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According to the the documentation, BufferedReader.read() and BufferedWriter.write() should raise io.BlockingIOError if the file is in non-blocking mode and the operation cannot succeed without blocking.
However, BufferedReader.read() returns None (which is what RawIOBase.read() is documented as doing), and BufferedWriter.write() raises IOError with a message like
 raw write() returned invalid length -1 (should have been 
 between 0 and 5904)
I tested this on Linux with Python 2.6, 2.7 and 3.x.
Attached is a unit test.

BufferedReader.readinto() should also raise BlockingIOError according to the docs. Updated unittest checks for that also.
BTW, The documentation for BufferedIOBase.read() says that BlockingIOError should be raised if nothing can be read in non-blocking mode. BufferedReader inherits from BufferedIOBase and overrides the read() method. This is the documentation for BufferedReader.read():
 read([n])
 Read and return n bytes, or if n is not given or negative, 
 until EOF or if the read call would block in non-blocking mode.
This sentence is complete gobbledygook, and it makes no mention of what should happen if nothing can be read in non-blocking mode. So I presume behaviour for BufferedReader.read() should match the documented behaviour for BufferedIOBase.read().

Wierdly, it looks like BlockingIO is not raised anywhere in the code for the C implementation of io.
Even more wierdly, in the Python implementation of io, BlockingIOError is only ever raised by except clauses which have already caught BlockingIOError. So, of course, these clauses are dead code.
The only code in CPython which can ever succesfully raise BlockingIOError is MockNonBlockWriterIO.write() in test/test_io.py.
I don't know what the correct behaviour is for flush() and close() if you get EAGAIN. I think flush() should raise an error rather than blocking, and that close() should delegate to self.raw.close() before raising the error.
The docs say that read(), readinto() and write() can raise BlockingIOError. But what should readall() and readline() do? Should we just try to emulate whatever Python's old libc IO system did (with BlockingIOError replacing IOError(EAGAIN))?

> Wierdly, it looks like BlockingIO is not raised anywhere in the code
> for the C implementation of io.
That would explain why it isn't raised :)
This is a hairy issue: read(n) is documented as returning either n bytes or nothing. But what if less than n bytes are available non-blocking? Currently we return a partial read. readline() behaviour is especially problematic:
>>> fcntl.fcntl(r, fcntl.F_SETFL, os.O_NDELAY)
0
>>> rf = open(r, mode='rb')
>>> os.write(w, b'xy')
2
>>> rf.read(3)
b'xy'
>>> os.write(w, b'xy')
2
>>> rf.readline()
b'xy'
We should probably raise BlockingIOError in these cases, but that complicates the implementation quite a bit: where do we buffer the partial data? The internal (fixed size) buffer might not be large enough.
write() is a bit simpler, since BlockingIOError has a "characters_written" attribute which is meant to inform you of the partial success: we can just reuse that. That said, BlockingIOError could grow a "partial_read" attribute containing the read result...
Of course, we may also question whether it's useful to use buffered I/O objects around non-blocking file descriptors; if you do non-blocking I/O, you generally want to be in control, which means not having any implicit buffer between you and the OS.
(this may be a topic for python-dev)

> This is a hairy issue
Indeed.
Performing partial read/write may sound imperfect, but using buffered I/O around non-blockind FD is definitely not a good idea.
Also, the advantage of the current approach is that at least, no data is ever lost (and changing the behavior to raise a BlockingIOError might break some code out there in the wild).
Note that Java's BufferedInputStream and ReadableByteChannel also return partial reads.
So I'm somewhat inclined to keep the current behavior (it would however probably be a good idea to update the documentation to warn about this limitation, though).

> Also, the advantage of the current approach is that at least, no data
> is ever lost
But what about the buggy readline() behaviour?

> Note that Java's BufferedInputStream and ReadableByteChannel also 
> return partial reads.
Apparently, they are specified to, even for blocking streams (which I find a bit weird, and the language in the docs seems deliberately vague). Python's buffered read(), though, is specified to return the requested number of bytes (unless EOF happens).

No one has suggested raising BlockingIOError and DISCARDING the data when a partial read has occurred. The docs seem to imply that the partially read data should be returned since they only say that BlockingIOError should be raised if there is NOTHING to read. Clearly this should all be spelt out properly.
That leaves the question of whether, when there is NOTHING to read, BlockingIOError should be raised (as the docs say) or None should be returned (as is done now). I don't mind either way as long as the docs match reality.
The part which really needs addressing is partial writes. Currently, if a write fails with EAGAIN then IOError is raised and there is no way to work out how much data was written/buffered. The docs say that BlockingIOError should be raised with the e.args[2] set to indicate the number of bytes written/buffered. This at least should be fixed.
I will work on a patch.

> But what about the buggy readline() behaviour?
Just tell people that if the return value is a string which does not end in '\n' then it might caused by EOF or EAGAIN. They can just call readline() again to check which.

The third arg of BlockingIOError is used in two quite different ways.
In write(s) it indicates the number of bytes of s which have been "consumed" (ie written to the raw file or buffered).
But in flush() and flush_unlocked() (in _pyio) it indicates the number of bytes from the internal buffer which have been written to the raw file.
I think this explains the following comment in write():
 # We're full, so let's pre-flush the buffer
 try:
 self._flush_unlocked()
 except BlockingIOError as e:
 # We can't accept anything else.
 # XXX Why not just let the exception pass through?
 raise BlockingIOError(e.errno, e.strerror, 0)
I don't think flush() should try to tell us how many bytes were flushed: we only need to know whether we need to try again.

> Apparently, they are specified to, even for blocking streams (which 
> I find a bit weird, and the language in the docs seems deliberately
> vague). 
"""
As an additional convenience, it attempts to read as many bytes as possible by repeatedly invoking the read method of the underlying stream. This iterated read continues until one of the following conditions becomes true:
The specified number of bytes have been read,
The read method of the underlying stream returns -1, indicating end-of-file, or
The available method of the underlying stream returns zero, indicating that further input requests would block.
"""
As I understand it, it will return the number of bytes asked, unless EOF or EAGAIN/EWOULDBLOCK. It would seem reasonable to me to add the same note for non-blocking FDs to Python's read().
>> But what about the buggy readline() behaviour?
> Just tell people that if the return value is a string which does not 
> end in '\n' then it might caused by EOF or EAGAIN. They can just call 
> readline() again to check which.
Sounds reasonable.
> No one has suggested raising BlockingIOError and DISCARDING the data 
> when a partial read has occurred.
The problem is that if we raise BlockingIOError, we can only buffer a limited amount of data.
> The docs seem to imply that the partially read data should be returned 
> since they only say that BlockingIOError should be raised if there is 
> NOTHING to read. Clearly this should all be spelt out properly.
Agreed.
> That leaves the question of whether, when there is NOTHING to 
> read, BlockingIOError should be raised (as the docs say) or None
> should be returned (as is done now).
I don't have a string feeling: if we don't raise BlockingIOError on partial reads, then it probably makes sense to keep None.

Currently a BlockingIOError exception raised by flush() sets
characters_written to the number of bytes fushed from the internal
buffer. This is undocument (although there is a unit test which tests
for it) and causes confusion because characters_written has conflicting
meanings depending on whether the exception was raised by flush() or
write(). I would propose setting characters_written to zero on
BlockingIOError exceptions raised by flush(). Are there any reasons not
to make this change?
Also, the docs say that the raw file wrapped by
BufferedReader/BufferedWriter should implement RawIOBase. This means
that self.raw.write() should return None instead of raising
BlockingIOError. But the implementation tries to cope with
BlockingIOError coming from a raw write. In fact, the
MockNonBlockWriterIO class in unit tests is used as a raw file, but its
write() method raises BlockingIOError.
It would simplify matters a lot to insist that raw files implement
RawIOBase properly.
BTW, when I try to change characters_written of an existing
BlockingIOError exception using the pointer returned by
_buffered_check_blocking_error(), it appears not to work: the exception
continues to have characters_written == 0 -- not sure why...

> >> But what about the buggy readline() behaviour?
> > Just tell people that if the return value is a string which does not 
> > end in '\n' then it might caused by EOF or EAGAIN. They can just call 
> > readline() again to check which.
> 
> Sounds reasonable.
But then what's the point of using buffered I/O at all? If it can't
offer anything more than raw I/O, I'd rather do something like raise a
RuntimeError("buffered I/O doesn't work with non-blocking streams") when
the raw stream returns None. Returning partial results on a buffered's
readline() is not something we should ever do.
(actually, raw I/O readline() is probably buggy as well)

> But then what's the point of using buffered I/O at all? If it can't
> offer anything more than raw I/O, I'd rather do something like raise
> a RuntimeError("buffered I/O doesn't work with non-blocking streams")
> when the raw stream returns None.
Well, ideally it should be an UnsupportedOperation, but that's an option. The only think I didn't like about this is that we should ideally raise this error upon the first method call, not when - and if - we receive EAGAIN.
Another possibility would be that, since lines are usually reasonably sized, they should fit in the buffer (which is 8KB by default). So we could do the extra effort of buffering the data and return it once the line is complete: if the buffer fills up before we got the whole line, then we could raise a RuntimeError("Partial line read"). Note that I didn't check if it's easily feasible (i.e. we should avoid introducing kludges in the I/O layer just to handle thi corner case).
> Returning partial results on a buffered's readline() is not something
> we should ever do.
Yeah, I know.
Java made the choice of making readline() block, which is IMHO even worse (I mean, it defeats the whole point of non-blocking I/O...).

> Another possibility would be that, since lines are usually reasonably
> sized, they should fit in the buffer (which is 8KB by default). So we 
> could do the extra effort of buffering the data and return it once the 
> line is complete: if the buffer fills up before we got the whole line, 
> then we could raise a RuntimeError("Partial line read"). Note that I 
> didn't check if it's easily feasible (i.e. we should avoid introducing 
> kludges in the I/O layer just to handle thi corner case).
Discarding data rarely is worse than always throwing an exception.

The attached patch makes BufferedWrite.write() raise BlockingIOError when the raw file is non-blocking and the write would block.

> write() is a bit simpler, since BlockingIOError has 
> a "characters_written" attribute which is meant to inform you of the 
> partial success: we can just reuse that. That said, BlockingIOError 
> could grow a "partial_read" attribute containing the read result...
Now that I think about it, it's probably the best solution:
always raise a BlockingIOError in case of partial write, with characters_written set correctly (sbt's patch).
And do the same thing on partial read/readline, and return the partially read data as an attribute of BlockingIOError (we could also return a characters_read that would indicate the exact number of bytes read: then the user could call read()/read_into() with exactly characters_read).
That could certainly break existing - sloppy - code, but this would be more much consistent than the current behavior.

Testing the patch a bit more thoroughly, I found that data received from the readable end of the pipe can be corrupted by the C implementation. This seems to be because two of the previously dormant codepaths did not properly maintain the necessary invariants.
I got the failures to go away by adding
 self->pos += avail;
in two places. However, I really do not know what all the attributes mean. (Should self->raw_pos also be modified...?) Someone familiar with the code would need to check whether things are being done properly. This new patch adds some XXX comments in places in bufferedio.c which I am unsure about.

Hi,
> Testing the patch a bit more thoroughly, I found that data received
> from the readable end of the pipe can be corrupted by the C
> implementation. This seems to be because two of the previously
> dormant codepaths did not properly maintain the necessary invariants.
Ouch. Were they only non-blocking codepaths?
> in two places. However, I really do not know what all the attributes
> mean. (Should self->raw_pos also be modified...?)
raw_pos is the position which the underlying raw stream is currently at.
It only needs to be modified when a successful write(), read() or seek()
is done on the raw stream.
Another comment: you set errno to EAGAIN, but it is not sure that was
the actual errno raised by the raw stream (although that's quite
likely). You might want to reflect the actual C errno (but you'd better
set it to 0 before the system call, then).

> Ouch. Were they only non-blocking codepaths?
Yes.
> raw_pos is the position which the underlying raw stream is currently
> at. It only needs to be modified when a successful write(), read()
> or seek() is done on the raw stream.
Do you mean self->raw_pos should give the same answer as self.raw.tell()? (But that seems to be the definition of self->abs_pos.) Or is it the buffer offset which corresponds to self.raw.tell()?

> Do you mean self->raw_pos should give the same answer as
> self.raw.tell()? (But that seems to be the definition of
> self->abs_pos.) Or is it the buffer offset which corresponds to
> self.raw.tell()?
The latter.

Here is an updated patch which uses the real errno.
It also gets rid of the restore_pos argument of _bufferedwriter_flush_unlocked() which is always set to false -- 
I guess buffered_flush_and_rewind_unlocked() is used instead.

Thanks again. Just a nit: the tests should be in MiscIOTest, since they don't directly instantiate the individual classes. Also, perhaps it would be nice to check that the exception's "errno" attribute is EAGAIN.

> Thanks again. Just a nit: the tests should be in MiscIOTest, since 
> they don't directly instantiate the individual classes. Also, perhaps 
> it would be nice to check that the exception's "errno" attribute is 
> EAGAIN.
Done.

Thanks. Who should I credit? "sbt"?

> Thanks. Who should I credit? "sbt"?
Yeah, thanks.

New changeset ac2c4c62b486 by Antoine Pitrou in branch '3.2':
Issue #13322: Fix BufferedWriter.write() to ensure that BlockingIOError is
http://hg.python.org/cpython/rev/ac2c4c62b486
New changeset 3cd1985ed04f by Antoine Pitrou in branch 'default':
Issue #13322: Fix BufferedWriter.write() to ensure that BlockingIOError is
http://hg.python.org/cpython/rev/3cd1985ed04f
New changeset e84e17643eeb by Antoine Pitrou in branch '2.7':
Issue #13322: Fix BufferedWriter.write() to ensure that BlockingIOError is
http://hg.python.org/cpython/rev/e84e17643eeb 

The patches only fix write? What about read?
http://bugs.python.org/issue13858 

My experiments with buffered and unbuffered readers wrapping a non-blocking TCP socket, with no data received:
Method Buffered impl. Buffered doc. SocketIO impl. RawIOBase doc.
========= ============== =============== ============== ==============
read None BlockingIOError None None
read1 b"" [unclear]
readinto None BlockingIOError None None
readinto1 None BlockingIOError
readall None [unclear]
peek b"" [unclear]
readline b"" [unspecified] OSError [unspecified]
readlines [] [unspecified] OSError [unspecified]
__next__ StopIteration [unspecified] OSError [unspecified]
The non-blocking behaviour of BufferedReader matches the RawIOBase documentation better than its own documentation. I’m not sure which way it should be fixed. Is this a documentation bug or an implementation bug?
I propose to change the read1() and peek() methods to behave like the others (whether than be returning None or raising BlockingIOError). It would be nice to have a way to differentiate non-blocking data being unavailable from hard EOF, at least for non-interactive mode, and the added consistency would be nice.
A non-blocking BufferedReader use case: to be able to peek one byte of a HTTP response stream to see if the connection has been closed. Plain sockets support MSG_PEEK, but SSL sockets don’t, and a BufferedReader is already being used. Later when actually parsing the response, the reader is set to blocking mode.

Looking at test_io.BufferedReaderTest.test_read_non_blocking(), at Lib/test/test_io.py:1037, there are explicit tests for ‘peek(1) == b"" ’ and ‘read() is None’. The peek() test was added in revision 3049ac17e256, in 2009 (large merge of "io" implementation in C; can’t find more detailed history). The read() test was added in revision 21233c2e5d09 in 2007, with a remark about a "tentative decision to drop nonblocking I/O support from the buffering layers".
My suggestion is to make the read/into/1/all/peek() methods all return None if no non-blocking data is available, and return a short non-empty result if some data was available but not enough to satisfy the equivalent blocking call. However, this would invove changing the behaviour of BufferedReader.read1() and peek(); would that be allowed?
The readline() based methods could probably work similarly, but that would be another issue and a bigger change, because the equivalent RawIOBase methods do not return None.

I’m adding nonblock-none.patch, which changes all the simple buffered read methods to return None when there is no non-blocking data available. I’d be interested to see if other people thought this was a sensible change.
* Documented existing behaviour of C "io" module buffered read(), readinto/1() returning None
* io.BufferedIOBase.read() and io.FileIO.readall() doc strings already mention returning None
* Removed false claims of buffered reads returning short for interactive streams, sockets and pipes
* Fixed _pyio return values to match C "io" module returning None
* Fixed BufferedIOBase.readinto/1() implementation to handle read/1() returning None
* Changed buffered read1() and peek() to return None

See also Issue 1191964, discussing inconsistencies and how to differentiate the four non-blocking special results in a new API:
* read signals permanent EOF
* read signals no data temporarily available
* write signals a permanently broken pipe or connection
* write signals no buffer space temporarily available

For added fun: at least one part of the standard library doesn't expect None returns from read() in the buffering layer.
>>> import os
>>> r, w = os.pipe2(os.O_NONBLOCK)
>>> f = os.fdopen(r, 'r')
>>> f.read()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/izbyshev/workspace/cpython/Lib/codecs.py", line 321, in decode
 data = self.buffer + input
TypeError: can't concat NoneType to bytes
Note that nonblock-none.patch doesn't seem to address that.

Generally I doubt anyone is using the non-blocking semantics of the Python 3 I/O stack. People doing non-blocking I/O generally do it with sockets instead, which tend to reproduce quite literally the POSIX behaviour and error codes.

Yes, your claim is confirmed by the fact that there have been little interest in this issue since 2011. Still, non-blocking behavior is incorrectly specified in the docs and is inconsistent (as investigated by Martin). And obscure errors like in my example or in Issue 13858 show that I/O stack is confused too. To prevent people from tripping on it, would you consider recommending against usage of I/O stack for non-blocking operations in io module docs?

Yes, I think adding a note in the docs is reasonable.

Issue 35762 was opened specifically about Izbyshev’s case: TextIOWrapper behaviour with a non-blocking file. Calling "os.fdopen" with mode='r' (text mode) returns a TextIOWrapper object.

I closed bpo-35762 as a duplicate of this issue: subprocess.Popen with universal_newlines and nonblocking streams fails with "can't concat NoneType to bytes".

I closed bpo-26292 as a duplicate of this issue: Raw I/O writelines() broken for non-blocking I/O.

I closed bpo-24560 as a duplicate of this issue: codecs.StreamReader doesn't work with nonblocking streams: TypeError: can't concat bytes to NoneType.

See also bpo-32561: Add API to io objects for non-blocking reads/writes.

TextIOWrapper, and maybe also BufferedRead, may raise an exception if the underlying file descriptor is configured in non-blocking mode. It may require an additional syscall() to query the FD properties, which may slowdown the creation of file objects in Python :-/

I have experienced both ״TypeError: can't concat NoneType to bytes״, and the fact BufferedIO returns None.
@pitrou @izbyshev contrary to your belief, I think there is at least some interest in this issue. Every few months another ticket is opened about a different aspect of the same underlying problem.