[ENH] Add DataFrame method to explode a list-like column (GH #16538)

Sometimes a values column is presented with list-like values on one row.
Instead we may want to split each individual value onto its own row,
keeping the same mapping to the other key columns. While it's possible
to chain together existing pandas operations (in fact that's exactly
what this implementation is) to do this, the sequence of operations
is not obvious. By contrast this is available as a built-in operation
in say Spark and is a fairly common use case.

asv_bench/benchmarks/reshape.py

@@ -184,4 +184,22 @@ def time_qcut_datetime(self, bins):
         pd.qcut(self.datetime_series, bins)
 
 
+class Explode(object):
+    param_names = ['n_rows', 'max_list_length']
+    params = [[100, 1000, 10000], [3, 5, 10]]
+
+    def setup(self, n_rows, max_list_length):
+        import string
+        num_letters = np.random.randint(0, max_list_length, n_rows)
+        key_column = [','.join([np.random.choice(list(string.ascii_letters))
+                                for _ in range(k)])
+                      for k in num_letters]
+        value_column = np.random.randn(n_rows)
+        self.frame = pd.DataFrame({'key': key_column,
+                                   'value': value_column})
+
+    def time_explode(self, n_rows, max_list_length):
+        self.frame.explode('key', sep=',')
+
+
 from .pandas_vb_common import setup  # noqa: F401

doc/source/reshaping.rst

@@ -801,3 +801,40 @@ Note to subdivide over multiple columns we can pass in a list to the
 
    df.pivot_table(
        values=['val0'], index='row', columns=['item', 'col'], aggfunc=['mean'])
+
+.. _reshaping.explode:
+
+Exploding a List-like Column
+----------------------------
+
+.. ipython:: python
+   :suppress:
+
+   keys = ['panda1', 'panda2', 'panda3']
+   values = [['eats', 'shoots'], ['shoots', 'leaves'], ['eats', 'leaves']]
+   df = pd.DataFrame({'keys': keys, 'values': values})
+   exploded = df.explode('values')
+   values = ['eats,shoots', 'shoots,leaves', 'eats,shoots,leaves']
+   df2 = pd.DataFrame({'keys': keys, 'values': values})
+
+Sometimes the value column is list-like:
+
+.. ipython:: python
+
+   df
+
+But we actually want to put each value onto its own row.
+For this purpose we can use ``DataFrame.explode``:
+
+.. ipython:: python
+
+   df.explode('values')
+
+For convenience, we can use the optional keyword ``sep`` to automatically
+split a string column before exploding:
+
+.. ipython:: python
+
+   df2
+
+   df2.explode('values', sep=',')

doc/source/whatsnew/v0.24.0.rst

@@ -31,6 +31,7 @@ New features
 - :func:`read_feather` now accepts ``columns`` as an argument, allowing the user to specify which columns should be read. (:issue:`24025`)
 - :func:`DataFrame.to_html` now accepts ``render_links`` as an argument, allowing the user to generate HTML with links to any URLs that appear in the DataFrame.
   See the :ref:`section on writing HTML <io.html>` in the IO docs for example usage. (:issue:`2679`)
+- :func:`DataFrame.explode` to split list-like values onto individual rows. See :ref:`section on Exploding list-like column <reshaping.html>` in docs for more information (:issue:`16538`)
 
 .. _whatsnew_0240.values_api:
 

pandas/core/frame.py

@@ -5980,6 +5980,49 @@ def melt(self, id_vars=None, value_vars=None, var_name=None,
                     var_name=var_name, value_name=value_name,
                     col_level=col_level)
 
+    def explode(self, col_name, sep=None, dtype=None):
+        """
+        Create a new DataFrame where each element in each row
+        of a list-like column `col_name` is expanded to its own row
+
+        .. versionadded:: 0.24.0
+
+        Parameters
+        ----------
+        col_name : str
+            Name of the column to be exploded
+        sep : str, default None
+            Convenience to split a string `col_name` before exploding
+        dtype : str or dtype, default None
+            Optionally coerce the dtype of exploded column
+-
+        Examples
+        --------
+        >>> df = pd.DataFrame({'k': ['a,b', 'c,d'], 'v': [0, 1]})
+        >>> df.explode('k', sep=',')
+           k  v
+        0  a  0
+        0  b  0
+        1  c  1
+        1  d  1
+        """
+        col = self[col_name]
+        if len(self) == 0:
+            return self.copy()
+        if sep:
+            col_expanded = col.str.split(sep, expand=True)
+        else:
+            col_expanded = col.apply(Series)
+        col_stacked = (col_expanded
+                       .stack()
+                       .reset_index(level=-1, drop=True)
+                       .rename(col_name))
+        if dtype:
+            col_stacked = col_stacked.astype(dtype)
+        return (col_stacked.to_frame()
+                .join(self.drop(col_name, axis=1))
+                .reindex(self.columns, axis=1))
+
     # ----------------------------------------------------------------------
     # Time series-related
 

pandas/tests/frame/test_reshape.py

@@ -918,6 +918,90 @@ def test_unstack_swaplevel_sortlevel(self, level):
         tm.assert_frame_equal(result, expected)
 
 
+def test_explode():
+    # GH 16538
+
+    # Automatically do str.split
+    columns = ['a', 'b', 'c']
+    df = pd.DataFrame([['foo,bar', 'x', 42],
+                       ['fizz,buzz', 'y', 43]],
+                      columns=columns)
+    rs = df.explode('a', sep=',')
+    xp = pd.DataFrame({'a': ['foo', 'bar', 'fizz', 'buzz'],
+                       'b': ['x', 'x', 'y', 'y'],
+                       'c': [42, 42, 43, 43]},
+                      index=[0, 0, 1, 1])
+    tm.assert_frame_equal(rs, xp)
+
+    # Coerce dtype
+    df = pd.DataFrame([[[0, 1, 4], 'x', 42],
+                       [[2, 3], 'y', 43]],
+                      columns=columns)
+    rs = df.explode('a', dtype='int')
+    xp = pd.DataFrame({'a': np.array([0, 1, 4, 2, 3], dtype='int'),
+                       'b': ['x', 'x', 'x', 'y', 'y'],
+                       'c': [42, 42, 42, 43, 43]},
+                      index=[0, 0, 0, 1, 1])
+    tm.assert_frame_equal(rs, xp)
+
+    # NaN's and empty lists are omitted
+    # TODO: option to preserve explicit NAs instead
+    df = pd.DataFrame([[[], 'x', 42],
+                       [[2.0, np.nan], 'y', 43]],
+                      columns=columns)
+    rs = df.explode('a')
+    xp = pd.DataFrame({'a': [2.0],
+                       'b': ['y'],
+                       'c': [43]},
+                      index=[1])
+    tm.assert_frame_equal(rs, xp)
+
+    # Not everything is a list
+    df = pd.DataFrame([[[0, 1, 4], 'x', 42],
+                       [3, 'y', 43]],
+                      columns=columns)
+    rs = df.explode('a', dtype='int')
+    xp = pd.DataFrame({'a': np.array([0, 1, 4, 3], dtype='int'),
+                       'b': ['x', 'x', 'x', 'y'],
+                       'c': [42, 42, 42, 43]},
+                      index=[0, 0, 0, 1])
+    tm.assert_frame_equal(rs, xp)
+
+    # Nothing is a list
+    df = pd.DataFrame([[0, 'x', 42],
+                       [3, 'y', 43]],
+                      columns=columns)
+    rs = df.explode('a')
+    xp = pd.DataFrame({'a': [0, 3],
+                       'b': ['x', 'y'],
+                       'c': [42, 43]},
+                      index=[0, 1])
+    tm.assert_frame_equal(rs, xp)
+
+    # Empty frame
+    rs = pd.DataFrame(columns=['a', 'b']).explode('a')
+    xp = pd.DataFrame(columns=['a', 'b'])
+    tm.assert_frame_equal(rs, xp)
+
+    # Bad column name
+    pytest.raises(KeyError, df.explode, 'badcolumnname')
+
+    # Multi-index
+    columns = ['a', 'b', 'c']
+    idx = pd.MultiIndex.from_tuples([(0, 'a'), (1, 'b')])
+    df = pd.DataFrame([['foo,bar', 'x', 42],
+                       ['fizz,buzz', 'y', 43]],
+                      columns=columns,
+                      index=idx)
+    rs = df.explode('a', sep=',')
+    idx = pd.MultiIndex.from_tuples([(0, 'a'), (0, 'a'), (1, 'b'), (1, 'b')])
+    xp = pd.DataFrame({'a': ['foo', 'bar', 'fizz', 'buzz'],
+                       'b': ['x', 'x', 'y', 'y'],
+                       'c': [42, 42, 43, 43]},
+                      index=idx)
+    tm.assert_frame_equal(rs, xp)
+
+
 def test_unstack_fill_frame_object():
     # GH12815 Test unstacking with object.
     data = pd.Series(['a', 'b', 'c', 'a'], dtype='object')