CLN: memory_map do not silently fail

doc/source/whatsnew/v1.4.0.rst

@@ -746,6 +746,8 @@ I/O
 - Bug in :func:`read_csv` raising ``AttributeError`` when attempting to read a .csv file and infer index column dtype from an nullable integer type (:issue:`44079`)
 - :meth:`DataFrame.to_csv` and :meth:`Series.to_csv` with ``compression`` set to ``'zip'`` no longer create a zip file containing a file ending with ".zip". Instead, they try to infer the inner file name more smartly. (:issue:`39465`)
 - Bug in :func:`read_csv` when passing simultaneously a parser in ``date_parser`` and ``parse_dates=False``, the parsing was still called (:issue:`44366`)
+- Bug in :func:`read_csv` silently ignoring errors when failling to create a memory-mapped file (:issue:`44766`)
+-
 
 Period
 ^^^^^^

pandas/io/common.py

@@ -1040,26 +1040,15 @@ def _maybe_memory_map(
             handle = open(handle, mode)
         handles.append(handle)
 
-    try:
-        # error: Argument 1 to "_MMapWrapper" has incompatible type "Union[IO[Any],
-        # RawIOBase, BufferedIOBase, TextIOBase, mmap]"; expected "IO[Any]"
-        wrapped = cast(
-            BaseBuffer,
-            _MMapWrapper(handle, encoding, errors, decode),  # type: ignore[arg-type]
-        )
-        # error: "BaseBuffer" has no attribute "close"
-        handle.close()  # type: ignore[attr-defined]
-        handles.remove(handle)
-        handles.append(wrapped)
-        handle = wrapped
-    except Exception:
-        # we catch any errors that may have occurred
-        # because that is consistent with the lower-level
-        # functionality of the C engine (pd.read_csv), so
-        # leave the file handler as is then
-        memory_map = False
-
-    return handle, memory_map, handles
+    # error: Argument 1 to "_MMapWrapper" has incompatible type "Union[IO[Any],
+    # RawIOBase, BufferedIOBase, TextIOBase, mmap]"; expected "IO[Any]"
+    wrapped = cast(
+        BaseBuffer,
+        _MMapWrapper(handle, encoding, errors, decode),  # type: ignore[arg-type]
+    )
+    handles.append(wrapped)
+
+    return wrapped, memory_map, handles
 
 
 def file_exists(filepath_or_buffer: FilePath | BaseBuffer) -> bool:

pandas/tests/io/parser/common/test_file_buffer_url.py

@@ -347,25 +347,6 @@ def test_read_csv_file_handle(all_parsers, io_class, encoding):
     assert not handle.closed
 
 
-def test_memory_map_file_handle_silent_fallback(all_parsers, compression):
-    """
-    Do not fail for buffers with memory_map=True (cannot memory map BytesIO).
-
-    GH 37621
-    """
-    parser = all_parsers
-    expected = DataFrame({"a": [1], "b": [2]})
-
-    handle = BytesIO()
-    expected.to_csv(handle, index=False, compression=compression, mode="wb")
-    handle.seek(0)
-
-    tm.assert_frame_equal(
-        parser.read_csv(handle, memory_map=True, compression=compression),
-        expected,
-    )
-
-
 def test_memory_map_compression(all_parsers, compression):
     """
     Support memory map for compressed files.

pandas/tests/io/parser/test_read_fwf.py

@@ -699,15 +699,15 @@ def test_encoding_mmap(memory_map):
     GH 23254.
     """
     encoding = "iso8859_1"
-    data = BytesIO(" 1 A Ä 2\n".encode(encoding))
-    df = read_fwf(
-        data,
-        header=None,
-        widths=[2, 2, 2, 2],
-        encoding=encoding,
-        memory_map=memory_map,
-    )
-    data.seek(0)
+    with tm.ensure_clean() as path:
+        Path(path).write_bytes(" 1 A Ä 2\n".encode(encoding))
+        df = read_fwf(
+            path,
+            header=None,
+            widths=[2, 2, 2, 2],
+            encoding=encoding,
+            memory_map=memory_map,
+        )
     df_reference = DataFrame([[1, "A", "Ä", 2]])
     tm.assert_frame_equal(df, df_reference)
 

pandas/tests/io/test_common.py

@@ -7,6 +7,7 @@
 from io import (
     BytesIO,
     StringIO,
+    UnsupportedOperation,
 )
 import mmap
 import os
@@ -602,3 +603,8 @@ def test_errno_attribute():
     with pytest.raises(FileNotFoundError, match="\\[Errno 2\\]") as err:
         pd.read_csv("doesnt_exist")
         assert err.errno == errno.ENOENT
+
+
+def test_fail_mmap():
+    with pytest.raises(UnsupportedOperation, match="fileno"):
+        icom.get_handle(BytesIO(), "rb", memory_map=True)