Feature selection using custom python expression functions

[qgib]

Author Name: Simran Khare (Simran Khare)
Original Redmine Issue: 16424
Affected QGIS version: 2.18.5
Redmine category:expressions

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This is regarding selecting features by expression in QGIS 2.18.x. The selection does not seem to work when we try to access the feature attributes in our custom expression function.

For example, we would expect the following function to select all features with non-null osm_ids. Upon executing the function in the expression engine, the output preview is 'true', but no features are selected.

@qgsfunction(args=0, group='Custom')
def fun3(value1, feature, parent):
if feature['osm_id']:
return True
else:
return False

The above function works fine in QGIS 2.14.12, but not in 2.18.5.

Attached is the pbf file and some Python functions I tested with. Please add the point vector layer from 'tulersee.pbf' and call fun3() to reproduce the error.

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• turlersee.pbf (Simran Khare)
• test.py (Simran Khare)

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• easy_fix was configured as 0
• regression was configured as 0

[qgib]

Author Name: Simran Khare (Simran Khare)

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This is not a bug, but a slight difference in custom python expression functions in QGIS 2.18.
In version 2.18, any feature attribute that we intend to use within the function must be passed as an argument to the function.

So, for example, a function that accesses the 'osm_id' field should be modified as follows and called as fun("osm_id"). In this case the features are selected.

@qgsfunction(args='auto', group='Custom')
def fun(field, feature, parent):

if feature['osm_id']:

return True
else:
return False

Thanks to this answer: https://gis.stackexchange.com/a/238226/94852

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• status_id was changed from Open to Closed