Menulis Skrip Pythin untuk Framework Pemrosessan

Peringatan

A new version of this tutorial is available at Writing Python Scripts for Processing Framework (QGIS3)

One can write standalone pyqgis scripts that can be run via the Python Console in QGIS. With a few tweaks, you can make your standalone scripts run via the Processing Framework. This has several advantages. First, taking user input and writing output files is far easier because Processing Framework offers standardized user interface for these. Second, having your script in the Processing Toolbox also allows it to be part of any Processing Model or be run as a Batch job with multiple inputs. This tutorial will show how to write a custom python script that can be part of the Processing Framework in QGIS.

Tinjauan Tugas

Our script will perform a dissolve operation based on a field chosen by the user. It will also sum up values of another field for the dissolved features. In the example, we will dissolve a World shapefile based on a SUBREGION attribute and sum up POP_EST field to calculate total population in the dissolved region.

Catatan

Jika anda mencoba untuk melakukan operasi dissolve dengan statistiknya, anda dapat menggunakan plugin DissolveWithStats . Skrip ini adalah sebuah demonstrasi bagaimana mengimplementasikan fungsi serupa melalui sebuah skript processing.

Mendapatkan data

Kita akan menggunakan dataset Admin 0 - Countries dari Natural Earth.

Unduh Admin 0 - countries shapefile..

Sumber Data: [NATURALEARTH]

Untuk kenyamanan, anda bisa secara langsung mengunduh sebuah kopi dari kedua dataset dari link di bawah:

ne_10_admin_0_countries.zip

Prosedur

  1. Buka QGIS dan akses Layers ‣ Add Vector ‣ Add Vector Layer . Jelajahi file unduhan ne_10_admin_0_countries.zip dan buka layer ne_10_admin_0_countries . Akses Processing ‣ Toolbox.

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  1. Telusuri grup Scripts pada Processing Toolbox dan pilih Create a new script.

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  1. Untuk sebuah skript python agar bisa dikenal sebagai skrip processing, permulaan skrip adalah spesifikasi dari indput dan output. Ini akan digunakan untuk mengkonstruksi user interface untuk menjalankan skript. Anda dapat belajar lebih banyak tentang format dari hal ini dari QGIS Processing Documentation . Masukkan lines berikut pada Script editor . Disini kita menspesifikasi 3 input dari user: dissolve_layer, dissolve_field dan sum_field . Perhatikan bahwa kita menambahkan dissolve_layer setelah kedua field memasukkan definisi. Ini berarti bahwa input akan semacam dikumpulkan dengan pilihan field dari dissolve_layer . Kita juga akan menentukan output_layer sebagai layer output. Klik tombol Save

##dissolve_layer=vector
##dissolve_field=field dissolve_layer
##sum_field=field dissolve_layer
##output_layer=output vector
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  1. Name the script dissolve_with_sum and save it at the default location under .qgis2 ‣ processing ‣ scripts folder.

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  1. Back in the Script editor, click Run algorithm button to preview the user interface.

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  1. You can see that just by adding a few lines, we have a nice user interface for the user to specify the inputs. It is also consistent with all other Processing algorithms, so there is no learning curve involved in using your custom algorithm.

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  1. In the Script editor, enter the following code. You will notice that we are using some special methods such as processing.getObject() and processing.features(). These are convenience wrappers that make it easy to work with data. You can learn more about these from Additional functions for handling data section of QGIS Processing Documentation. Click Save to save the newly entered code and then the X button to close the editor.

Catatan

This script uses python list comprehensions extensively. Take a look at this list comprehension tutorial to get familiar with the syntax.

from qgis.core import *
from PyQt4.QtCore import *

inlayer = processing.getObject(dissolve_layer)
dissolve_field_index = inlayer.fieldNameIndex(dissolve_field)
sum_field_index = inlayer.fieldNameIndex(sum_field)

# Find unique values present in the dissolve field
unique_values = set([f[dissolve_field] for f in
processing.features(inlayer)])

print unique_values
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  1. While writing code, it is important to be able to see errors and debug your code. Your processing scripts can be debugged easily via the built-in Python Console. In the main QGIS window, go to Plugins ‣ Python Console. Once the console is open, find your script in the Processing Toolbox and double-click it to launch it.

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  1. Select SUBREGION as the dissolve field. You may choose any field as the sum field as the script doesn't have any code yet to deal with it. Click Run.

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  1. You will see an error dialog. This is expected since the script is incomplete and doesn't generate any output yet.

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  1. In the main QGIS windows, you will see the debug output from the script printed in the console. This is useful way to add print statements and see intermediate variable values.

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  1. Let's go back to editing the script by right-clicking the script and select Edit script.

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  1. Enter the following code to complete the script. Note that we are using the existing dissolve algorithm in QGIS via processing using processing.runalg() method.

# Create a dictionary to hold values from the sum field
sum_unique_values = {}
attrs = [f.attributes() for f in processing.features(inlayer)]

for unique_value in unique_values:
    val_list = [ f_attr[sum_field_index] for f_attr in attrs if f_attr[dissolve_field_index] == unique_value]
    sum_unique_values[unique_value] = sum(val_list)

# Run the regular Dissolve algorithm
processing.runalg("qgis:dissolve", dissolve_layer, "false",
    dissolve_field, output_layer)

# Add a new attribute called 'SUM' in the output layer
outlayer = processing.getObject(output_layer)
provider = outlayer.dataProvider()
provider.addAttributes([QgsField('SUM', QVariant.Double)])
outlayer.updateFields()

# Set the value of the 'SUM' field for each feature
outlayer.startEditing()
new_field_index = outlayer.fieldNameIndex('SUM')
for f in processing.features(outlayer):
  outlayer.changeAttributeValue(f.id(), new_field_index, sum_unique_values[f[dissolve_field]])
outlayer.commitChanges()
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  1. Run the algorithm by selecting SUBREGION as the dissolve field and POP_EST as the sum field. Click Run.

Catatan

The processing algorithm may take upto 10 minutes to finish depending on your system.

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  1. Once the processing finishes, you can use the Identify tool and click on any polygon. You will see the newly added SUM field with the POP_EST values from all original polygons added up.

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  1. You will note that all other fields in the output are still present. When you dissolve many features to create a single feature, it doesn't make sense to keep the original fields in the output. Go back to the Script editor and add the following code to delete all fields except the SUM field and the field that was used to dissolve the original layer. Click Save button and close the window.

# Delete all fields except dissolve field and the newly created 'SUM' field.
outlayer.startEditing()

fields_to_delete = [fid for fid in range(len(provider.fields())) if fid != new_field_index and fid != dissolve_field_index]
provider.deleteAttributes(fields_to_delete)
outlayer.updateFields()

outlayer.commitChanges()
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  1. One of the hidden features of the Processing Framework is that all algorithms can work on selected features of a layer. This is very helpful when you want to run an algorithm on the subset of a layer. As our script uses processing.features() method to read features, it will respect the current selection. To demonstrate that, let's make a selection first. Click on the Select features using an expression button.

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  1. Enter the following expression to select features from North and South America and click Select.

"CONTINENT" = 'North America' OR "CONTINENT" = 'South America'
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  1. You will see the selected features highlighted in yellow. Right-click the dissolve_with_sum script and select Execute.

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  1. Select the inputs as before and click Run.

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  1. A new output layer will be added to QGIS. This will contain dissolved geometries only from the selected features in the input layer. You will also note that the output layer will contain only 2 fields as expected.

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  1. One last but important remmaining work is to document our algorithm. The Processing Framework has nice tools to write and access help. Go to the Script editor and click the Edit script help button.

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  1. Fill in the details for different elements and click OK. Now a detailed help will be available to all users of your script in the Help tab when they launch the algorithm.

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Below is the complete script for reference. You may modify it to suit your needs.

##dissolve_layer=vector
##dissolve_field=field dissolve_layer
##sum_field=field dissolve_layer
##output_layer=output vector

from qgis.core import *
from PyQt4.QtCore import *

inlayer = processing.getObject(dissolve_layer)
dissolve_field_index = inlayer.fieldNameIndex(dissolve_field)
sum_field_index = inlayer.fieldNameIndex(sum_field)

# Find unique values present in the dissolve field
unique_values = set([f[dissolve_field] for f in processing.features(inlayer)])

# Create a dictionary to hold values from the sum field
sum_unique_values = {}
attrs = [f.attributes() for f in processing.features(inlayer)]
 
for unique_value in unique_values:
    val_list = [ f_attr[sum_field_index] for f_attr in attrs if f_attr[dissolve_field_index] == unique_value]
    sum_unique_values[unique_value] = sum(val_list)

# Run the regular Dissolve algorithm
processing.runalg("qgis:dissolve", dissolve_layer, "false",
    dissolve_field, output_layer)

# Add a new attribute called 'SUM' in the output layer
outlayer = processing.getObject(output_layer)
provider = outlayer.dataProvider()
provider.addAttributes([QgsField('SUM', QVariant.Double)])
outlayer.updateFields()

# Set the value of the 'SUM' field for each feature
outlayer.startEditing()
new_field_index = outlayer.fieldNameIndex('SUM')
for f in processing.features(outlayer):
  outlayer.changeAttributeValue(f.id(), new_field_index, sum_unique_values[f[dissolve_field]])
outlayer.commitChanges()

# Delete all fields except dissolve field and the newly created 'SUM' field
outlayer.startEditing()
fields_to_delete = [fid for fid in range(len(provider.fields())) if fid != new_field_index and fid != dissolve_field_index]
provider.deleteAttributes(fields_to_delete)
outlayer.updateFields()
outlayer.commitChanges()

If you want to report any issues with this tutorial, please comment below. (requires GitHub account)