qgis - PyQGIS: Change attribute value doesn't work, results in NULL

I am writing a QGIS plugin, which at one point attempts to change several attribute values on a layer. However, in the end no changes are made. If I punch similar code into the built-in Python Console, it works as expected. Code (paraphrased):

grid = self.dlg.gridMapLayerComboBox.currentLayer()
# layer from GUI
prov = grid.dataProvider()
grid.startEditing()

if prov.fieldNameIndex("NewField") == -1:   
    prov.addAttributes( [ QgsField("NewField", QVariant.Int)])
if prov.fieldNameIndex("NewField2") == -1:
    prov.addAttributes( [ QgsField("NewField2", QVariant.Int)])


grid.updateFields()
fldIdx = prov.fieldNameIndex("NewField")
fldIdx2 = prov.fieldNameIndex("NewField2")

for feature in grid.getFeatures():
    ID = feature.id()
    grid.changeAttributeValue(ID, fldIdx, 42)
    grid.changeAttributeValue(ID, fldIdx2, 101)
    grid.updateFeature(feature)


grid.commitChanges()

So there are several similar questions on the site that seem like they should answer my question and I believe I've followed advice in the solutions, but to no avail. There is something I am just not seeing!

How to change the value of an attribute using QgsFeature in PyQGIS? Got Null values when updating attributes with PyQgis

This does not work either:

feature.setAttribute(fldIdx, 42)

Answer

The example code above should work (coincidently with or without the 'grid.updateFeature(feature)'). The problem was in my actual code I was trying to pass Numpy datatypes and rather than throwing an TypeError, changeAttributeValue() just silently failed.

Always remember to check your variable types when debugging!