dask.dataframe.multi.concat(dfs, axis=0, join='outer', interleave_partitions=False, ignore_unknown_divisions=False, ignore_order=False, **kwargs)[source]

Concatenate DataFrames along rows.

  • When axis=0 (default), concatenate DataFrames row-wise:

    • If all divisions are known and ordered, concatenate DataFrames keeping divisions. When divisions are not ordered, specifying interleave_partition=True allows concatenate divisions each by each.

    • If any of division is unknown, concatenate DataFrames resetting its division to unknown (None)

  • When axis=1, concatenate DataFrames column-wise:

    • Allowed if all divisions are known.

    • If any of division is unknown, it raises ValueError.


List of dask.DataFrames to be concatenated

axis{0, 1, ‘index’, ‘columns’}, default 0

The axis to concatenate along

join{‘inner’, ‘outer’}, default ‘outer’

How to handle indexes on other axis

interleave_partitionsbool, default False

Whether to concatenate DataFrames ignoring its order. If True, every divisions are concatenated each by each.

ignore_unknown_divisionsbool, default False

By default a warning is raised if any input has unknown divisions. Set to True to disable this warning.

ignore_orderbool, default False

Whether to ignore order when doing the union of categoricals.


This differs in from pd.concat in the when concatenating Categoricals with different categories. Pandas currently coerces those to objects before concatenating. Coercing to objects is very expensive for large arrays, so dask preserves the Categoricals by taking the union of the categories.


If all divisions are known and ordered, divisions are kept.

>>> import dask.dataframe as dd
>>> a                                               
dd.DataFrame<x, divisions=(1, 3, 5)>
>>> b                                               
dd.DataFrame<y, divisions=(6, 8, 10)>
>>> dd.concat([a, b])                               
dd.DataFrame<concat-..., divisions=(1, 3, 6, 8, 10)>

Unable to concatenate if divisions are not ordered.

>>> a                                               
dd.DataFrame<x, divisions=(1, 3, 5)>
>>> b                                               
dd.DataFrame<y, divisions=(2, 3, 6)>
>>> dd.concat([a, b])                               
ValueError: All inputs have known divisions which cannot be concatenated
in order. Specify interleave_partitions=True to ignore order

Specify interleave_partitions=True to ignore the division order.

>>> dd.concat([a, b], interleave_partitions=True)   
dd.DataFrame<concat-..., divisions=(1, 2, 3, 5, 6)>

If any of division is unknown, the result division will be unknown

>>> a                                               
dd.DataFrame<x, divisions=(None, None)>
>>> b                                               
dd.DataFrame<y, divisions=(1, 4, 10)>
>>> dd.concat([a, b])                               
dd.DataFrame<concat-..., divisions=(None, None, None, None)>

By default concatenating with unknown divisions will raise a warning. Set ignore_unknown_divisions=True to disable this:

>>> dd.concat([a, b], ignore_unknown_divisions=True)
dd.DataFrame<concat-..., divisions=(None, None, None, None)>

Different categoricals are unioned

>>> dd.concat([
...     dd.from_pandas(pd.Series(['a', 'b'], dtype='category'), 1),
...     dd.from_pandas(pd.Series(['a', 'c'], dtype='category'), 1),
... ], interleave_partitions=True).dtype
CategoricalDtype(categories=['a', 'b', 'c'], ordered=False)