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I've wrote a plugin and when I was tested it I found a plugin's case of use that it has to handle with null values. I've had some research and doesnt found much about it in PyQGIS3 documentation, but I read about NULL from qgis.core and write the code below:

from qgis.core import QgsProject, NULL

layer = QgsProject.instance().mapLayersByName('My_Layer_Name')[0]
selection = layer.selectedFeatures()
n_sel = layer.selectedFeatureCount()

if n_sel > 0:
    layer.startEditing()
    for feature in selection:
        pos = feature['offset_quad']
        if pos == NULL:
            pos = 0
        else:
            pos = int(feature['offset_quad'])
    # do some stuf here

I'd like to know if there is a better "pythonic" way to do this test for null values.

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6
  • 1
    Have you tried if pos == None? What sort of feature data are you working with? Not all data types support null (None) values. Commented Mar 22, 2019 at 2:17
  • 1
    Python syntax is if pos is None. I Think your code is ok, easy to understand. (You could shorten it with pos = 0 if pos is None else int(pos)) Commented Mar 22, 2019 at 6:34
  • @MichaelStimson, thanks for your comment. As a pyqgis newbie, I've already read that has some difference between none and null values in python, but I need to read more. So, I've tried your suggestion and works fine, thanks. Commented Mar 23, 2019 at 12:20
  • @BERA, I do print('null') if pos is None else print('not null') and returns not null. I'm missing something? I liked this way of shorten conditionals, where I can find some docs to learn more about it? Commented Mar 23, 2019 at 12:31
  • 2
    I have encountered an issue with this. In a case where pos returns {QVariant}NULL the statement if pos != None evaluates to False but if pos is not None returns True. Apparently a {QVariant}NULL equals None although it is not None. This creates an issue in variance with PEP8. Commented Sep 12, 2019 at 18:50

1 Answer 1

Reset to default
7

I guess a somewhat more pythonic method could be to use:

if not pos:

So something like:

layer = QgsProject.instance().mapLayersByName('My_Layer_Name')[0]
selection = layer.selectedFeatures()
n_sel = layer.selectedFeatureCount()

if n_sel > 0:
    for feature in selection:
        pos = feature['tesr']
        if not pos:
            pos = 0
        else:
            pos = int(feature['tesr'])
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3
  • 1
    Thanks @Joseph. Following your suggestion and @BERA way of shorten, I code this: pos = 0 if not pos else int(pos) and works fine. Commented Mar 23, 2019 at 12:42
  • Great answer. Always good to remember 'going Pythonic' can avert many package-specific problems - like what exactly the Null (NULL?) value is in QGIS anyways.. Commented Jul 23, 2020 at 19:44
  • 2
    not pos evaluates to True if pos is 0, an empty string or or or. This is not what you wanted but rather a bug! OP's original pos == NULL is actually the right approach. Commented Sep 24, 2022 at 20:30

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