Index.reduction(chunk, aggregate=None, combine=None, meta=_NoDefault.no_default, token=None, split_every=None, chunk_kwargs=None, aggregate_kwargs=None, combine_kwargs=None, **kwargs)

Generic row-wise reductions.


Function to operate on each partition. Should return a pandas.DataFrame, pandas.Series, or a scalar.

aggregatecallable, optional

Function to operate on the concatenated result of chunk. If not specified, defaults to chunk. Used to do the final aggregation in a tree reduction.

The input to aggregate depends on the output of chunk. If the output of chunk is a:

  • scalar: Input is a Series, with one row per partition.

  • Series: Input is a DataFrame, with one row per partition. Columns are the rows in the output series.

  • DataFrame: Input is a DataFrame, with one row per partition. Columns are the columns in the output dataframes.

Should return a pandas.DataFrame, pandas.Series, or a scalar.

combinecallable, optional

Function to operate on intermediate concatenated results of chunk in a tree-reduction. If not provided, defaults to aggregate. The input/output requirements should match that of aggregate described above.

metapd.DataFrame, pd.Series, dict, iterable, tuple, optional

An empty pd.DataFrame or pd.Series that matches the dtypes and column names of the output. This metadata is necessary for many algorithms in dask dataframe to work. For ease of use, some alternative inputs are also available. Instead of a DataFrame, a dict of {name: dtype} or iterable of (name, dtype) can be provided (note that the order of the names should match the order of the columns). Instead of a series, a tuple of (name, dtype) can be used. If not provided, dask will try to infer the metadata. This may lead to unexpected results, so providing meta is recommended. For more information, see dask.dataframe.utils.make_meta.

tokenstr, optional

The name to use for the output keys.

split_everyint, optional

Group partitions into groups of this size while performing a tree-reduction. If set to False, no tree-reduction will be used, and all intermediates will be concatenated and passed to aggregate. Default is 8.

chunk_kwargsdict, optional

Keyword arguments to pass on to chunk only.

aggregate_kwargsdict, optional

Keyword arguments to pass on to aggregate only.

combine_kwargsdict, optional

Keyword arguments to pass on to combine only.


All remaining keywords will be passed to chunk, combine, and aggregate.


>>> import pandas as pd
>>> import dask.dataframe as dd
>>> df = pd.DataFrame({'x': range(50), 'y': range(50, 100)})
>>> ddf = dd.from_pandas(df, npartitions=4)

Count the number of rows in a DataFrame. To do this, count the number of rows in each partition, then sum the results:

>>> res = ddf.reduction(lambda x: x.count(),
...                     aggregate=lambda x: x.sum())
>>> res.compute()
x    50
y    50
dtype: int64

Count the number of rows in a Series with elements greater than or equal to a value (provided via a keyword).

>>> def count_greater(x, value=0):
...     return (x >= value).sum()
>>> res = ddf.x.reduction(count_greater, aggregate=lambda x: x.sum(),
...                       chunk_kwargs={'value': 25})
>>> res.compute()

Aggregate both the sum and count of a Series at the same time:

>>> def sum_and_count(x):
...     return pd.Series({'count': x.count(), 'sum': x.sum()},
...                      index=['count', 'sum'])
>>> res = ddf.x.reduction(sum_and_count, aggregate=lambda x: x.sum())
>>> res.compute()
count      50
sum      1225
dtype: int64

Doing the same, but for a DataFrame. Here chunk returns a DataFrame, meaning the input to aggregate is a DataFrame with an index with non-unique entries for both ‘x’ and ‘y’. We groupby the index, and sum each group to get the final result.

>>> def sum_and_count(x):
...     return pd.DataFrame({'count': x.count(), 'sum': x.sum()},
...                         columns=['count', 'sum'])
>>> res = ddf.reduction(sum_and_count,
...                     aggregate=lambda x: x.groupby(level=0).sum())
>>> res.compute()
   count   sum
x     50  1225
y     50  3725