dask.dataframe.Series.map_overlap

dask.dataframe.Series.map_overlap

Series.map_overlap(func, before, after, *args, **kwargs)

Apply a function to each partition, sharing rows with adjacent partitions.

This can be useful for implementing windowing functions such as df.rolling(...).mean() or df.diff().

Parameters
funcfunction

Function applied to each partition.

beforeint, timedelta or string timedelta

The rows to prepend to partition i from the end of partition i - 1.

afterint, timedelta or string timedelta

The rows to append to partition i from the beginning of partition i + 1.

args, kwargs

Positional and keyword arguments to pass to the function. Positional arguments are computed on a per-partition basis, while keyword arguments are shared across all partitions. The partition itself will be the first positional argument, with all other arguments passed after. Arguments can be Scalar, Delayed, or regular Python objects. DataFrame-like args (both dask and pandas) will be repartitioned to align (if necessary) before applying the function; see align_dataframes to control this behavior.

enforce_metadatabool, default True

Whether to enforce at runtime that the structure of the DataFrame produced by func actually matches the structure of meta. This will rename and reorder columns for each partition, and will raise an error if this doesn’t work, but it won’t raise if dtypes don’t match.

transform_divisionsbool, default True

Whether to apply the function onto the divisions and apply those transformed divisions to the output.

align_dataframesbool, default True

Whether to repartition DataFrame- or Series-like args (both dask and pandas) so their divisions align before applying the function. This requires all inputs to have known divisions. Single-partition inputs will be split into multiple partitions.

If False, all inputs must have either the same number of partitions or a single partition. Single-partition inputs will be broadcast to every partition of multi-partition inputs.

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.

Notes

Given positive integers before and after, and a function func, map_overlap does the following:

  1. Prepend before rows to each partition i from the end of partition i - 1. The first partition has no rows prepended.

  2. Append after rows to each partition i from the beginning of partition i + 1. The last partition has no rows appended.

  3. Apply func to each partition, passing in any extra args and kwargs if provided.

  4. Trim before rows from the beginning of all but the first partition.

  5. Trim after rows from the end of all but the last partition.

Examples

Given a DataFrame, Series, or Index, such as:

>>> import pandas as pd
>>> import dask.dataframe as dd
>>> df = pd.DataFrame({'x': [1, 2, 4, 7, 11],
...                    'y': [1., 2., 3., 4., 5.]})
>>> ddf = dd.from_pandas(df, npartitions=2)

A rolling sum with a trailing moving window of size 2 can be computed by overlapping 2 rows before each partition, and then mapping calls to df.rolling(2).sum():

>>> ddf.compute()
    x    y
0   1  1.0
1   2  2.0
2   4  3.0
3   7  4.0
4  11  5.0
>>> ddf.map_overlap(lambda df: df.rolling(2).sum(), 2, 0).compute()
      x    y
0   NaN  NaN
1   3.0  3.0
2   6.0  5.0
3  11.0  7.0
4  18.0  9.0

The pandas diff method computes a discrete difference shifted by a number of periods (can be positive or negative). This can be implemented by mapping calls to df.diff to each partition after prepending/appending that many rows, depending on sign:

>>> def diff(df, periods=1):
...     before, after = (periods, 0) if periods > 0 else (0, -periods)
...     return df.map_overlap(lambda df, periods=1: df.diff(periods),
...                           periods, 0, periods=periods)
>>> diff(ddf, 1).compute()
     x    y
0  NaN  NaN
1  1.0  1.0
2  2.0  1.0
3  3.0  1.0
4  4.0  1.0

If you have a DatetimeIndex, you can use a pd.Timedelta for time- based windows or any pd.Timedelta convertible string:

>>> ts = pd.Series(range(10), index=pd.date_range('2017', periods=10))
>>> dts = dd.from_pandas(ts, npartitions=2)
>>> dts.map_overlap(lambda df: df.rolling('2D').sum(),
...                 pd.Timedelta('2D'), 0).compute()
2017-01-01     0.0
2017-01-02     1.0
2017-01-03     3.0
2017-01-04     5.0
2017-01-05     7.0
2017-01-06     9.0
2017-01-07    11.0
2017-01-08    13.0
2017-01-09    15.0
2017-01-10    17.0
Freq: D, dtype: float64

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