Backport PR #38141 on branch 1.2.x (BUG: do not stringify file-like objects)

doc/source/whatsnew/v1.2.0.rst

@@ -746,6 +746,7 @@ I/O
 - :meth:`read_fwf` was inferring compression with ``compression=None`` which was not consistent with the other :meth:``read_*`` functions (:issue:`37909`)
 - :meth:`DataFrame.to_html` was ignoring ``formatters`` argument for ``ExtensionDtype`` columns (:issue:`36525`)
 - Bumped minimum xarray version to 0.12.3 to avoid reference to the removed ``Panel`` class (:issue:`27101`)
+- :meth:`DataFrame.to_csv` was re-opening file-like handles that also implement ``os.PathLike`` (:issue:`38125`)
 
 Period
 ^^^^^^

pandas/io/common.py

@@ -152,6 +152,7 @@ def validate_header_arg(header) -> None:
 
 def stringify_path(
     filepath_or_buffer: FilePathOrBuffer[AnyStr],
+    convert_file_like: bool = False,
 ) -> FileOrBuffer[AnyStr]:
     """
     Attempt to convert a path-like object to a string.
@@ -169,12 +170,15 @@ def stringify_path(
     Objects supporting the fspath protocol (python 3.6+) are coerced
     according to its __fspath__ method.
 
-    For backwards compatibility with older pythons, pathlib.Path and
-    py.path objects are specially coerced.
-
     Any other object is passed through unchanged, which includes bytes,
     strings, buffers, or anything else that's not even path-like.
     """
+    if not convert_file_like and is_file_like(filepath_or_buffer):
+        # GH 38125: some fsspec objects implement os.PathLike but have already opened a
+        # file. This prevents opening the file a second time. infer_compression calls
+        # this function with convert_file_like=True to infer the compression.
+        return cast(FileOrBuffer[AnyStr], filepath_or_buffer)
+
     if isinstance(filepath_or_buffer, os.PathLike):
         filepath_or_buffer = filepath_or_buffer.__fspath__()
     return _expand_user(filepath_or_buffer)
@@ -462,7 +466,7 @@ def infer_compression(
     # Infer compression
     if compression == "infer":
         # Convert all path types (e.g. pathlib.Path) to strings
-        filepath_or_buffer = stringify_path(filepath_or_buffer)
+        filepath_or_buffer = stringify_path(filepath_or_buffer, convert_file_like=True)
         if not isinstance(filepath_or_buffer, str):
             # Cannot infer compression of a buffer, assume no compression
             return None

pandas/io/parsers.py

@@ -74,7 +74,7 @@
 from pandas.core.series import Series
 from pandas.core.tools import datetimes as tools
 
-from pandas.io.common import IOHandles, get_handle, stringify_path, validate_header_arg
+from pandas.io.common import IOHandles, get_handle, validate_header_arg
 from pandas.io.date_converters import generic_parser
 
 # BOM character (byte order mark)
@@ -774,7 +774,7 @@ class TextFileReader(abc.Iterator):
 
     def __init__(self, f, engine=None, **kwds):
 
-        self.f = stringify_path(f)
+        self.f = f
 
         if engine is not None:
             engine_specified = True
@@ -859,14 +859,14 @@ def _get_options_with_defaults(self, engine):
 
     def _check_file_or_buffer(self, f, engine):
         # see gh-16530
-        if is_file_like(f):
+        if is_file_like(f) and engine != "c" and not hasattr(f, "__next__"):
             # The C engine doesn't need the file-like to have the "__next__"
             # attribute. However, the Python engine explicitly calls
             # "__next__(...)" when iterating through such an object, meaning it
             # needs to have that attribute
-            if engine != "c" and not hasattr(f, "__next__"):
-                msg = "The 'python' engine cannot iterate through this file buffer."
-                raise ValueError(msg)
+            raise ValueError(
+                "The 'python' engine cannot iterate through this file buffer."
+            )
 
     def _clean_options(self, options, engine):
         result = options.copy()

pandas/tests/io/test_common.py

@@ -85,6 +85,13 @@ def test_stringify_path_fspath(self):
         result = icom.stringify_path(p)
         assert result == "foo/bar.csv"
 
+    def test_stringify_file_and_path_like(self):
+        # GH 38125: do not stringify file objects that are also path-like
+        fsspec = pytest.importorskip("fsspec")
+        with tm.ensure_clean() as path:
+            with fsspec.open(f"file://{path}", mode="wb") as fsspec_obj:
+                assert fsspec_obj == icom.stringify_path(fsspec_obj)
+
     @pytest.mark.parametrize(
         "extension,expected",
         [