ENH: Add case_when method

doc/source/reference/series.rst

@@ -177,6 +177,7 @@ Reindexing / selection / label manipulation
    :toctree: api/
 
    Series.align
+   Series.case_when
    Series.drop
    Series.droplevel
    Series.drop_duplicates

doc/source/whatsnew/v2.2.0.rst

@@ -188,6 +188,26 @@ For a full list of ADBC drivers and their development status, see the `ADBC Driv
 Implementation Status <https://arrow.apache.org/adbc/current/driver/status.html>`_
 documentation.
 
+.. _whatsnew_220.enhancements.case_when:
+
+Create a pandas Series based on one or more conditions
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The :meth:`Series.case_when` function has been added to create a Series object based on one or more conditions. (:issue:`39154`)
+
+.. ipython:: python
+
+   import pandas as pd
+
+   df = pd.DataFrame(dict(a=[1, 2, 3], b=[4, 5, 6]))
+   default=pd.Series('default', index=df.index)
+   default.case_when(
+        caselist=[
+            (df.a == 1, 'first'),                              # condition, replacement
+            (df.a.gt(1) & df.b.eq(5), 'second'),  # condition, replacement
+        ],
+   )
+
 .. _whatsnew_220.enhancements.to_numpy_ea:
 
 ``to_numpy`` for NumPy nullable and Arrow types converts to suitable NumPy dtype

pandas/core/series.py

@@ -67,6 +67,9 @@
 from pandas.core.dtypes.astype import astype_is_view
 from pandas.core.dtypes.cast import (
     LossySetitemError,
+    construct_1d_arraylike_from_scalar,
+    find_common_type,
+    infer_dtype_from,
     maybe_box_native,
     maybe_cast_pointwise_result,
 )
@@ -84,7 +87,10 @@
     CategoricalDtype,
     ExtensionDtype,
 )
-from pandas.core.dtypes.generic import ABCDataFrame
+from pandas.core.dtypes.generic import (
+    ABCDataFrame,
+    ABCSeries,
+)
 from pandas.core.dtypes.inference import is_hashable
 from pandas.core.dtypes.missing import (
     isna,
@@ -113,6 +119,7 @@
 from pandas.core.arrays.sparse import SparseAccessor
 from pandas.core.arrays.string_ import StringDtype
 from pandas.core.construction import (
+    array as pd_array,
     extract_array,
     sanitize_array,
 )
@@ -5629,6 +5636,121 @@ def between(
 
         return lmask & rmask
 
+    def case_when(
+        self,
+        caselist: list[
+            tuple[
+                ArrayLike | Callable[[Series], Series | np.ndarray | Sequence[bool]],
+                ArrayLike | Scalar | Callable[[Series], Series | np.ndarray],
+            ],
+        ],
+    ) -> Series:
+        """
+        Replace values where the conditions are True.
+
+        Parameters
+        ----------
+        caselist : A list of tuples of conditions and expected replacements
+            Takes the form:  ``(condition0, replacement0)``,
+            ``(condition1, replacement1)``, ... .
+            ``condition`` should be a 1-D boolean array-like object
+            or a callable. If ``condition`` is a callable,
+            it is computed on the Series
+            and should return a boolean Series or array.
+            The callable must not change the input Series
+            (though pandas doesn`t check it). ``replacement`` should be a
+            1-D array-like object, a scalar or a callable.
+            If ``replacement`` is a callable, it is computed on the Series
+            and should return a scalar or Series. The callable
+            must not change the input Series
+            (though pandas doesn`t check it).
+
+            .. versionadded:: 2.2.0
+
+        Returns
+        -------
+        Series
+
+        See Also
+        --------
+        Series.mask : Replace values where the condition is True.
+
+        Examples
+        --------
+        >>> c = pd.Series([6, 7, 8, 9], name='c')
+        >>> a = pd.Series([0, 0, 1, 2])
+        >>> b = pd.Series([0, 3, 4, 5])
+
+        >>> c.case_when(caselist=[(a.gt(0), a),  # condition, replacement
+        ...                       (b.gt(0), b)])
+        0    6
+        1    3
+        2    1
+        3    2
+        Name: c, dtype: int64
+        """
+        if not isinstance(caselist, list):
+            raise TypeError(
+                f"The caselist argument should be a list; instead got {type(caselist)}"
+            )
+
+        if not caselist:
+            raise ValueError(
+                "provide at least one boolean condition, "
+                "with a corresponding replacement."
+            )
+
+        for num, entry in enumerate(caselist):
+            if not isinstance(entry, tuple):
+                raise TypeError(
+                    f"Argument {num} must be a tuple; instead got {type(entry)}."
+                )
+            if len(entry) != 2:
+                raise ValueError(
+                    f"Argument {num} must have length 2; "
+                    "a condition and replacement; "
+                    f"instead got length {len(entry)}."
+                )
+        caselist = [
+            (
+                com.apply_if_callable(condition, self),
+                com.apply_if_callable(replacement, self),
+            )
+            for condition, replacement in caselist
+        ]
+        default = self.copy()
+        conditions, replacements = zip(*caselist)
+        common_dtypes = [infer_dtype_from(arg)[0] for arg in [*replacements, default]]
+        if len(set(common_dtypes)) > 1:
+            common_dtype = find_common_type(common_dtypes)
+            updated_replacements = []
+            for condition, replacement in zip(conditions, replacements):
+                if is_scalar(replacement):
+                    replacement = construct_1d_arraylike_from_scalar(
+                        value=replacement, length=len(condition), dtype=common_dtype
+                    )
+                elif isinstance(replacement, ABCSeries):
+                    replacement = replacement.astype(common_dtype)
+                else:
+                    replacement = pd_array(replacement, dtype=common_dtype)
+                updated_replacements.append(replacement)
+            replacements = updated_replacements
+            default = default.astype(common_dtype)
+
+        counter = reversed(range(len(conditions)))
+        for position, condition, replacement in zip(
+            counter, conditions[::-1], replacements[::-1]
+        ):
+            try:
+                default = default.mask(
+                    condition, other=replacement, axis=0, inplace=False, level=None
+                )
+            except Exception as error:
+                raise ValueError(
+                    f"Failed to apply condition{position} and replacement{position}."
+                ) from error
+        return default
+
     # error: Cannot determine type of 'isna'
     @doc(NDFrame.isna, klass=_shared_doc_kwargs["klass"])  # type: ignore[has-type]
     def isna(self) -> Series:

pandas/tests/series/methods/test_case_when.py

@@ -0,0 +1,148 @@
+import numpy as np
+import pytest
+
+from pandas import (
+    DataFrame,
+    Series,
+    array as pd_array,
+    date_range,
+)
+import pandas._testing as tm
+
+
+@pytest.fixture
+def df():
+    """
+    base dataframe for testing
+    """
+    return DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+
+
+def test_case_when_caselist_is_not_a_list(df):
+    """
+    Raise ValueError if caselist is not a list.
+    """
+    msg = "The caselist argument should be a list; "
+    msg += "instead got.+"
+    with pytest.raises(TypeError, match=msg):  # GH39154
+        df["a"].case_when(caselist=())
+
+
+def test_case_when_no_caselist(df):
+    """
+    Raise ValueError if no caselist is provided.
+    """
+    msg = "provide at least one boolean condition, "
+    msg += "with a corresponding replacement."
+    with pytest.raises(ValueError, match=msg):  # GH39154
+        df["a"].case_when([])
+
+
+def test_case_when_odd_caselist(df):
+    """
+    Raise ValueError if no of caselist is odd.
+    """
+    msg = "Argument 0 must have length 2; "
+    msg += "a condition and replacement; instead got length 3."
+
+    with pytest.raises(ValueError, match=msg):
+        df["a"].case_when([(df["a"].eq(1), 1, df.a.gt(1))])
+
+
+def test_case_when_raise_error_from_mask(df):
+    """
+    Raise Error from within Series.mask
+    """
+    msg = "Failed to apply condition0 and replacement0."
+    with pytest.raises(ValueError, match=msg):
+        df["a"].case_when([(df["a"].eq(1), [1, 2])])
+
+
+def test_case_when_single_condition(df):
+    """
+    Test output on a single condition.
+    """
+    result = Series([np.nan, np.nan, np.nan]).case_when([(df.a.eq(1), 1)])
+    expected = Series([1, np.nan, np.nan])
+    tm.assert_series_equal(result, expected)
+
+
+def test_case_when_multiple_conditions(df):
+    """
+    Test output when booleans are derived from a computation
+    """
+    result = Series([np.nan, np.nan, np.nan]).case_when(
+        [(df.a.eq(1), 1), (Series([False, True, False]), 2)]
+    )
+    expected = Series([1, 2, np.nan])
+    tm.assert_series_equal(result, expected)
+
+
+def test_case_when_multiple_conditions_replacement_list(df):
+    """
+    Test output when replacement is a list
+    """
+    result = Series([np.nan, np.nan, np.nan]).case_when(
+        [([True, False, False], 1), (df["a"].gt(1) & df["b"].eq(5), [1, 2, 3])]
+    )
+    expected = Series([1, 2, np.nan])
+    tm.assert_series_equal(result, expected)
+
+
+def test_case_when_multiple_conditions_replacement_extension_dtype(df):
+    """
+    Test output when replacement has an extension dtype
+    """
+    result = Series([np.nan, np.nan, np.nan]).case_when(
+        [
+            ([True, False, False], 1),
+            (df["a"].gt(1) & df["b"].eq(5), pd_array([1, 2, 3], dtype="Int64")),
+        ],
+    )
+    expected = Series([1, 2, np.nan], dtype="Float64")
+    tm.assert_series_equal(result, expected)
+
+
+def test_case_when_multiple_conditions_replacement_series(df):
+    """
+    Test output when replacement is a Series
+    """
+    result = Series([np.nan, np.nan, np.nan]).case_when(
+        [
+            (np.array([True, False, False]), 1),
+            (df["a"].gt(1) & df["b"].eq(5), Series([1, 2, 3])),
+        ],
+    )
+    expected = Series([1, 2, np.nan])
+    tm.assert_series_equal(result, expected)
+
+
+def test_case_when_non_range_index():
+    """
+    Test output if index is not RangeIndex
+    """
+    rng = np.random.default_rng(seed=123)
+    dates = date_range("1/1/2000", periods=8)
+    df = DataFrame(
+        rng.standard_normal(size=(8, 4)), index=dates, columns=["A", "B", "C", "D"]
+    )
+    result = Series(5, index=df.index, name="A").case_when([(df.A.gt(0), df.B)])
+    expected = df.A.mask(df.A.gt(0), df.B).where(df.A.gt(0), 5)
+    tm.assert_series_equal(result, expected)
+
+
+def test_case_when_callable():
+    """
+    Test output on a callable
+    """
+    # https://numpy.org/doc/stable/reference/generated/numpy.piecewise.html
+    x = np.linspace(-2.5, 2.5, 6)
+    ser = Series(x)
+    result = ser.case_when(
+        caselist=[
+            (lambda df: df < 0, lambda df: -df),
+            (lambda df: df >= 0, lambda df: df),
+        ]
+    )
+    expected = np.piecewise(x, [x < 0, x >= 0], [lambda x: -x, lambda x: x])
+    tm.assert_series_equal(result, Series(expected))