kharesimran/custom-expression-functions

Custom Python expression functions for QGIS

custom-expression-functions

Custom Python expression functions for QGIS.

max_incremented.py

1. max_incremented('fieldName')

   Given the name of the field (fieldName), returns 1 + the maximum value for the field. Can be used to automatically generate primary keys (eg. id, fid) when adding features in QGIS. Paste the following expression in the Default value box in the Edit Widget Properties dialog.

   Example usage: max_incremented('fid')

   To avoid the user from editing the automatically generated id, uncheck the Editable checkbox in the Edit Widget Properties dialog.

hstore_functions.py

Converts the hstore attribute to a Python dictionary with the tags as keys and tag values as values.
Expects a string value which contains key/values like this:
'"name"=>"elcom", "website"=>"http://www.elcom-ag.ch", "addr:housenumber"=>"48"' or 'name=>elcom,website=http://www.elcom-ag.ch,addr:housenumber=>48'
This is then parsed into a Python dict for the input key, key-value pair or hstore. The program can handle both inputs, with or without spaces between the key-value pairs.

The file hstore_functions.py comprises the following expression functions:

1. hstore_get_value(hstore_attr, key)

   Given the key, this function returns its value from the hstore string. If the key is not present, it returns null.

   Example usage: hstore_get_value('other_tags', 'tourism')

   Upon executing the above query in the expression engine, all features for which this function returns a non-null value will be selected, i.e. all features that contain the key tourism in their hstore.

2. hstore_exist(hstore_attr, key)

   This function returns whether the given key exists in the hstore string. It returns a boolean, True or False.

   Example usage: hstore_exist('other_tags', 'tourism')

   Upon executing the above query in the expression engine, all features which contain the key tourism in their hstore will be selected.

3. hstore_contains_key_value(hstore_attr, key_value)

   This function returns whether the given key value pair exists in the hstore string. It returns a boolean, True or False.

   Example usage: hstore_contains_key_value('other_tags', 'tourism=>information')

   Upon executing the above query in the expression engine, all features which contain the key value pair tourism=>information in their hstore will be selected.

4. hstore_contains_hstore(hstore_attr, hstore_input_string)

   This function returns whether the given hstore string exists in the feature's hstore attribute. It returns a boolean, True or False.

   Example usage: hstore_contains_hstore('other_tags', 'tourism=>information,information=>guidepost')

   Upon executing the above query in the expression engine, all features which contain both key value pairs tourism=>information and information=>guidepost in their hstore will be selected. The function also accepts and returns the correct result for input hstore strings that have spaces or double quotes surrounding the keys and values.

The code also contains the function hstore_to_dict() which accepts an hstore string and returns a Python dictionary. This function is called by 1, 2, 3 and 4 above.