monkeypatch.py

# Django 2.0.1
# This probably has something todo tith this: https://docs.python.org/dev/library/unittest.mock.html#where-to-patch
# I can't figure out the right way to patch the usage of this function.
# this prints True:
print(hasattr(backend.transactions.fetchers.generic_exchange,
            "update_exchange_tx_generic")) 

# but my lambda is never called and throws no error
monkeypatch.setattr(backend.transactions.fetchers.generic_exchange,
                    "update_exchange_tx_generic",
                    lambda account: print("lambda"))

user.py

# this is the user script of the function
import graphene

from backend.transactions.fetchers.generic_exchange import update_exchange_tx_generic

class AccountRefreshTransactionsMutation(graphene.relay.ClientIDMutation):
    @classmethod
    def mutate(cls, root, info, input) -> "AccountRefreshTransactionsMutation":
        # ...
        update_exchange_tx_generic(account)
        # ...