Repository: pytest-dev/pyfakefs

Issue:
Bug: FakeFileOpen objects don't implement the mode property required by typing.io.IO
**How To Reproduce**
```
from pyfakefs.fake_filesystem import FakeFilesystem
from pyfakefs.fake_open import FakeFileOpen
from pyfakefs.fake_pathlib import FakePathlibModule

fake_fs = FakeFilesystem()
fake_open = FakeFileOpen(fake_fs)
fake_pathlib = FakePathlibModule(fake_fs)

f = open("test.txt")
assert f.mode  # This works fine

fake_pathlib.Path("test.txt").write_text(f.read())
f_ = fake_open("test.txt")
assert f_.mode  # This fails
```

**Expected behavior:**
Fake file objects should properly implement the mode property as required by the typing.io.IO abstract class:

```
class IO(Generic[AnyStr]):
    @property
    @abstractmethod
    def mode(self) -> str:
        pass
```

**Your environment**
python3.9
pyfakefs==5.8.0

Hints from issue comments:
@mrbean-bremen: Thanks, I will have a look!

Solution patch:
diff --git a/CHANGES.md b/CHANGES.md
index 3ee6596..3aeb569 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -27,6 +27,7 @@ The released versions correspond to PyPI releases.
   (see [#1121](../../issues/1121))
 * fixed workaround for recursion with pytest under Windows to ignore capitalization
   of pytest executable (see [#1096](../../issues/1096))
+* added missing `mode` property to fake file wrapper (see [#1162](../../issues/1096))
 
 ### Infrastructure
 * adapt test for increased default buffer size in Python 3.14a6
diff --git a/pyfakefs/fake_file.py b/pyfakefs/fake_file.py
index 6da4791..2b2fa8a 100644
--- a/pyfakefs/fake_file.py
+++ b/pyfakefs/fake_file.py
@@ -751,9 +751,8 @@ class FakeFileWrapper:
         self,
         file_object: FakeFile,
         file_path: AnyStr,
-        update: bool,
-        read: bool,
-        append: bool,
+        open_modes: _OpenModes,
+        allow_update: bool,
         delete_on_close: bool,
         filesystem: "FakeFilesystem",
         newline: Optional[str],
@@ -768,9 +767,8 @@ class FakeFileWrapper:
     ):
         self.file_object = file_object
         self.file_path = file_path  # type: ignore[var-annotated]
-        self._append = append
-        self._read = read
-        self.allow_update = update
+        self.open_modes = open_modes
+        self.allow_update = allow_update
         self._closefd = closefd
         self._file_epoch = file_object.epoch
         self.raw_io = raw_io
@@ -801,8 +799,8 @@ class FakeFileWrapper:
         self._flush_pos = 0
         if contents:
             self._flush_pos = len(contents)
-            if update:
-                if not append:
+            if self.allow_update:
+                if not self.open_modes.append:
                     self._io.seek(0)
                 else:
                     self._io.seek(self._flush_pos)
@@ -890,6 +888,21 @@ class FakeFileWrapper:
         """Simulate the `closed` attribute on file."""
         return not self._is_open()
 
+    @property
+    def mode(self) -> str:
+        if self.open_modes.append:
+            m = "ab" if self._binary else "a"
+            return m + "+" if self.open_modes.can_read else m
+        if self.open_modes.truncate:
+            if self._binary:
+                return "rb+" if self.open_modes.can_read else "wb"
+            return "w+" if self.open_modes.can_read else "w"
+        if self.open_modes.must_not_exist:
+            m = "xb" if self._binary else "x"
+            return m + "+" if self.open_modes.can_read else m
+        m = "rb" if self._binary else "r"
+        return m + "+" if self.open_modes.can_write else m
+
     def _try_flush(self, old_pos: int) -> None:
         """Try to flush and reset the position if it fails."""
         flush_pos = self._flush_pos
@@ -910,7 +923,7 @@ class FakeFileWrapper:
         self._check_open_file()
 
         if self.allow_update:
-            if self._append:
+            if self.open_modes.append:
                 contents = self._io.getvalue()
                 self._sync_io()
                 old_contents = self.file_object.byte_contents
@@ -951,14 +964,14 @@ class FakeFileWrapper:
                         open_file is not self
                         and isinstance(open_file, FakeFileWrapper)
                         and self.file_object == open_file.file_object
-                        and not open_file._append
+                        and not open_file.open_modes.append
                     ):
                         open_file._sync_io()
 
     def seek(self, offset: int, whence: int = 0) -> None:
         """Move read/write pointer in 'file'."""
         self._check_open_file()
-        if not self._append:
+        if not self.open_modes.append:
             self._io.seek(offset, whence)
         else:
             self._read_seek = offset
@@ -976,7 +989,7 @@ class FakeFileWrapper:
         if not self.is_stream:
             self.flush()
 
-        if not self._append:
+        if not self.open_modes.append:
             return self._io.tell()
         if self._read_whence:
             write_seek = self._io.tell()
@@ -1001,7 +1014,7 @@ class FakeFileWrapper:
         self._io.seek(0)
         self._io.truncate()
         self._io.putvalue(contents)
-        if not self._append:
+        if not self.open_modes.append:
             self._io.seek(whence)
 
     def _read_wrappers(self, name: str) -> Callable:
@@ -1117,7 +1130,7 @@ class FakeFileWrapper:
                 self._try_flush(old_pos)
                 if not flush_all:
                     ret_value = io_attr(*args, **kwargs)
-            if self._append:
+            if self.open_modes.append:
                 self._read_seek = self._io.tell()
                 self._read_whence = 0
             return ret_value
@@ -1132,7 +1145,7 @@ class FakeFileWrapper:
                         open_file is not self
                         and isinstance(open_file, FakeFileWrapper)
                         and self.file_object == open_file.file_object
-                        and cast(FakeFileWrapper, open_file)._append
+                        and cast(FakeFileWrapper, open_file).open_modes.append
                     ):
                         open_file._read_seek += size
 
@@ -1146,7 +1159,7 @@ class FakeFileWrapper:
 
         def truncate_wrapper(*args, **kwargs):
             """Wrap truncate call to call flush after truncate."""
-            if self._append:
+            if self.open_modes.append:
                 self._io.seek(self._read_seek, self._read_whence)
             size = io_attr(*args, **kwargs)
             self.flush()
@@ -1179,7 +1192,7 @@ class FakeFileWrapper:
 
         if reading or writing:
             self._check_open_file()
-        if not self._read and reading:
+        if not self.open_modes.can_read and reading:
             return self._read_error()
         if not self.opened_as_fd and not self.allow_update and writing:
             return self._write_error()
@@ -1192,7 +1205,7 @@ class FakeFileWrapper:
                 self.file_object.st_atime = helpers.now()
         if truncate:
             return self._truncate_wrapper()
-        if self._append:
+        if self.open_modes.append:
             if reading:
                 return self._read_wrappers(name)
             elif not writing:
@@ -1234,12 +1247,12 @@ class FakeFileWrapper:
             raise ValueError("I/O operation on closed file")
 
     def __iter__(self) -> Union[Iterator[str], Iterator[bytes]]:
-        if not self._read:
+        if not self.open_modes.can_read:
             self._raise("File is not open for reading")
         return self._io.__iter__()
 
     def __next__(self):
-        if not self._read:
+        if not self.open_modes.can_read:
             self._raise("File is not open for reading")
         return next(self._io)
 
diff --git a/pyfakefs/fake_open.py b/pyfakefs/fake_open.py
index d3dcd4e..0bd90a8 100644
--- a/pyfakefs/fake_open.py
+++ b/pyfakefs/fake_open.py
@@ -160,7 +160,7 @@ class FakeFileOpen:
                 flushed if buffer size is exceeded. The default (-1) uses a
                 system specific default buffer size. Text line mode (e.g.
                 buffering=1 in text mode) is not supported.
-            encoding: The encoding used to encode unicode strings / decode
+            encoding: The encoding used to encode Unicode strings / decode
                 bytes.
             errors: (str) Defines how encoding errors are handled.
             newline: Controls universal newlines, passed to stream object.
@@ -270,9 +270,8 @@ class FakeFileOpen:
         fakefile = FakeFileWrapper(
             file_object,
             file_path,
-            update=open_modes.can_write and can_write,
-            read=open_modes.can_read,
-            append=open_modes.append,
+            open_modes=open_modes,
+            allow_update=open_modes.can_write and can_write,
             delete_on_close=self._delete_on_close,
             filesystem=self.filesystem,
             newline=newline,


Test patch:
diff --git a/pyfakefs/tests/fake_open_test.py b/pyfakefs/tests/fake_open_test.py
index fb49505..3f9bb4d 100644
--- a/pyfakefs/tests/fake_open_test.py
+++ b/pyfakefs/tests/fake_open_test.py
@@ -263,8 +263,10 @@ class FakeFileOpenTest(FakeFileOpenTestBase):
         self.os.mkdir(file_dir)
         contents = b"Binary contents"
         with self.open(file_path, "xb") as fake_file:
+            self.assertEqual("xb", fake_file.mode)
             fake_file.write(contents)
         with self.open(file_path, "rb") as fake_file:
+            self.assertEqual("rb", fake_file.mode)
             self.assertEqual(contents, fake_file.read())
 
     def test_overwrite_existing_file(self):
@@ -302,11 +304,27 @@ class FakeFileOpenTest(FakeFileOpenTestBase):
         self.assertTrue(self.os.path.exists(file_path))
         with self.open(file_path, "r", encoding="utf8") as fake_file:
             self.assertEqual("old contents", fake_file.read())
+            self.assertEqual("r", fake_file.mode)
         # actual tests
         with self.open(file_path, "w+", encoding="utf8") as fake_file:
             fake_file.write("new contents")
             fake_file.seek(0)
             self.assertTrue("new contents", fake_file.read())
+            self.assertEqual("w+", fake_file.mode)
+
+    def test_open_with_wplus_binary(self):
+        file_path = self.make_path("wplus_file_b")
+        self.create_file(file_path, contents=b"old contents")
+        self.assertTrue(self.os.path.exists(file_path))
+        with self.open(file_path, "rb") as fake_file:
+            self.assertEqual(b"old contents", fake_file.read())
+            self.assertEqual("rb", fake_file.mode)
+        # actual tests
+        with self.open(file_path, "wb+") as fake_file:
+            fake_file.write(b"new contents")
+            fake_file.seek(0)
+            self.assertTrue(b"new contents", fake_file.read())
+            self.assertEqual("rb+", fake_file.mode)
 
     def test_open_with_wplus_truncation(self):
         # set up
@@ -319,6 +337,7 @@ class FakeFileOpenTest(FakeFileOpenTestBase):
         with self.open(file_path, "w+", encoding="utf8") as fake_file:
             fake_file.seek(0)
             self.assertEqual("", fake_file.read())
+            self.assertEqual("w+", fake_file.mode)
 
     def test_open_with_append_flag(self):
         contents = [
@@ -335,6 +354,7 @@ class FakeFileOpenTest(FakeFileOpenTestBase):
                 fake_file.read(0)
             with self.assertRaises(io.UnsupportedOperation):
                 fake_file.readline()
+            self.assertEqual("a", fake_file.mode)
             expected_len = len("".join(contents))
             expected_len += len(contents) * (len(self.os.linesep) - 1)
             self.assertEqual(expected_len, fake_file.tell())
@@ -344,6 +364,22 @@ class FakeFileOpenTest(FakeFileOpenTestBase):
         with self.open(file_path, encoding="utf8") as fake_file:
             self.assertEqual(contents + additional_contents, fake_file.readlines())
 
+    def test_open_with_append_flag_binary(self):
+        contents = b"Just some boring stuff... "
+        additional_contents = b"some excitement added"
+        file_path = self.make_path("append-binary")
+        self.create_file(file_path, contents=contents)
+        with self.open(file_path, "ab") as fake_file:
+            with self.assertRaises(io.UnsupportedOperation):
+                fake_file.read(0)
+            self.assertEqual("ab", fake_file.mode)
+            self.assertEqual(len(contents), fake_file.tell())
+            fake_file.seek(0)
+            self.assertEqual(0, fake_file.tell())
+            fake_file.write(additional_contents)
+        with self.open(file_path, "rb") as fake_file:
+            self.assertEqual(contents + additional_contents, fake_file.read())
+
     def check_append_with_aplus(self):
         file_path = self.make_path("aplus_file")
         self.create_file(file_path, contents="old contents")
@@ -357,6 +393,7 @@ class FakeFileOpenTest(FakeFileOpenTestBase):
                 self.filesystem, delete_on_close=True
             )
         with self.open(file_path, "a+", encoding="utf8") as fake_file:
+            self.assertEqual("a+", fake_file.mode)
             self.assertEqual(12, fake_file.tell())
             fake_file.write("new contents")
             self.assertEqual(24, fake_file.tell())
@@ -391,6 +428,12 @@ class FakeFileOpenTest(FakeFileOpenTestBase):
         with self.open(file_path, "a+", encoding="utf8") as fake_file:
             self.assertEqual("", fake_file.read())
 
+    def test_read_empty_file_with_aplus_binary(self):
+        file_path = self.make_path("aplus_file")
+        with self.open(file_path, "ab+") as fake_file:
+            self.assertEqual(b"", fake_file.read())
+            self.assertEqual("ab+", fake_file.mode)
+
     def test_read_with_rplus(self):
         # set up
         file_path = self.make_path("rplus_file")
@@ -401,11 +444,27 @@ class FakeFileOpenTest(FakeFileOpenTestBase):
         # actual tests
         with self.open(file_path, "r+", encoding="utf8") as fake_file:
             self.assertEqual("old contents here", fake_file.read())
+            self.assertEqual("r+", fake_file.mode)
             fake_file.seek(0)
             fake_file.write("new contents")
             fake_file.seek(0)
             self.assertEqual("new contents here", fake_file.read())
 
+    def test_read_with_rplus_binary(self):
+        file_path = self.make_path("rplus_binary")
+        self.create_file(file_path, contents=b"old contents here")
+        self.assertTrue(self.os.path.exists(file_path))
+        with self.open(file_path, "rb") as fake_file:
+            self.assertEqual(b"old contents here", fake_file.read())
+
+        with self.open(file_path, "rb+") as fake_file:
+            self.assertEqual(b"old contents here", fake_file.read())
+            self.assertEqual("rb+", fake_file.mode)
+            fake_file.seek(0)
+            fake_file.write(b"new contents")
+            fake_file.seek(0)
+            self.assertEqual(b"new contents here", fake_file.read())
+
     def create_with_permission(self, file_path, perm_bits):
         self.create_file(file_path)
         self.os.chmod(file_path, perm_bits)
@@ -900,6 +959,7 @@ class FakeFileOpenTest(FakeFileOpenTestBase):
         file_obj = self.filesystem.get_object(filename)
         with self.open(fd, "wb", closefd=False) as fp:
             fp.write(b"test")
+            self.assertEqual("wb", fp.mode)
         self.assertTrue(self.filesystem.has_open_file(file_obj))
         self.os.close(fd)
         self.assertFalse(self.filesystem.has_open_file(file_obj))
@@ -1028,6 +1088,7 @@ class FakeFileOpenWithOpenerTest(FakeFileOpenTestBase):
 
         file_path = self.make_path("bar")
         with self.open(file_path, "x", encoding="utf8", opener=self.opener) as f:
+            self.assertEqual("x", f.mode)
             assert f.write("bar") == 3
             with self.assertRaises(OSError):
                 f.read()
@@ -1042,6 +1103,7 @@ class FakeFileOpenWithOpenerTest(FakeFileOpenTestBase):
 
         file_path = self.make_path("bar")
         with self.open(file_path, "x+", encoding="utf8", opener=self.opener) as f:
+            self.assertEqual("x+", f.mode)
             assert f.write("bar") == 3
             assert f.read() == ""
         with self.open(file_path, encoding="utf8") as f: