Deleting all Features of a Vector Layer in pyQGIS
You could use the following code which is heavily based on the answer from this post: How to delete selected features in QGIS using Python
layer = iface.activeLayer()
with edit(layer):
for feat in layer.getFeatures():
layer.deleteFeature(feat.id())
Edit:
Thanks to @GermánCarrillo, a more efficient method could be to delete all features at once:
layer = iface.activeLayer()
with edit(layer):
listOfIds = [feat.id() for feat in layer.getFeatures()]
layer.deleteFeatures( listOfIds )
In QGIS3 you can use truncate()
bool QgsVectorDataProvider::truncate()
Removes all features from the layer.