BUG: iter(ser.str) did not raise TypeError (#54174)

* BUG: iter(ser.str) did not raise TypeError

* pass through

* Change test

* Typing

Author: mroeschke

doc/source/whatsnew/v2.1.0.rst

@@ -492,7 +492,7 @@ Conversion
 
 Strings
 ^^^^^^^
--
+- Bug in :meth:`Series.str` that did not raise a ``TypeError`` when iterated (:issue:`54173`)
 -
 
 Interval

pandas/core/strings/accessor.py

@@ -48,7 +48,10 @@
 from pandas.core.construction import extract_array
 
 if TYPE_CHECKING:
-    from collections.abc import Hashable
+    from collections.abc import (
+        Hashable,
+        Iterator,
+    )
 
     from pandas import (
         DataFrame,
@@ -243,6 +246,9 @@ def __getitem__(self, key):
         result = self._data.array._str_getitem(key)
         return self._wrap_result(result)
 
+    def __iter__(self) -> Iterator:
+        raise TypeError(f"'{type(self).__name__}' object is not iterable")
+
     def _wrap_result(
         self,
         result,
@@ -438,22 +444,26 @@ def _get_series_list(self, others):
             others = DataFrame(others, index=idx)
             return [others[x] for x in others]
         elif is_list_like(others, allow_sets=False):
-            others = list(others)  # ensure iterators do not get read twice etc
-
-            # in case of list-like `others`, all elements must be
-            # either Series/Index/np.ndarray (1-dim)...
-            if all(
-                isinstance(x, (ABCSeries, ABCIndex))
-                or (isinstance(x, np.ndarray) and x.ndim == 1)
-                for x in others
-            ):
-                los: list[Series] = []
-                while others:  # iterate through list and append each element
-                    los = los + self._get_series_list(others.pop(0))
-                return los
-            # ... or just strings
-            elif all(not is_list_like(x) for x in others):
-                return [Series(others, index=idx)]
+            try:
+                others = list(others)  # ensure iterators do not get read twice etc
+            except TypeError:
+                # e.g. ser.str, raise below
+                pass
+            else:
+                # in case of list-like `others`, all elements must be
+                # either Series/Index/np.ndarray (1-dim)...
+                if all(
+                    isinstance(x, (ABCSeries, ABCIndex))
+                    or (isinstance(x, np.ndarray) and x.ndim == 1)
+                    for x in others
+                ):
+                    los: list[Series] = []
+                    while others:  # iterate through list and append each element
+                        los = los + self._get_series_list(others.pop(0))
+                    return los
+                # ... or just strings
+                elif all(not is_list_like(x) for x in others):
+                    return [Series(others, index=idx)]
         raise TypeError(
             "others must be Series, Index, DataFrame, np.ndarray "
             "or list-like (either containing only strings or "

pandas/tests/dtypes/test_inference.py

@@ -150,8 +150,9 @@ def shape(self):
     ((_ for _ in []), True, "generator-empty"),
     (Series([1]), True, "Series"),
     (Series([], dtype=object), True, "Series-empty"),
-    (Series(["a"]).str, False, "StringMethods"),
-    (Series([], dtype="O").str, False, "StringMethods-empty"),
+    # Series.str will still raise a TypeError if iterated
+    (Series(["a"]).str, True, "StringMethods"),
+    (Series([], dtype="O").str, True, "StringMethods-empty"),
     (Index([1]), True, "Index"),
     (Index([]), True, "Index-empty"),
     (DataFrame([[1]]), True, "DataFrame"),

pandas/tests/strings/test_strings.py

@@ -27,6 +27,13 @@ def test_startswith_endswith_non_str_patterns(pattern):
         ser.str.endswith(pattern)
 
 
+def test_iter_raises():
+    # GH 54173
+    ser = Series(["foo", "bar"])
+    with pytest.raises(TypeError, match="'StringMethods' object is not iterable"):
+        iter(ser.str)
+
+
 # test integer/float dtypes (inferred by constructor) and mixed