Issue 20856: bz2.peek always peeks all the remaining bytes ignoring n argument (original) (raw)

Created on 2014-03-06 08:56 by vajrasky, last changed 2022-04-11 14:57 by admin. This issue is now closed.

Messages (4)
msg212796 - (view)	Author: Vajrasky Kok (vajrasky) *	Date: 2014-03-06 08:56
# Bug demo TEXT_LINES = [ b'cutecat\n', b'promiscuousbonobo\n', ] TEXT = b''.join(TEXT_LINES) import bz2 filename = '/tmp/demo.bz2' with open(filename, 'wb') as f: f.write(bz2.compress(TEXT)) with bz2.BZ2File(filename) as bz2f: pdata = bz2f.peek(n=7) print(pdata) It outputs b'cutecat\npromiscuousbonobo\n', not b'cutecat'. Here is the patch to fix the bug.
msg212797 - (view)	Author: Serhiy Storchaka (serhiy.storchaka) *	Date: 2014-03-06 09:13
This is documented behavior. .. method:: peek([n]) Return buffered data without advancing the file position. At least one byte of data will be returned (unless at EOF). The exact number of bytes returned is unspecified.
msg212802 - (view)	Author: Vajrasky Kok (vajrasky) *	Date: 2014-03-06 10:05
Just curious, why the exact number of bytes returned is unspecified in bz2 (in other words, n argument is ignored)? gzip uses n argument.
msg212804 - (view)	Author: Serhiy Storchaka (serhiy.storchaka) *	Date: 2014-03-06 10:31
Because it is unspecified in io.BufferedReader.peek() and in many classes implemented the io.BufferedReader interface. .. method:: peek([size]) Return bytes from the stream without advancing the position. At most one single read on the raw stream is done to satisfy the call. The number of bytes returned may be less or more than requested. I agree that this is weird, but this is a much larger issue than just bz2. We can't just "fix" this for bz2. This worths a discussion on Python-Dev.