[docs] [issue15068] fileinput requires two EOF when reading stdin

[Serhiy Storchaka]

Serhiy Storchaka <storchaka at gmail.com> added the comment:

>  The patch I've attached checks if the number of bytes read from the file is less than the size of the buffer (which means that the file has ended).

>From io.RawIOBase.read docs:

"""
Read up to n bytes from the object and return them. As a convenience, if
n is unspecified or -1, readall() is called. Otherwise, only one system
call is ever made. Fewer than n bytes may be returned if the operating
system call returns fewer than n bytes.

If 0 bytes are returned, and n was not 0, this indicates end of file.
"""

This is not an arbitrary assumption. In particular, when reading from a
terminal with line buffering (you can edit the line until you press
Enter) on C level you read only a whole line (if line length is not
greater than buffer length) and 0 bytes you will receive only by
pressing ^D or ^Z at the beginning of the line. Same for pipes and
sockets. On Python level there are many third-party implementations of
file-like objects which rely on this behavior, you cannot rewrite all of
them.

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<http://bugs.python.org/issue15068>
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