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Not flushed file gives next results:
rion@rionhost ~/temp $ python2 test.py 
Traceback (most recent call last):
 File "test.py", line 10, in <module>
 m = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
mmap.error: [Errno 22] Invalid argument
rion@rionhost ~/temp $ python3 test.py 
Traceback (most recent call last):
 File "test.py", line 10, in <module>
 m = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
ValueError: mmap offset is greater than file size
testcase:
import mmap
from tempfile import TemporaryFile
f = TemporaryFile()
f.write(b"hello world")
#f.flush()
m = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
just uncomment f.flush() and try again. feel the difference.
suggestion: force flush on file descriptor when passed to mmap.