import dask.dataframe as dd
import pandas as pd
import numpy as np
df = pd.DataFrame({"a": np.arange(100)})
ddf = dd.from_pandas(df, npartitions=4)

# silly function. it just append the mean for every columns to every partition
def fun(x):
    x = pd.concat([x, x.mean().to_frame().T], ignore_index=True)
    return x

out = ddf.map_partitions(lambda x: fun(x)).compute()