[Python-Dev] (#19562) Asserts in Python stdlib code (datetime.py)

Terry Reedy tjreedy at udel.edu
Sat Nov 16 02:51:30 CET 2013

propose to change the first assert in Lib/datetime.py
   assert 1 <= month <= 12, month
   assert 1 <= month <= 12,'month must be in 1..12'
to match the next two asserts out of the *53* in the file. I think that 
is the wrong direction of change, but that is not my question here.

Should stdlib code use assert at all?

If user input can trigger an assert, then the code should raise a normal 
exception that will not disappear with -OO.

If the assert is testing program logic, then it seems that the test 
belongs in the test file, in this case, test/test_datetime.py. For 
example, consider the following (backwards) code.

_DI4Y   = _days_before_year(5)
# A 4-year cycle has an extra leap day over what we'd get from pasting
# together 4 single years.
assert _DI4Y == 4 * 365 + 1

To me, the constant should be directly set to its known value.
_DI4Y = 4*365 + 1.
The function should then be tested in test_datetime.
   self.assertEqual(dt._days_before_year(5), dt._DI4Y)

Is there any policy on use of assert in stdlib production code?

Terry Jan Reedy

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