Can't delete features from layer using QGIS' Python API

Question

I've only recently started using QGIS and it's Python API. Currently I've loaded two OpenStreetMap datasets. I would like to remove lines with from the second set where the name is already present in the first. There is a catch: the first dataset uses a field called name1 while the second dataset uses a field called Name.

What I've tried so far is fairly naive and non-pythonic:

1. Store a list of all the values called name1 from the first dataset
2. Iterate through all the fields in the second dataset and if Name attribute is contained in the name1 values array above, call QgsVectorLayer's deleteFeature() passing the feature's id()

In code, the layer I want to delete from is called Cropped and the list of all values called name1 from the first dataset is called roads so what I tried looks like this:

Cropped.startEditing()
fields = Cropped.getFeatures()
for f in fields:
    name = f.attribute('Name')
    count = roads.count(name)
    if count > 0:
        print 'removing ',name,count
        Cropped.deleteFeature(f.id())
Cropped.commitChanges()

startEditing() and commitChanges() return True, but all Cropped.deleteFeature(f.id()) calls return False.

I've also spotted this existing post on How to delete selected features in Qgis using Python which seems to tackle the exact problem. I am using QGIS version 2.14.0-Essen so I have tried this, based on the answer:

with edit(Cropped):
    for fid in fids:
        Cropped.deleteFeature(fid)

Where fids is a list of field ids that match the condition above. I get False for every single call.

Any hints/tips on what I'm doing wrong/missing ?

Answer 1

Some vector formats are read-only in QGIS. So, first check if your Cropped layer is read-only:

Cropped.isReadOnly()

If not, then you could use this code snippet to delete all features whose Name attribute value is already in the roads list (note the handy function to delete all those features in a single step):

features = Cropped.getFeatures()
ids = [ f.id() for f in features if f.attribute( 'Name' ) in roads ]
with edit( Cropped ):
    Cropped.deleteFeatures( ids )