REF: move _str_extract function in accessor.py to array method (pandas-dev#41663)

Author: simonjayhawkins

pandas/core/arrays/categorical.py

@@ -2453,7 +2453,9 @@ def replace(self, to_replace, value, inplace: bool = False):
 
     # ------------------------------------------------------------------------
     # String methods interface
-    def _str_map(self, f, na_value=np.nan, dtype=np.dtype("object")):
+    def _str_map(
+        self, f, na_value=np.nan, dtype=np.dtype("object"), convert: bool = True
+    ):
         # Optimization to apply the callable `f` to the categories once
         # and rebuild the result by `take`ing from the result with the codes.
         # Returns the same type as the object-dtype implementation though.

pandas/core/arrays/string_.py

@@ -410,7 +410,9 @@ def _cmp_method(self, other, op):
     # String methods interface
     _str_na_value = StringDtype.na_value
 
-    def _str_map(self, f, na_value=None, dtype: Dtype | None = None):
+    def _str_map(
+        self, f, na_value=None, dtype: Dtype | None = None, convert: bool = True
+    ):
         from pandas.arrays import BooleanArray
 
         if dtype is None:

pandas/core/arrays/string_arrow.py

@@ -742,7 +742,9 @@ def value_counts(self, dropna: bool = True) -> Series:
 
     _str_na_value = ArrowStringDtype.na_value
 
-    def _str_map(self, f, na_value=None, dtype: Dtype | None = None):
+    def _str_map(
+        self, f, na_value=None, dtype: Dtype | None = None, convert: bool = True
+    ):
         # TODO: de-duplicate with StringArray method. This method is moreless copy and
         # paste.
 

pandas/core/strings/accessor.py

@@ -13,10 +13,7 @@
 import numpy as np
 
 import pandas._libs.lib as lib
-from pandas._typing import (
-    ArrayLike,
-    FrameOrSeriesUnion,
-)
+from pandas._typing import FrameOrSeriesUnion
 from pandas.util._decorators import Appender
 
 from pandas.core.dtypes.common import (
@@ -160,7 +157,6 @@ class StringMethods(NoNewAttributesMixin):
     # TODO: Dispatch all the methods
     # Currently the following are not dispatched to the array
     # * cat
-    # * extract
     # * extractall
 
     def __init__(self, data):
@@ -243,7 +239,7 @@ def _wrap_result(
         self,
         result,
         name=None,
-        expand=None,
+        expand: bool | None = None,
         fill_value=np.nan,
         returns_string=True,
     ):
@@ -2385,10 +2381,7 @@ def extract(
         2    NaN
         dtype: object
         """
-        from pandas import (
-            DataFrame,
-            array as pd_array,
-        )
+        from pandas import DataFrame
 
         if not isinstance(expand, bool):
             raise ValueError("expand must be True or False")
@@ -2400,8 +2393,6 @@ def extract(
         if not expand and regex.groups > 1 and isinstance(self._data, ABCIndex):
             raise ValueError("only one regex group is supported with Index")
 
-        # TODO: dispatch
-
         obj = self._data
         result_dtype = _result_dtype(obj)
 
@@ -2415,8 +2406,8 @@ def extract(
                 result = DataFrame(columns=columns, dtype=result_dtype)
 
             else:
-                result_list = _str_extract(
-                    obj.array, pat, flags=flags, expand=returns_df
+                result_list = self._data.array._str_extract(
+                    pat, flags=flags, expand=returns_df
                 )
 
                 result_index: Index | None
@@ -2431,9 +2422,7 @@ def extract(
 
         else:
             name = _get_single_group_name(regex)
-            result_arr = _str_extract(obj.array, pat, flags=flags, expand=returns_df)
-            # not dispatching, so we have to reconstruct here.
-            result = pd_array(result_arr, dtype=result_dtype)
+            result = self._data.array._str_extract(pat, flags=flags, expand=returns_df)
         return self._wrap_result(result, name=name)
 
     @forbid_nonstring_types(["bytes"])
@@ -3121,33 +3110,6 @@ def _get_group_names(regex: re.Pattern) -> list[Hashable]:
     return [names.get(1 + i, i) for i in range(regex.groups)]
 
 
-def _str_extract(arr: ArrayLike, pat: str, flags=0, expand: bool = True):
-    """
-    Find groups in each string in the array using passed regular expression.
-
-    Returns
-    -------
-    np.ndarray or list of lists is expand is True
-    """
-    regex = re.compile(pat, flags=flags)
-
-    empty_row = [np.nan] * regex.groups
-
-    def f(x):
-        if not isinstance(x, str):
-            return empty_row
-        m = regex.search(x)
-        if m:
-            return [np.nan if item is None else item for item in m.groups()]
-        else:
-            return empty_row
-
-    if expand:
-        return [f(val) for val in np.asarray(arr)]
-
-    return np.array([f(val)[0] for val in np.asarray(arr)], dtype=object)
-
-
 def str_extractall(arr, pat, flags=0):
     regex = re.compile(pat, flags=flags)
     # the regex must contain capture groups.

pandas/core/strings/base.py

@@ -230,3 +230,7 @@ def _str_split(self, pat=None, n=-1, expand=False):
     @abc.abstractmethod
     def _str_rsplit(self, pat=None, n=-1):
         pass
+
+    @abc.abstractmethod
+    def _str_extract(self, pat: str, flags: int = 0, expand: bool = True):
+        pass

pandas/core/strings/object_array.py

@@ -32,7 +32,9 @@ def __len__(self):
         # For typing, _str_map relies on the object being sized.
         raise NotImplementedError
 
-    def _str_map(self, f, na_value=None, dtype: Dtype | None = None):
+    def _str_map(
+        self, f, na_value=None, dtype: Dtype | None = None, convert: bool = True
+    ):
         """
         Map a callable over valid element of the array.
 
@@ -47,6 +49,8 @@ def _str_map(self, f, na_value=None, dtype: Dtype | None = None):
             for object-dtype and Categorical and ``pd.NA`` for StringArray.
         dtype : Dtype, optional
             The dtype of the result array.
+        convert : bool, default True
+            Whether to call `maybe_convert_objects` on the resulting ndarray
         """
         if dtype is None:
             dtype = np.dtype("object")
@@ -60,9 +64,9 @@ def _str_map(self, f, na_value=None, dtype: Dtype | None = None):
 
         arr = np.asarray(self, dtype=object)
         mask = isna(arr)
-        convert = not np.all(mask)
+        map_convert = convert and not np.all(mask)
         try:
-            result = lib.map_infer_mask(arr, f, mask.view(np.uint8), convert)
+            result = lib.map_infer_mask(arr, f, mask.view(np.uint8), map_convert)
         except (TypeError, AttributeError) as e:
             # Reraise the exception if callable `f` got wrong number of args.
             # The user may want to be warned by this, instead of getting NaN
@@ -88,7 +92,7 @@ def g(x):
             return result
         if na_value is not np.nan:
             np.putmask(result, mask, na_value)
-            if result.dtype == object:
+            if convert and result.dtype == object:
                 result = lib.maybe_convert_objects(result)
         return result
 
@@ -410,3 +414,28 @@ def _str_lstrip(self, to_strip=None):
 
     def _str_rstrip(self, to_strip=None):
         return self._str_map(lambda x: x.rstrip(to_strip))
+
+    def _str_extract(self, pat: str, flags: int = 0, expand: bool = True):
+        regex = re.compile(pat, flags=flags)
+        na_value = self._str_na_value
+
+        if not expand:
+
+            def g(x):
+                m = regex.search(x)
+                return m.groups()[0] if m else na_value
+
+            return self._str_map(g, convert=False)
+
+        empty_row = [na_value] * regex.groups
+
+        def f(x):
+            if not isinstance(x, str):
+                return empty_row
+            m = regex.search(x)
+            if m:
+                return [na_value if item is None else item for item in m.groups()]
+            else:
+                return empty_row
+
+        return [f(val) for val in np.asarray(self)]