Load and Save Data with Dask DataFrames
=======================================

.. meta::
    :description: Learn how to create DataFrames and store them. Create a Dask DataFrame from various data storage formats like CSV, HDF, Apache Parquet, and others.

You can create a Dask DataFrame from various data storage formats
like CSV, HDF, Apache Parquet, and others. For most formats, this data can live on various
storage systems including local disk, network file systems (NFS), the Hadoop
Distributed File System (HDFS), Google Cloud Storage, and Amazon S3
(excepting HDF, which is only available on POSIX like file systems).

See the :doc:`DataFrame overview page <dataframe>` for more on
``dask.dataframe`` scope, use, and limitations and
:doc:`DataFrame Best Practices <dataframe-best-practices>` for more tips
and solutions to common problems.

API
---

The following functions provide access to convert between Dask DataFrames,
file formats, and other Dask or Python collections.

.. currentmodule:: dask.dataframe

File Formats:

.. autosummary::
    read_csv
    read_parquet
    read_hdf
    read_orc
    read_json
    read_sql_table
    read_sql_query
    read_sql
    read_table
    read_fwf
    from_array
    to_csv
    to_parquet
    to_hdf
    to_sql

Dask Collections:

.. autosummary::
    from_delayed
    from_dask_array
    from_map
    dask.bag.core.Bag.to_dataframe
    DataFrame.to_delayed
    to_records
    to_bag

Pandas:

.. autosummary::
    from_pandas
    DataFrame.from_dict

Other File Formats:

- `Snowflake <https://github.com/coiled/dask-snowflake>`_
- `Bigquery <https://github.com/coiled/dask-bigquery>`_
- `Delta Lake <https://github.com/dask-contrib/dask-deltatable>`_

Creating
--------

Read from CSV
~~~~~~~~~~~~~

You can use :func:`read_csv` to read one or more CSV files into a Dask DataFrame.
It supports loading multiple files at once using globstrings:

.. code-block:: python

   >>> df = dd.read_csv('myfiles.*.csv')

You can break up a single large file with the ``blocksize`` parameter:

.. code-block:: python

   >>> df = dd.read_csv('largefile.csv', blocksize=25e6)  # 25MB chunks

Changing the ``blocksize`` parameter will change the number of partitions (see the explanation on
:ref:`partitions <dataframe-design-partitions>`). A good rule of thumb when working with
Dask DataFrames is to keep your partitions under 100MB in size.

Read from Parquet
~~~~~~~~~~~~~~~~~

Similarly, you can use :func:`read_parquet` for reading one or more Parquet files.
You can read in a single Parquet file:

.. code-block:: python

   >>> df = dd.read_parquet("path/to/mydata.parquet")

Or a directory of local Parquet files:

.. code-block:: python

   >>> df = dd.read_parquet("path/to/my/parquet/")

For more details on working with Parquet files, including tips and best practices, see the documentation
on :doc:`dataframe-parquet`.

Read from cloud storage
~~~~~~~~~~~~~~~~~~~~~~~

Dask can read data from a variety of data stores including cloud object stores.
You can do this by prepending a protocol like ``s3://`` to paths
used in common data access functions like ``dd.read_csv``:

.. code-block:: python

   >>> df = dd.read_csv('s3://bucket/path/to/data-*.csv')
   >>> df = dd.read_parquet('gcs://bucket/path/to/data-*.parq')

For remote systems like Amazon S3 or Google Cloud Storage, you may need to provide credentials.
These are usually stored in a configuration file, but in some cases you may want to pass
storage-specific options through to the storage backend.
You can do this with the ``storage_options`` parameter:

.. code-block:: python

   >>> df = dd.read_csv('s3://bucket-name/my-data-*.csv',
   ...                  storage_options={'anon': True})
   >>> df = dd.read_parquet('gs://dask-nyc-taxi/yellowtrip.parquet',
   ...                      storage_options={'token': 'anon'})

See the documentation on connecting to :ref:`Amazon S3 <connect-to-remote-data-s3>` or
:ref:`Google Cloud Storage <connect-to-remote-data-gc>`.

Mapping from a function
~~~~~~~~~~~~~~~~~~~~~~~

For cases that are not covered by the functions above, but *can* be
captured by a simple ``map`` operation, :func:`from_map` is likely to be
the most convenient means for DataFrame creation. For example, this
API can be used to convert an arbitrary PyArrow ``Dataset`` object into a
DataFrame collection by mapping fragments to DataFrame partitions:

.. code-block:: python

   >>> import pyarrow.dataset as ds
   >>> dataset = ds.dataset("hive_data_path", format="orc", partitioning="hive")
   >>> fragments = dataset.get_fragments()
   >>> func = lambda frag: frag.to_table().to_pandas()
   >>> df = dd.from_map(func, fragments)


Dask Delayed
~~~~~~~~~~~~

:doc:`Dask delayed<delayed>` is particularly useful when simple ``map`` operations aren’t sufficient to capture the complexity of your data layout. It lets you construct Dask DataFrames out of arbitrary Python function calls, which can be
helpful to handle custom data formats or bake in particular logic around loading data.
See the :doc:`documentation on using dask.delayed with collections<delayed-collections>`.

Storing
-------

Writing files locally
~~~~~~~~~~~~~~~~~~~~~

You can save files locally, assuming each worker can access the same file system. The workers could be located on the same machine, or a network file system can be mounted and referenced at the same path location for every worker node.
See the documentation on :ref:`accessing data locally <connect-to-remote-data-local>`.

Writing to remote locations
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Dask can write to a variety of data stores including cloud object stores.
For example, you can write a ``dask.dataframe`` to an Azure storage blob as:

.. code-block:: python

   >>> d = {'col1': [1, 2, 3, 4], 'col2': [5, 6, 7, 8]}
   >>> df = dd.from_pandas(pd.DataFrame(data=d), npartitions=2)
   >>> dd.to_parquet(df=df,
   ...               path='abfs://CONTAINER/FILE.parquet'
   ...               storage_options={'account_name': 'ACCOUNT_NAME',
   ...                                'account_key': 'ACCOUNT_KEY'}

See the :doc:`how-to guide on connecting to remote data <how-to/connect-to-remote-data>`.
