Jeffrey Kintscher <websur...@surf2c.net> added the comment:
I adapted the test code on StackOverflow to show the expected and actual values.
I get this output using Python 2.7.16:
opening myfile with wb
writing 3 bytes to file
('f.tell(): expecting 3, got', 3)
closing myfile
opening myfile with a+b
('f.tell(): expecting 3, got', 3)
writing 3 bytes to file
('f.tell(): expecting 6, got', 6)
('f.seek(0): expecting 0, got', None)
('f.tell(): expecint 0, got', 0)
writing 3 bytes to file
('f.tell(): expecting 9, got', 9)
writing 3 bytes to file
('f.tell(): expecting 12, got', 12)
('f.seek(0, io.SEEK_CUR): expecting 12, got', None)
('f.tell(): expecting 12, got', 12)
('f.seek(0): expecting 0, got', None)
("f.read(): expecting b'abcdefghijkl', got", 'abcdefghijkl')
closing file
removing file
As described, I get different results using Python 3.7.3 as well as the master
branch (Python 3.8.0a4+):
closing myfile
opening myfile with a+b
f.tell(): expecting 3, got 3
writing 3 bytes to file
f.tell(): expecting 6, got 6
f.seek(0): expecting 0, got 0
f.tell(): expecint 0, got 0
writing 3 bytes to file
f.tell(): expecting 9, got 3
writing 3 bytes to file
f.tell(): expecting 12, got 6
f.seek(0, io.SEEK_CUR): expecting 12, got 12
f.tell(): expecting 12, got 12
f.seek(0): expecting 0, got 0
f.read(): expecting b'abcdefghijkl', got b'abcdefghijkl'
closing file
removing file
I wrote an equivalent C program using the stream I/O functions and get the same
results as Python 2.7.16:
opening file with wb
writing 3 bytes to file
ftell(f): expecting 3, got 3
closing file
opening file with a+b
ftell(f): expecting 3, got 3
writing 3 bytes to filftell(f): expecting 6, got 6
fseek(f, 0, SEEK_SET): expecting 0, got 0
ftell(f): expecting 0, got 0
writing 3 bytes to file
ftell(f): expecting 9, got 9
writing 3 bytes to file
ftell(f): expecting 12, got 12
fseek(f, 0, SEEK_CUR): expecting 0, got 0
ftell(f): expecting 12, got 12
fseek(f, 0, SEEK_SET): expecting 0, got 0
fread(buf, 1, 256, f): expecting 12, got 12
expecting 'abcdefghijkl', got 'abcdefghijkl'
closing file
removing file
I consider this behavior to be a bug because Python 2.7 and C agree, but Python
3.7 behaves differently. Requiring that developers know and work around
non-standard quirky behavior leads to extra work and error-prone code.
----------
nosy: +Jeffrey.Kintscher
Added file: https://bugs.python.org/file48368/bpo-36411.py
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<https://bugs.python.org/issue36411>
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