Firestore transaction fails only silently

I am trying to:

1. read a value from a document
2. update a document.

However, I would like the transaction to fail if another process made a modification to the same document between 1 and 2.

I simulated it with the debugger (stopping after 1, make a modification on the document and resume). As a result, the transaction does not seem to be applied but I do not get any error. I would like a real failure with an exception. How can I achieve that?

transaction = db.transaction()
doc_ref = db.collection('mycollection').document('mydoc')

    @firestore.transactional
    def update_in_transaction(transaction, doc_ref):
        snapshot = doc_ref.get(transaction=transaction)
        my_value = snapshot.get(u'my_value')
        transaction.update(doc_ref, {
            u'my_value': 'someothervalue'
        })

update_in_transaction(transaction, doc_ref)