io.UnsupportedOperation: File is not open for reading on io.TextIOWrapper

[davidlbaird]

Describe the bug
Found when calling pandas DataFrame.to_csv on an open file handle. An error is raised when
opening a file with mode='w'.

self = <pyfakefs.tests.fake_open_test.FakeFileOpenTest testMethod=test_io_text_wrapper>

    def test_io_text_wrapper(self):
        file_path = self.make_path("foo")
        with self.open(file_path, mode="w") as f:
>         handle = self.fake_io_module.TextIOWrapper(f)

[tests/fake_open_test.py:1014](redacted): 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
fake_file.py:1219: in read_error
    self._raise("File is not open for reading.")
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <pyfakefs.fake_file.FakeFileWrapper object at 0x336ebe0cf5c0>
message = 'File is not open for reading.'

    def _raise(self, message: str) -> NoReturn:
        if self.raw_io:
            self._filesystem.raise_os_error(errno.EBADF, self.file_path)
>       raise io.UnsupportedOperation(message)
E       io.UnsupportedOperation: File is not open for reading.

fake_file.py:823: UnsupportedOperation

The error is not raised if opening the file with mode='w+'.

How To Reproduce
I added a test function in fake_open_test.py

class FakeFileOpenTest(FakeFileOpenTestBase):

  def test_io_text_wrapper(self):
        file_path = self.make_path("foo")
        with self.open(file_path, mode="w") as f:
          handle = self.fake_io_module.TextIOWrapper(f)
          handle.write("test")

The test passes when the "w" is changed to "w+'.

Your environment

Linux-6.16.12-1rodete2-amd64-x86_64-with-glibc2.27
Python 3.12.11 (stable, redacted, redacted) [Clang 9999.0.0 (7d381f2a5634d1e41b61299839d652cc4a021898)]
pyfakefs 6.0.0
pytest 8.3.4

[davidlbaird]

After some poking around, I found that __getattr__ in fake_file.py is called by the above code with name set to readable. The code further down returns a call to _read_error. I am not sure what the caller is, though. I see that io.IOBase does support both readable and writable methods, which would cause problems for the __getattr__ code in fake_file.py.

Adding the following method to fake_file.py resolves the unit test function:

class FakeFileWrapper:

    def readable(self) -> bool:
        return self.open_modes.can_read

[mrbean-bremen]

Thanks - this looks like an omission that hasn't been caught so far. Probably also need to implement writable.
Do you want to create a PR?