Refactor string methods for StringArray + return IntegerArray for numeric results

doc/source/user_guide/text.rst

@@ -13,7 +13,7 @@ Text Data Types
 
 .. versionadded:: 1.0.0
 
-There are two main ways to store text data
+There are two ways to store text data in pandas:
 
 1. ``object`` -dtype NumPy array.
 2. :class:`StringDtype` extension type.
@@ -63,7 +63,40 @@ Or ``astype`` after the ``Series`` or ``DataFrame`` is created
    s
    s.astype("string")
 
-Everything that follows in the rest of this document applies equally to
+.. _text.differences:
+
+Behavior differences
+^^^^^^^^^^^^^^^^^^^^
+
+These are places where the behavior of ``StringDtype`` objects differ from
+``object`` dtype
+
+l. For ``StringDtype``, :ref:`string accessor methods<api.series.str>`
+   that return **numeric** output will always return a nullable integer dtype,
+   rather than either int or float dtype, depending on the presence of NA values.
+
+   .. ipython:: python
+
+      s = pd.Series(["a", None, "b"], dtype="string")
+      s
+      s.str.count("a")
+      s.dropna().str.count("a")
+
+   Both outputs are ``Int64`` dtype. Compare that with object-dtype
+
+   .. ipython:: python
+
+      s.astype(object).str.count("a")
+      s.astype(object).dropna().str.count("a")
+
+   When NA values are present, the output dtype is float64.
+
+2. Some string methods, like :meth:`Series.str.decode` are not available
+   on ``StringArray`` because ``StringArray`` only holds strings, not
+   bytes.
+
+
+Everything else that follows in the rest of this document applies equally to
 ``string`` and ``object`` dtype.
 
 .. _text.string_methods:

doc/source/whatsnew/v1.0.0.rst

@@ -63,7 +63,7 @@ Previously, strings were typically stored in object-dtype NumPy arrays.
    ``StringDtype`` is currently considered experimental. The implementation
    and parts of the API may change without warning.
 
-The text extension type solves several issues with object-dtype NumPy arrays:
+The ``'string'`` extension type solves several issues with object-dtype NumPy arrays:
 
 1. You can accidentally store a *mixture* of strings and non-strings in an
    ``object`` dtype array. A ``StringArray`` can only store strings.

pandas/_libs/lib.pyx

@@ -2208,31 +2208,47 @@ def maybe_convert_objects(ndarray[object] objects, bint try_float=0,
     return objects
 
 
+_no_default = object()
+
+
 @cython.boundscheck(False)
 @cython.wraparound(False)
-def map_infer_mask(ndarray arr, object f, const uint8_t[:] mask, bint convert=1):
+def map_infer_mask(ndarray arr, object f, const uint8_t[:] mask, bint convert=1,
+                   object na_value=_no_default, object dtype=object):
     """
     Substitute for np.vectorize with pandas-friendly dtype inference
 
     Parameters
     ----------
     arr : ndarray
     f : function
+    mask : ndarray
+        uint8 dtype ndarray indicating values not to apply `f` to.
+    convert : bool, default True
+        Whether to call `maybe_convert_objects` on the resulting ndarray
+    na_value : Any, optional
+        The result value to use for masked values. By default, the
+        input value is used
+    dtype : numpy.dtype
+        The numpy dtype to use for the result ndarray.
 
     Returns
     -------
     mapped : ndarray
     """
     cdef:
         Py_ssize_t i, n
-        ndarray[object] result
+        ndarray result
         object val
 
     n = len(arr)
-    result = np.empty(n, dtype=object)
+    result = np.empty(n, dtype=dtype)
     for i in range(n):
         if mask[i]:
-            val = arr[i]
+            if na_value is _no_default:
+                val = arr[i]
+            else:
+                val = na_value
         else:
             val = f(arr[i])
 

pandas/core/strings.py

@@ -2,7 +2,7 @@
 from functools import wraps
 import re
 import textwrap
-from typing import Dict, List
+from typing import TYPE_CHECKING, Any, Callable, Dict, List
 import warnings
 
 import numpy as np
@@ -15,10 +15,14 @@
     ensure_object,
     is_bool_dtype,
     is_categorical_dtype,
+    is_extension_array_dtype,
     is_integer,
+    is_integer_dtype,
     is_list_like,
+    is_object_dtype,
     is_re,
     is_scalar,
+    is_string_dtype,
 )
 from pandas.core.dtypes.generic import (
     ABCDataFrame,
@@ -28,9 +32,14 @@
 )
 from pandas.core.dtypes.missing import isna
 
+from pandas._typing import ArrayLike, Dtype
 from pandas.core.algorithms import take_1d
 from pandas.core.base import NoNewAttributesMixin
 import pandas.core.common as com
+from pandas.core.construction import extract_array
+
+if TYPE_CHECKING:
+    from pandas.arrays import StringArray
 
 _cpython_optimized_encoders = (
     "utf-8",
@@ -109,9 +118,78 @@ def cat_safe(list_of_columns: List, sep: str):
 
 def _na_map(f, arr, na_result=np.nan, dtype=object):
     # should really _check_ for NA
+    if is_extension_array_dtype(arr.dtype):
+        # just StringDtype
+        arr = extract_array(arr)
+        return _stringarray_map(f, arr, na_value=na_result, dtype=dtype)
     return _map(f, arr, na_mask=True, na_value=na_result, dtype=dtype)
 
 
+def _stringarray_map(
+    func: Callable[[str], Any], arr: "StringArray", na_value: Any, dtype: Dtype
+) -> ArrayLike:
+    """
+    Map a callable over valid elements of a StringArrray.
+
+    Parameters
+    ----------
+    func : Callable[[str], Any]
+        Apply to each valid element.
+    arr : StringArray
+    na_value : Any
+        The value to use for missing values. By default, this is
+        the original value (NA).
+    dtype : Dtype
+        The result dtype to use. Specifying this aviods an intermediate
+        object-dtype allocation.
+
+    Returns
+    -------
+    ArrayLike
+        An ExtensionArray for integer or string dtypes, otherwise
+        an ndarray.
+
+    """
+    from pandas.arrays import IntegerArray, StringArray
+
+    mask = isna(arr)
+
+    assert isinstance(arr, StringArray)
+    arr = np.asarray(arr)
+
+    if is_integer_dtype(dtype):
+        na_value_is_na = isna(na_value)
+        if na_value_is_na:
+            na_value = 1
+        result = lib.map_infer_mask(
+            arr,
+            func,
+            mask.view("uint8"),
+            convert=False,
+            na_value=na_value,
+            dtype=np.dtype("int64"),
+        )
+
+        if not na_value_is_na:
+            mask[:] = False
+
+        return IntegerArray(result, mask)
+
+    elif is_string_dtype(dtype) and not is_object_dtype(dtype):
+        # i.e. StringDtype
+        result = lib.map_infer_mask(
+            arr, func, mask.view("uint8"), convert=False, na_value=na_value
+        )
+        return StringArray(result)
+    # TODO: BooleanArray
+    else:
+        # This is when the result type is object. We reach this when
+        # -> We know the result type is truly object (e.g. .encode returns bytes
+        #    or .findall returns a list).
+        # -> We don't know the result type. E.g. `.get` can return anything.
+        return lib.map_infer_mask(arr, func, mask.view("uint8"))
+
+
 def _map(f, arr, na_mask=False, na_value=np.nan, dtype=object):
     if not len(arr):
         return np.ndarray(0, dtype=dtype)
@@ -634,7 +712,7 @@ def str_replace(arr, pat, repl, n=-1, case=None, flags=0, regex=True):
             raise ValueError("Cannot use a callable replacement when regex=False")
         f = lambda x: x.replace(pat, repl, n)
 
-    return _na_map(f, arr)
+    return _na_map(f, arr, dtype=str)
 
 
 def str_repeat(arr, repeats):
@@ -685,7 +763,7 @@ def scalar_rep(x):
             except TypeError:
                 return str.__mul__(x, repeats)
 
-        return _na_map(scalar_rep, arr)
+        return _na_map(scalar_rep, arr, dtype=str)
     else:
 
         def rep(x, r):
@@ -1150,7 +1228,7 @@ def str_join(arr, sep):
     4                    NaN
     dtype: object
     """
-    return _na_map(sep.join, arr)
+    return _na_map(sep.join, arr, dtype=str)
 
 
 def str_findall(arr, pat, flags=0):
@@ -1381,7 +1459,7 @@ def str_pad(arr, width, side="left", fillchar=" "):
     else:  # pragma: no cover
         raise ValueError("Invalid side")
 
-    return _na_map(f, arr)
+    return _na_map(f, arr, dtype=str)
 
 
 def str_split(arr, pat=None, n=None):
@@ -1487,7 +1565,7 @@ def str_slice(arr, start=None, stop=None, step=None):
     """
     obj = slice(start, stop, step)
     f = lambda x: x[obj]
-    return _na_map(f, arr)
+    return _na_map(f, arr, dtype=str)
 
 
 def str_slice_replace(arr, start=None, stop=None, repl=None):
@@ -1578,7 +1656,7 @@ def f(x):
             y += x[local_stop:]
         return y
 
-    return _na_map(f, arr)
+    return _na_map(f, arr, dtype=str)
 
 
 def str_strip(arr, to_strip=None, side="both"):
@@ -1603,7 +1681,7 @@ def str_strip(arr, to_strip=None, side="both"):
         f = lambda x: x.rstrip(to_strip)
     else:  # pragma: no cover
         raise ValueError("Invalid side")
-    return _na_map(f, arr)
+    return _na_map(f, arr, dtype=str)
 
 
 def str_wrap(arr, width, **kwargs):
@@ -1667,7 +1745,7 @@ def str_wrap(arr, width, **kwargs):
 
     tw = textwrap.TextWrapper(**kwargs)
 
-    return _na_map(lambda s: "\n".join(tw.wrap(s)), arr)
+    return _na_map(lambda s: "\n".join(tw.wrap(s)), arr, dtype=str)
 
 
 def str_translate(arr, table):
@@ -1687,7 +1765,7 @@ def str_translate(arr, table):
     -------
     Series or Index
     """
-    return _na_map(lambda x: x.translate(table), arr)
+    return _na_map(lambda x: x.translate(table), arr, dtype=str)
 
 
 def str_get(arr, i):
@@ -3025,7 +3103,7 @@ def normalize(self, form):
         import unicodedata
 
         f = lambda x: unicodedata.normalize(form, x)
-        result = _na_map(f, self._parent)
+        result = _na_map(f, self._parent, dtype=str)
         return self._wrap_result(result)
 
     _shared_docs[
@@ -3223,31 +3301,37 @@ def rindex(self, sub, start=0, end=None):
         lambda x: x.lower(),
         name="lower",
         docstring=_shared_docs["casemethods"] % _doc_args["lower"],
+        dtype=str,
     )
     upper = _noarg_wrapper(
         lambda x: x.upper(),
         name="upper",
         docstring=_shared_docs["casemethods"] % _doc_args["upper"],
+        dtype=str,
     )
     title = _noarg_wrapper(
         lambda x: x.title(),
         name="title",
         docstring=_shared_docs["casemethods"] % _doc_args["title"],
+        dtype=str,
     )
     capitalize = _noarg_wrapper(
         lambda x: x.capitalize(),
         name="capitalize",
         docstring=_shared_docs["casemethods"] % _doc_args["capitalize"],
+        dtype=str,
     )
     swapcase = _noarg_wrapper(
         lambda x: x.swapcase(),
         name="swapcase",
         docstring=_shared_docs["casemethods"] % _doc_args["swapcase"],
+        dtype=str,
     )
     casefold = _noarg_wrapper(
         lambda x: x.casefold(),
         name="casefold",
         docstring=_shared_docs["casemethods"] % _doc_args["casefold"],
+        dtype=str,
     )
 
     _shared_docs[