I ran my_activity.py 5 times like this:

for i in 1 2 3 4 5 ; do
  ~/depot_tools/my_activity.py -b 3/1/18 -e 8/31/18 -u pkl@chromium.org > pkl-$i.txt
done

This gives 5 output files:
-rw-r--r--  1 pkl  eng      0 Aug 28 07:18 pkl-1.txt
-rw-r--r--  1 pkl  eng  28011 Aug 28 07:19 pkl-2.txt
-rw-r--r--  1 pkl  eng  24017 Aug 28 07:19 pkl-3.txt
-rw-r--r--  1 pkl  eng  21366 Aug 28 07:19 pkl-4.txt
-rw-r--r--  1 pkl  eng  21366 Aug 28 07:19 pkl-5.txt

I do not expect 5 consecutive runs would give 5 different output.

The first file is 0-length because it had an exception like this:
WARNING:root:Consider installing python-keyring
Traceback (most recent call last):
  File "/Users/pkl/depot_tools/my_activity.py", line 1096, in <module>
    sys.exit(main())
  File "/Users/pkl/depot_tools/my_activity.py", line 1051, in main
    my_activity.get_issues()
  File "/Users/pkl/depot_tools/my_activity.py", line 711, in get_issues
    self.monorail_issue_search, monorail_projects.keys())
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/pool.py", line 251, in map
    return self.map_async(func, iterable, chunksize).get()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/pool.py", line 567, in get
    raise self._value
ssl.SSLError: ('The read operation timed out',)

The other 4 runs have warnings like this (which I think are expected):
WARNING:root:Consider installing python-keyring
ERROR:root:Access Errors:
ERROR:root:Looking up 'review.coreboot.org': Bad Request: User pkl not found
ERROR:root:Looking up 'pdfium-review.googlesource.com': Bad Request: User pkl not found