BytesIO object casted into TextIOWrapper does not have fileno attribute.

Question

I have the following code:

>>> import io
>>> b = io.BytesIO(b"Hello World")
>>> f = io.TextIOWrapper(b)
>>> f.fileno()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
io.UnsupportedOperation: fileno

However, when I load a file in, there is a fileno attribute:

>>> f = open("test.py")
>>> f.fileno()
3

Is there a way to create a fileno attribute for the first case, where I am casting a BytesIO object to the TextIOWrapper object?

Answer 1

Well, fileno is not available because there is no file.

The fileno() method returns an integer, representing the position of an open file in the operating system's table of process-related files. If you don't actually open a file, the operating system won't give you a file number.

Your program's standard input, output and error streams (those you read with input and write with print) are numbered 0, 1 and 2. Subsequent open files are usually given sequential numbers by the system.

This cannot be faked reliably: anything you return from fileno() when no actual file is backing the object is a lie. This is why the implementation chose to raise UnsupportedOperation. No return makes sense, except perhaps None.

If it's absolutely imperative that you have a fileno() for your string content, you could do this:

• Open a file for read+write
• Write your string
• Rewind the file to the beginning

There must be a better design, however, that won't force you to use this workaround.