Change exception raised from fileno() (and others) to io.UnsupportedOperation

[metasyn]

In capture, we define a method fileno for the DontReadFromInput:
https://github.com/pytest-dev/pytest/blob/master/_pytest/capture.py#L450

However, python docs suggest this shouldn't be implemented:

file.fileno()

Return the integer “file descriptor” that is used by the underlying implementation to request I/O operations from the operating system. This can be useful for other, lower level interfaces that use file descriptors, such as the fcntl module or os.read() and friends.

Note File-like objects which do not have a real file descriptor should not provide this method!

https://docs.python.org/2/library/stdtypes.html

Is there a particular reason it is provided?

[RonnyPfannschmidt]

@metasyn most likely to get better errors, but at close evaluation it looks a bit like its a bad idea for code thatd does checks - i wonder if there are other users that work that way

i checked io.StringIO.fileno and its implemented, so i believe its a good idea to check what is done, not what is said (and perhaps use a more specific exception

[nicoddemus]

Indeed:

Python 3.5.2 | packaged by conda-forge | (default, Jul 26 2016, 02:06:09) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import io
>>> io.StringIO().fileno()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
io.UnsupportedOperation: fileno

Python 2.7.11 (v2.7.11:6d1b6a68f775, Dec  5 2015, 20:32:19) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import io
>>> io.StringIO().fileno()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
io.UnsupportedOperation: fileno

(Same for 2.6)

I guess to follow suit we should raise io.UnsupportedOperation as well, instead of the current ValueError?

[RonnyPfannschmidt]

Yup

[RonnyPfannschmidt]

For compat we should check/ensure its a valueerror subclass

[nicoddemus]

According to the docs:

exception io.UnsupportedOperation
An exception inheriting OSError and ValueError that is raised when an unsupported operation is called on a stream.

So we are all set. 👍

[shubheksha]

I'd like to work on this! From what I understood by going through the above discussion is: I need to change the type of exception in fileno method from ValueError to io.UnsupportedOperation. Is that correct?

[RonnyPfannschmidt]

yup, in case of trouble on python2.6 we will assist

[metasyn]

Sorry slow response here - thanks for your thoughts ya'll. Sounds like a simple change.
@shubheksha are you still interested in doing this? If not, I'd be happy to make a PR.

[RonnyPfannschmidt]

there is no PR so far, so just make one ^^