How to transform Dask.DataFrame to pd.DataFrame?

Question

How can I transform my resulting dask.DataFrame into pandas.DataFrame (let's say I am done with heavy lifting, and just want to apply sklearn to my aggregate result)?

Solution 1

You can call the .compute() method to transform a dask.dataframe to a pandas dataframe:

df = df.compute()

Solution 2

MRocklin's answer is correct and this answer gives more details on when it's appropriate to convert from a Dask DataFrame to and Pandas DataFrame (and how to predict when it'll cause problems).

Each partition in a Dask DataFrame is a Pandas DataFrame. Running df.compute() will coalesce all the underlying partitions in the Dask DataFrame into a single Pandas DataFrame. That'll cause problems if the size of the Pandas DataFrame is bigger than the RAM on your machine.

If df has 30 GB of data and your computer has 16 GB of RAM, then df.compute() will blow up with a memory error. If df only has 1 GB of data, then you'll be fine.

You can run df.memory_usage(deep=True).sum() to compute the amount of memory that your DataFrame is using. This'll let you know if your DataFrame is sufficiently small to be coalesced into a single Pandas DataFrame.

Repartioning changes the number of underlying partitions in a Dask DataFrame. df.repartition(1).partitions[0] is conceptually similar to df.compute().

Converting to a Pandas DataFrame is especially possible after performing a big filtering operation. If you filter a 100 billion row dataset down to 10 thousand rows, then you can probably just switch to the Pandas API.

Comments

[Alexey Grigorev]

Would it be possible to rename it to something more intuitive, e.g. to_pandas()?

[MRocklin]

Probably not, no. .compute() is pretty standard among all dask collections.

[NirIzr]

.to_compute() is actually quite intuitive to anyone working with dask.

[Pyd]

@MRocklin, I am reading all the csv's from a folder and I cannot explicitly mention each column names and its dtypes. and merging all df's to single df on a common column. when I m doing df.compute, I get ValueError: The columns in the computed data do not match the columns in the provided metadata, how to handle this

[sunt05]

@pyd, check the meta in read_csv, which can be provided by a normal pandas.read_csv(); but you need to make sure such meta info is consistent across all the files you are reading in.

[closedloop]

In my experience this just returns a dataframe with only the columns names transposed in a single row.