iterate over GroupBy object in dask

问题

Is it possible, to iterate over a dask GroupBy object to get access to the underlying dataframes? I tried:

import dask.dataframe as dd
import pandas as pd
pdf = pd.DataFrame({'A':[1,2,3,4,5], 'B':['1','1','a','a','a']})
ddf = dd.from_pandas(pdf, npartitions = 3)
groups = ddf.groupby('B')
for name, df in groups:
    print(name)

However, this results in an error: KeyError: 'Column not found: 0'

More generally speaking, what kind of interactions does the dask GroupBy object allow, except from the apply method?

回答1:

you could iterate through groups doing this with dask, maybe there is a better way but this works for me.

import dask.dataframe as dd
import pandas as pd
pdf = pd.DataFrame({'A':[1, 2, 3, 4, 5], 'B':['1','1','a','a','a']})
ddf = dd.from_pandas(pdf, npartitions = 3)
groups = ddf.groupby('B')

for group in pdf['B'].unique():
    print groups.get_group(group)

this would return

dd.DataFrame<dataframe-groupby-get_group-e3ebb5d5a6a8001da9bb7653fface4c1, divisions=(0, 2, 4, 4)>
dd.DataFrame<dataframe-groupby-get_group-022502413b236592cf7d54b2dccf10a9, divisions=(0, 2, 4, 4)>

回答2:

Generally iterating over Dask.dataframe objects is not recommended. It is inefficient. Instead you might want to try constructing a function and mapping that function over the resulting groups using groupby.apply

来源：https://stackoverflow.com/questions/39731098/iterate-over-groupby-object-in-dask