QGis
- Exploring a QGIS API in the Python Console
- Creating a plugin structure with Plugin Builder
- A simple plugin example
- Setting up a debugging environment
- Debugging session example
- Summary
- 11. PyQGIS Scripting
- Where to learn Python basics
- How to load layers
- Vector structure
- Iterating over features
- Editing features
- Running Processing Toolbox algorithms
- Running an external algorithm or command
- Interacting with the map canvas
- Summary
In this paragraph, we'll introduce you to how to loop in Python and how to apply loops to explore the content of the layer loaded in the previous paragraph. Write the following snippet in the Python Console, taking special care with the code indentation:
for feature in layer.getFeatures():
print "Feature %d has attributes and geometry:" % feature.id()
print feature.attributes()
print feature.geometry().asPoint()This will print a pattern like the following:
Feature with id 21 has attributes and geometry: [22, u'US00342', 858.0, u'Airport/Airfield', u'PATL', u'TATALINA LRRS', u' Other'] (-328415,4.71636e+06)
Note
The layer.getFeatures() method returns an object that can be iterated inside a for Python instruction, getting a QgsFeature instance for every loop. The feature.attributes() method returns a list (inside the brackets, []) of the integer and Unicode strings (the u' values). The feature.geometry() method returns QgsGeometry that is converted in QgsPoint to be printed as a tuple (inside the () parenthesis) of coordinates.
It is strongly recommended that you explore the preceding classes. You can also practice by referring to the documentation at http://qgis.org/api/. Start by exploring the QgisInterface and QGis classes.
Indentation is an important part of the Python language; in fact, nesting the code in Python is done using indentation, as specified in the standard followed globally. You can find this at http://legacy.python.org/dev/peps/pep-0008/.
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