Saving an sklearn pipeline with its dependencies

I am using sklearn's Pipeline and FunctionTransformer with a custom function

from sklearn.externals import joblib
from sklearn.preprocessing import FunctionTransformer
from sklearn.pipeline import Pipeline

This is my code:

def f(x):
    return x*2
pipe = Pipeline([("times_2", FunctionTransformer(f))])
joblib.dump(pipe, "pipe.joblib")
del pipe
del f
pipe = joblib.load("pipe.joblib") # Causes an exception

And I get this error:

AttributeError: module '__ main__' has no attribute 'f'

How can this be resolved ?

from Saving an sklearn pipeline with its dependencies