Working with missing data

pandas uses different sentinel values to represent a missing (also referred to as NA)
depending on the data type.

``numpy.nan`` for NumPy data types. The disadvantage of using NumPy data types
is that the original data type will be coerced to ``np.float64`` or ``object``.

.. ipython:: python

   pd.Series([1, 2], dtype=np.int64).reindex([0, 1, 2])
   pd.Series([True, False], dtype=np.bool_).reindex([0, 1, 2])

:class:`NaT` for NumPy ``np.datetime64``, ``np.timedelta64``, and :class:`PeriodDtype`. For typing applications,
use :class:`api.typing.NaTType`.

.. ipython:: python

   pd.Series([1, 2], dtype=np.dtype("timedelta64[ns]")).reindex([0, 1, 2])
   pd.Series([1, 2], dtype=np.dtype("datetime64[ns]")).reindex([0, 1, 2])
   pd.Series(["2020", "2020"], dtype=pd.PeriodDtype("D")).reindex([0, 1, 2])

:class:`NA` for :class:`StringDtype`, :class:`Int64Dtype` (and other bit widths),
:class:`Float64Dtype` (and other bit widths), :class:`BooleanDtype` and :class:`ArrowDtype`.
These types will maintain the original data type of the data.
For typing applications, use :class:`api.typing.NAType`.

.. ipython:: python

   pd.Series([1, 2], dtype="Int64").reindex([0, 1, 2])
   pd.Series([True, False], dtype="boolean[pyarrow]").reindex([0, 1, 2])

To detect these missing value, use the :func:`isna` or :func:`notna` methods.

.. ipython:: python

   ser = pd.Series([pd.Timestamp("2020-01-01"), pd.NaT])
   ser
   pd.isna(ser)


.. note::

   :func:`isna` or :func:`notna` will also consider ``None`` a missing value.

   .. ipython:: python

      ser = pd.Series([1, None], dtype=object)
      ser
      pd.isna(ser)

.. warning::

   Equality comparisons between ``np.nan``, :class:`NaT`, and :class:`NA`
   do not act like ``None``

   .. ipython:: python

      None == None  # noqa: E711
      np.nan == np.nan
      pd.NaT == pd.NaT
      pd.NA == pd.NA

   Therefore, an equality comparison between a :class:`DataFrame` or :class:`Series`
   with one of these missing values does not provide the same information as
   :func:`isna` or :func:`notna`.

   .. ipython:: python

      ser = pd.Series([True, None], dtype="boolean[pyarrow]")
      ser == pd.NA
      pd.isna(ser)


.. _missing_data.NA:

:class:`NA` semantics
~~~~~~~~~~~~~~~~~~~~~

.. warning::

   Experimental: the behaviour of :class:`NA` can still change without warning.

Starting from pandas 1.0, an experimental :class:`NA` value (singleton) is
available to represent scalar missing values. The goal of :class:`NA` is provide a
"missing" indicator that can be used consistently across data types
(instead of ``np.nan``, ``None`` or ``pd.NaT`` depending on the data type).

For example, when having missing values in a :class:`Series` with the nullable integer
dtype, it will use :class:`NA`:

.. ipython:: python

    s = pd.Series([1, 2, None], dtype="Int64")
    s
    s[2]
    s[2] is pd.NA

Currently, pandas does not use those data types using :class:`NA` by default in
a :class:`DataFrame` or :class:`Series`, so you need to specify
the dtype explicitly. An easy way to convert to those dtypes is explained in the
:ref:`conversion section <missing_data.NA.conversion>`.

Propagation in arithmetic and comparison operations
---------------------------------------------------

In general, missing values *propagate* in operations involving :class:`NA`. When
one of the operands is unknown, the outcome of the operation is also unknown.

For example, :class:`NA` propagates in arithmetic operations, similarly to
``np.nan``:

.. ipython:: python

   pd.NA + 1
   "a" * pd.NA

There are a few special cases when the result is known, even when one of the
operands is ``NA``.

.. ipython:: python

   pd.NA ** 0
   1 ** pd.NA

In equality and comparison operations, :class:`NA` also propagates. This deviates
from the behaviour of ``np.nan``, where comparisons with ``np.nan`` always
return ``False``.

.. ipython:: python

   pd.NA == 1
   pd.NA == pd.NA
   pd.NA < 2.5

To check if a value is equal to :class:`NA`, use :func:`isna`

.. ipython:: python

   pd.isna(pd.NA)


.. note::

   An exception on this basic propagation rule are *reductions* (such as the
   mean or the minimum), where pandas defaults to skipping missing values. See the
   :ref:`calculation section <missing_data.calculations>` for more.

Logical operations
------------------

For logical operations, :class:`NA` follows the rules of the
`three-valued logic <https://en.wikipedia.org/wiki/Three-valued_logic>`__ (or
*Kleene logic*, similarly to R, SQL and Julia). This logic means to only
propagate missing values when it is logically required.

For example, for the logical "or" operation (``|``), if one of the operands
is ``True``, we already know the result will be ``True``, regardless of the
other value (so regardless the missing value would be ``True`` or ``False``).
In this case, :class:`NA` does not propagate:

.. ipython:: python

   True | False
   True | pd.NA
   pd.NA | True

On the other hand, if one of the operands is ``False``, the result depends
on the value of the other operand. Therefore, in this case :class:`NA`
propagates:

.. ipython:: python

   False | True
   False | False
   False | pd.NA

The behaviour of the logical "and" operation (``&``) can be derived using
similar logic (where now :class:`NA` will not propagate if one of the operands
is already ``False``):

.. ipython:: python

   False & True
   False & False
   False & pd.NA

.. ipython:: python

   True & True
   True & False
   True & pd.NA


``NA`` in a boolean context
---------------------------

Since the actual value of an NA is unknown, it is ambiguous to convert NA
to a boolean value.

.. ipython:: python
   :okexcept:

   bool(pd.NA)

This also means that :class:`NA` cannot be used in a context where it is
evaluated to a boolean, such as ``if condition: ...`` where ``condition`` can
potentially be :class:`NA`. In such cases, :func:`isna` can be used to check
for :class:`NA` or ``condition`` being :class:`NA` can be avoided, for example by
filling missing values beforehand.

A similar situation occurs when using :class:`Series` or :class:`DataFrame` objects in ``if``
statements, see :ref:`gotchas.truth`.

NumPy ufuncs
------------

:attr:`pandas.NA` implements NumPy's ``__array_ufunc__`` protocol. Most ufuncs
work with ``NA``, and generally return ``NA``:

.. ipython:: python

   np.log(pd.NA)
   np.add(pd.NA, 1)

.. warning::

   Currently, ufuncs involving an ndarray and ``NA`` will return an
   object-dtype filled with NA values.

   .. ipython:: python

      a = np.array([1, 2, 3])
      np.greater(a, pd.NA)

   The return type here may change to return a different array type
   in the future.

See :ref:`dsintro.numpy_interop` for more on ufuncs.

.. _missing_data.NA.conversion:

Conversion
^^^^^^^^^^

If you have a :class:`DataFrame` or :class:`Series` using ``np.nan``,
:meth:`DataFrame.convert_dtypes` and :meth:`Series.convert_dtypes`, respectively,
will convert your data to use the nullable data types supporting :class:`NA`,
such as :class:`Int64Dtype` or :class:`ArrowDtype`. This is especially helpful after reading
in data sets from IO methods where data types were inferred.

.. ipython:: python

   import io
   data = io.StringIO("a,b\n,True\n2,")
   df = pd.read_csv(data)
   df.dtypes
   df_conv = df.convert_dtypes()
   df_conv
   df_conv.dtypes

.. _missing.inserting:

Inserting missing data
~~~~~~~~~~~~~~~~~~~~~~

You can insert missing values by simply assigning to a :class:`Series` or :class:`DataFrame`.
The missing value sentinel used will be chosen based on the dtype.

.. ipython:: python

   ser = pd.Series([1., 2., 3.])
   ser.loc[0] = None
   ser

   ser = pd.Series([pd.Timestamp("2021"), pd.Timestamp("2021")])
   ser.iloc[0] = np.nan
   ser

   ser = pd.Series([True, False], dtype="boolean[pyarrow]")
   ser.iloc[0] = None
   ser

For ``object`` types, pandas will use the value given:

.. ipython:: python

   s = pd.Series(["a", "b", "c"], dtype=object)
   s.loc[0] = None
   s.loc[1] = np.nan
   s

.. _missing_data.calculations:

Calculations with missing data

:meth:`~DataFrame.dropna` drops rows or columns with missing data.

.. ipython:: python

   df = pd.DataFrame([[np.nan, 1, 2], [1, 2, np.nan], [1, 2, 3]])
   df
   df.dropna()
   df.dropna(axis=1)

   ser = pd.Series([1, pd.NA], dtype="int64[pyarrow]")
   ser.dropna()

Filling missing data
~~~~~~~~~~~~~~~~~~~~

.. _missing_data.fillna:

Filling by value
----------------

:meth:`~DataFrame.fillna` replaces NA values with non-NA data.

Replace NA with a scalar value

.. ipython:: python

   data = {"np": [1.0, np.nan, np.nan, 2], "arrow": pd.array([1.0, pd.NA, pd.NA, 2], dtype="float64[pyarrow]")}
   df = pd.DataFrame(data)
   df
   df.fillna(0)

When the data has object dtype, you can control what type of NA values are present.

.. ipython:: python

   df = pd.DataFrame({"a": [pd.NA, np.nan, None]}, dtype=object)
   df
   df.fillna(None)
   df.fillna(np.nan)
   df.fillna(pd.NA)

However when the dtype is not object, these will all be replaced with the proper NA value for the dtype.

.. ipython:: python

   data = {"np": [1.0, np.nan, np.nan, 2], "arrow": pd.array([1.0, pd.NA, pd.NA, 2], dtype="float64[pyarrow]")}
   df = pd.DataFrame(data)
   df
   df.fillna(None)
   df.fillna(np.nan)
   df.fillna(pd.NA)

Fill gaps forward or backward

.. ipython:: python

   df.ffill()
   df.bfill()

.. _missing_data.fillna.limit:

Limit the number of NA values filled

.. ipython:: python

   df.ffill(limit=1)

NA values can be replaced with corresponding value from a :class:`Series` or :class:`DataFrame`
where the index and column aligns between the original object and the filled object.

.. ipython:: python

   dff = pd.DataFrame(np.arange(30, dtype=np.float64).reshape(10, 3), columns=list("ABC"))
   dff.iloc[3:5, 0] = np.nan
   dff.iloc[4:6, 1] = np.nan
   dff.iloc[5:8, 2] = np.nan
   dff
   dff.fillna(dff.mean())

.. note::

   :meth:`DataFrame.where` can also be used to fill NA values. Same result as above.

   .. ipython:: python

      dff.where(pd.notna(dff), dff.mean(), axis="columns")


.. _missing_data.interpolate:

Interpolation
-------------

:meth:`DataFrame.interpolate` and :meth:`Series.interpolate` fills NA values
using various interpolation methods.

.. ipython:: python

   df = pd.DataFrame(
       {
           "A": [1, 2.1, np.nan, 4.7, 5.6, 6.8],
           "B": [0.25, np.nan, np.nan, 4, 12.2, 14.4],
       }
   )
   df
   df.interpolate()

   idx = pd.date_range("2020-01-01", periods=10, freq="D")
   data = np.random.default_rng(2).integers(0, 10, 10).astype(np.float64)
   ts = pd.Series(data, index=idx)
   ts.iloc[[1, 2, 5, 6, 9]] = np.nan

   ts
   @savefig series_before_interpolate.png
   ts.plot()

.. ipython:: python

   ts.interpolate()
   @savefig series_interpolate.png
   ts.interpolate().plot()

Interpolation relative to a :class:`Timestamp` in the :class:`DatetimeIndex`
is available by setting ``method="time"``

.. ipython:: python

   ts2 = ts.iloc[[0, 1, 3, 7, 9]]
   ts2
   ts2.interpolate()
   ts2.interpolate(method="time")

For a floating-point index, use ``method='values'``:

.. ipython:: python

   idx = [0.0, 1.0, 10.0]
   ser = pd.Series([0.0, np.nan, 10.0], idx)
   ser
   ser.interpolate()
   ser.interpolate(method="values")

If you have scipy_ installed, you can pass the name of a 1-d interpolation routine to ``method``.
as specified in the scipy interpolation documentation_ and reference guide_.
The appropriate interpolation method will depend on the data type.

.. tip::

   If you are dealing with a time series that is growing at an increasing rate,
   use ``method='barycentric'``.

   If you have values approximating a cumulative distribution function,
   use ``method='pchip'``.

   To fill missing values with goal of smooth plotting use ``method='akima'``.

   .. ipython:: python

      df = pd.DataFrame(
         {
            "A": [1, 2.1, np.nan, 4.7, 5.6, 6.8],
            "B": [0.25, np.nan, np.nan, 4, 12.2, 14.4],
         }
      )
      df
      df.interpolate(method="barycentric")
      df.interpolate(method="pchip")
      df.interpolate(method="akima")

When interpolating via a polynomial or spline approximation, you must also specify
the degree or order of the approximation:

.. ipython:: python

   df.interpolate(method="spline", order=2)
   df.interpolate(method="polynomial", order=2)

Comparing several methods.

.. ipython:: python

   np.random.seed(2)

   ser = pd.Series(np.arange(1, 10.1, 0.25) ** 2 + np.random.randn(37))
   missing = np.array([4, 13, 14, 15, 16, 17, 18, 20, 29])
   ser.iloc[missing] = np.nan
   methods = ["linear", "quadratic", "cubic"]

   df = pd.DataFrame({m: ser.interpolate(method=m) for m in methods})
   @savefig compare_interpolations.png
   df.plot()

Interpolating new observations from expanding data with :meth:`Series.reindex`.

.. ipython:: python

   ser = pd.Series(np.sort(np.random.uniform(size=100)))

   # interpolate at new_index
   new_index = ser.index.union(pd.Index([49.25, 49.5, 49.75, 50.25, 50.5, 50.75]))
   interp_s = ser.reindex(new_index).interpolate(method="pchip")
   interp_s.loc[49:51]

.. _scipy: https://scipy.org/
.. _documentation: https://docs.scipy.org/doc/scipy/reference/interpolate.html#univariate-interpolation
.. _guide: https://docs.scipy.org/doc/scipy/tutorial/interpolate.html

.. _missing_data.interp_limits:

Interpolation limits
^^^^^^^^^^^^^^^^^^^^

:meth:`~DataFrame.interpolate` accepts a ``limit`` keyword
argument to limit the number of consecutive ``NaN`` values
filled since the last valid observation

.. ipython:: python

   ser = pd.Series([np.nan, np.nan, 5, np.nan, np.nan, np.nan, 13, np.nan, np.nan])
   ser
   ser.interpolate()
   ser.interpolate(limit=1)

By default, ``NaN`` values are filled in a ``forward`` direction. Use
``limit_direction`` parameter to fill ``backward`` or from ``both`` directions.

.. ipython:: python

   ser.interpolate(limit=1, limit_direction="backward")
   ser.interpolate(limit=1, limit_direction="both")
   ser.interpolate(limit_direction="both")

By default, ``NaN`` values are filled whether they are surrounded by
existing valid values or outside existing valid values. The ``limit_area``
parameter restricts filling to either inside or outside values.

.. ipython:: python

   # fill one consecutive inside value in both directions
   ser.interpolate(limit_direction="both", limit_area="inside", limit=1)

   # fill all consecutive outside values backward
   ser.interpolate(limit_direction="backward", limit_area="outside")

   # fill all consecutive outside values in both directions
   ser.interpolate(limit_direction="both", limit_area="outside")

.. _missing_data.replace:

Replacing values
----------------

:meth:`Series.replace` and :meth:`DataFrame.replace` can be used similar to
:meth:`Series.fillna` and :meth:`DataFrame.fillna` to replace or insert missing values.

.. ipython:: python

   df = pd.DataFrame(np.eye(3))
   df
   df_missing = df.replace(0, np.nan)
   df_missing
   df_filled = df_missing.replace(np.nan, 2)
   df_filled

Replacing more than one value is possible by passing a list.

.. ipython:: python

   df_filled.replace([1, 44], [2, 28])

Replacing using a mapping dict.

.. ipython:: python

   df_filled.replace({1: 44, 2: 28})

.. _missing_data.replace_expression:

Regular expression replacement
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. note::

   Python strings prefixed with the ``r`` character such as ``r'hello world'``
   are `"raw" strings <https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals>`_.
   They have different semantics regarding backslashes than strings without this prefix.
   Backslashes in raw strings  will be interpreted as an escaped backslash, e.g., ``r'\' == '\\'``.

Replace the '.' with ``NaN``

.. ipython:: python

   d = {"a": list(range(4)), "b": list("ab.."), "c": ["a", "b", np.nan, "d"]}
   df = pd.DataFrame(d)
   df.replace(".", np.nan)

Replace the '.' with ``NaN`` with regular expression that removes surrounding whitespace

.. ipython:: python

   df.replace(r"\s*\.\s*", np.nan, regex=True)

Replace with a list of regexes.

.. ipython:: python

   df.replace([r"\.", r"(a)"], ["dot", r"\1stuff"], regex=True)

Replace with a regex in a mapping dict.

.. ipython:: python

   df.replace({"b": r"\s*\.\s*"}, {"b": np.nan}, regex=True)

Pass nested dictionaries of regular expressions that use the ``regex`` keyword.

.. ipython:: python

   df.replace({"b": {"b": r""}}, regex=True)
   df.replace(regex={"b": {r"\s*\.\s*": np.nan}})
   df.replace({"b": r"\s*(\.)\s*"}, {"b": r"\1ty"}, regex=True)

Pass a list of regular expressions that will replace matches with a scalar.

.. ipython:: python

   df.replace([r"\s*\.\s*", r"a|b"], "placeholder", regex=True)

All of the regular expression examples can also be passed with the
``to_replace`` argument as the ``regex`` argument. In this case the ``value``
argument must be passed explicitly by name or ``regex`` must be a nested
dictionary.

.. ipython:: python

   df.replace(regex=[r"\s*\.\s*", r"a|b"], value="placeholder")

.. note::

   A regular expression object from ``re.compile`` is a valid input as well.