Displaying milliseconds in Attribute Table using PyQGIS

Question

I have a datetime string with microsecond. I use the following script to get a QDateTime object and add it to a memory layer:

layer = QgsVectorLayer("Point?crs=EPSG:4326", "test", "memory")
layer.dataProvider().addAttributes([QgsField("dt", QVariant.DateTime)])
layer.updateFields()
QgsProject.instance().addMapLayer(layer, True)

s = "22/04/20 05:14:38:560000" # 560000 microsec = 560 millisec
# PyQt5 doesn't support microsec, so s[:-3]
dt = QDateTime.fromString(s[:-3], "yy/MM/dd HH:mm:ss:zzz").addYears(100)

feat = QgsFeature(layer.fields())
feat.setGeometry(QgsGeometry()) # empty geometry for test
feat["dt"] = dt

layer.dataProvider().addFeatures([feat])
layer.updateExtents()

But the millisecond part is missing in the attribute table.

How can I display the millisecond part in attribute table?

I do calculation at the level of milliseconds.

Answer 1

The comment by @Matt was already pointing in a correct direction. QgsDateTimeFieldFormatter is indeed needed to modify the display of DateTime values.

The following code example replaces the default DateTime formatter within the QgsFieldFormatterRegistry with a new formatter. The new formatter uses the given format string to parse any value.

class MyFormatter(QgsDateTimeFieldFormatter):
    
    config = {"display_format": "dd/MM/yyyy HH:mm:ss:zzz"} # your custom date time format
    
    def representValue(self, layer, fieldIndex, config, cache, value):
        return super().representValue(layer, fieldIndex, self.config, cache, value)
        

registry = QgsApplication.fieldFormatterRegistry()
registry.removeFieldFormatter('DateTime')
registry.addFieldFormatter(MyFormatter())

While writing this answer I just noticed that this approach removes the timezone string. I'm not sure at the moment on how to add it to the formatter.

Edit: As pointed out in the comments QgsDateTimeFieldFormatter formats all QDateTime, QDate and QTime values. In order to only modify the display of a single of those types implement a simple type check as below. QDate and QTime values should then stay the same.

def representValue(self, layer, fieldIndex, config, cache, value):
        if type(value) == QDateTime:
            return super().representValue(layer, fieldIndex, self.config, cache, value)
        
        return super().representValue(layer, fieldIndex, config, cache, value)