ENH: add expand kw to str.extract and str.get_dummies

pandas/core/strings.py

@@ -421,17 +421,21 @@ def str_extract(arr, pat, flags=0):
         Pattern or regular expression
     flags : int, default 0 (no flags)
         re module flags, e.g. re.IGNORECASE
+    expand : None or bool, default None
+        * If None, return Series/Index (one group) or DataFrame/MultiIndex (multiple groups)
+        * If True, return DataFrame/MultiIndex expanding dimensionality.
+        * If False, return Series/Index.
 
     Returns
     -------
-    extracted groups : Series (one group) or DataFrame (multiple groups)
+    extracted groups : Series/Index or DataFrame/MultiIndex of objects
         Note that dtype of the result is always object, even when no match is
         found and the result is a Series or DataFrame containing only NaN
         values.
 
     Examples
     --------
-    A pattern with one group will return a Series. Non-matches will be NaN.
+    A pattern with one group returns a Series. Non-matches will be NaN.
 
     >>> Series(['a1', 'b2', 'c3']).str.extract('[ab](\d)')
     0      1
@@ -463,11 +467,14 @@ def str_extract(arr, pat, flags=0):
     1      b     2
     2    NaN   NaN
 
-    """
-    from pandas.core.series import Series
-    from pandas.core.frame import DataFrame
-    from pandas.core.index import Index
+    Or you can specify ``expand=False`` to return Series.
 
+    >>> pd.Series(['a1', 'b2', 'c3']).str.extract('([ab])?(\d)', expand=False)
+    0      [a, 1]
+    1      [b, 2]
+    2    [nan, 3]
+    Name: [0, 1], dtype: object
+    """
     regex = re.compile(pat, flags=flags)
     # just to be safe, check this
     if regex.groups == 0:
@@ -487,18 +494,9 @@ def f(x):
         result = np.array([f(val)[0] for val in arr], dtype=object)
         name = _get_single_group_name(regex)
     else:
-        if isinstance(arr, Index):
-            raise ValueError("only one regex group is supported with Index")
-        name = None
         names = dict(zip(regex.groupindex.values(), regex.groupindex.keys()))
-        columns = [names.get(1 + i, i) for i in range(regex.groups)]
-        if arr.empty:
-            result = DataFrame(columns=columns, dtype=object)
-        else:
-            result = DataFrame([f(val) for val in arr],
-                               columns=columns,
-                               index=arr.index,
-                               dtype=object)
+        name = [names.get(1 + i, i) for i in range(regex.groups)]
+        result = np.array([f(val) for val in arr], dtype=object)
     return result, name
 
 
@@ -511,10 +509,13 @@ def str_get_dummies(arr, sep='|'):
     ----------
     sep : string, default "|"
         String to split on.
+    expand : bool, default True
+        * If True, return DataFrame/MultiIndex expanding dimensionality.
+        * If False, return Series/Index.
 
     Returns
     -------
-    dummies : DataFrame
+    dummies : Series/Index or DataFrame/MultiIndex of objects
 
     Examples
     --------
@@ -534,15 +535,15 @@ def str_get_dummies(arr, sep='|'):
     --------
     pandas.get_dummies
     """
-    from pandas.core.frame import DataFrame
     from pandas.core.index import Index
-
-    # GH9980, Index.str does not support get_dummies() as it returns a frame
+    # TODO: Add fillna GH 10089
     if isinstance(arr, Index):
-        raise TypeError("get_dummies is not supported for string methods on Index")
-
-    # TODO remove this hack?
-    arr = arr.fillna('')
+        # temp hack
+        values = arr.values
+        values[isnull(values)] = ''
+        arr = Index(values)
+    else:
+        arr = arr.fillna('')
     try:
         arr = sep + arr + sep
     except TypeError:
@@ -558,7 +559,7 @@ def str_get_dummies(arr, sep='|'):
     for i, t in enumerate(tags):
         pat = sep + t + sep
         dummies[:, i] = lib.map_infer(arr.values, lambda x: pat in x)
-    return DataFrame(dummies, arr.index, tags)
+    return dummies, tags
 
 
 def str_join(arr, sep):
@@ -1043,40 +1044,19 @@ def __iter__(self):
             i += 1
             g = self.get(i)
 
-    def _wrap_result(self, result, **kwargs):
-
-        # leave as it is to keep extract and get_dummies results
-        # can be merged to _wrap_result_expand in v0.17
-        from pandas.core.series import Series
-        from pandas.core.frame import DataFrame
-        from pandas.core.index import Index
-
-        if not hasattr(result, 'ndim'):
-            return result
-        name = kwargs.get('name') or getattr(result, 'name', None) or self.series.name
-
-        if result.ndim == 1:
-            if isinstance(self.series, Index):
-                # if result is a boolean np.array, return the np.array
-                # instead of wrapping it into a boolean Index (GH 8875)
-                if is_bool_dtype(result):
-                    return result
-                return Index(result, name=name)
-            return Series(result, index=self.series.index, name=name)
-        else:
-            assert result.ndim < 3
-            return DataFrame(result, index=self.series.index)
+    def _wrap_result(self, result, expand=False, name=None):
+        from pandas.core.index import Index, MultiIndex
 
-    def _wrap_result_expand(self, result, expand=False):
         if not isinstance(expand, bool):
             raise ValueError("expand must be True or False")
 
-        from pandas.core.index import Index, MultiIndex
+        if name is None:
+            name = getattr(result, 'name', None) or self.series.name
+
         if not hasattr(result, 'ndim'):
             return result
 
         if isinstance(self.series, Index):
-            name = getattr(result, 'name', None)
             # if result is a boolean np.array, return the np.array
             # instead of wrapping it into a boolean Index (GH 8875)
             if hasattr(result, 'dtype') and is_bool_dtype(result):
@@ -1092,10 +1072,12 @@ def _wrap_result_expand(self, result, expand=False):
             if expand:
                 cons_row = self.series._constructor
                 cons = self.series._constructor_expanddim
-                data = [cons_row(x) for x in result]
-                return cons(data, index=index)
+                data = [cons_row(x, index=name) for x in result]
+                return cons(data, index=index, columns=name,
+                            dtype=result.dtype)
             else:
-                name = getattr(result, 'name', None)
+                if result.ndim > 1:
+                    result = list(result)
                 cons = self.series._constructor
                 return cons(result, name=name, index=index)
 
@@ -1109,7 +1091,7 @@ def cat(self, others=None, sep=None, na_rep=None):
     @copy(str_split)
     def split(self, pat=None, n=-1, expand=False):
         result = str_split(self.series, pat, n=n)
-        return self._wrap_result_expand(result, expand=expand)
+        return self._wrap_result(result, expand=expand)
 
     _shared_docs['str_partition'] = ("""
     Split the string at the %(side)s occurrence of `sep`, and return 3 elements
@@ -1160,15 +1142,15 @@ def split(self, pat=None, n=-1, expand=False):
     def partition(self, pat=' ', expand=True):
         f = lambda x: x.partition(pat)
         result = _na_map(f, self.series)
-        return self._wrap_result_expand(result, expand=expand)
+        return self._wrap_result(result, expand=expand)
 
     @Appender(_shared_docs['str_partition'] % {'side': 'last',
         'return': '3 elements containing two empty strings, followed by the string itself',
         'also': 'partition : Split the string at the first occurrence of `sep`'})
     def rpartition(self, pat=' ', expand=True):
         f = lambda x: x.rpartition(pat)
         result = _na_map(f, self.series)
-        return self._wrap_result_expand(result, expand=expand)
+        return self._wrap_result(result, expand=expand)
 
     @copy(str_get)
     def get(self, i):
@@ -1309,9 +1291,9 @@ def wrap(self, width, **kwargs):
         return self._wrap_result(result)
 
     @copy(str_get_dummies)
-    def get_dummies(self, sep='|'):
-        result = str_get_dummies(self.series, sep)
-        return self._wrap_result(result)
+    def get_dummies(self, sep='|', expand=True):
+        result, name = str_get_dummies(self.series, sep)
+        return self._wrap_result(result, name=name, expand=expand)
 
     @copy(str_translate)
     def translate(self, table, deletechars=None):
@@ -1324,9 +1306,18 @@ def translate(self, table, deletechars=None):
     findall = _pat_wrapper(str_findall, flags=True)
 
     @copy(str_extract)
-    def extract(self, pat, flags=0):
+    def extract(self, pat, flags=0, expand=None):
         result, name = str_extract(self.series, pat, flags=flags)
-        return self._wrap_result(result, name=name)
+        if expand is None and hasattr(result, 'ndim'):
+            # to be compat with previous behavior
+            if len(result) == 0:
+                # for empty input
+                expand = True if isinstance(name, list) else False
+            elif result.ndim > 1:
+                expand = True
+            else:
+                expand = False
+        return self._wrap_result(result, name=name, expand=expand)
 
     _shared_docs['find'] = ("""
     Return %(side)s indexes in each strings in the Series/Index