Warnings are not shown when the source is already compiled to .pyc file.

$ find -name '*.py[co]' -exec rm '{}' +
$ ./python -Wa -m test test_unittest
Run tests sequentially
0:00:00 load avg: 0.54 [1/1] test_unittest
/home/serhiy/py/cpython/Lib/unittest/test/test_assertions.py:228: DeprecationWarning: invalid escape sequence \(
  ["^1 != 2 within 7 places \(1 difference\)$", "^oops$",
/home/serhiy/py/cpython/Lib/unittest/test/test_assertions.py:229: DeprecationWarning: invalid escape sequence \(
  "^1 != 2 within 7 places \(1 difference\)$",
/home/serhiy/py/cpython/Lib/unittest/test/test_assertions.py:230: DeprecationWarning: invalid escape sequence \(
  "^1 != 2 within 7 places \(1 difference\) : oops$"])
1 test OK.

Total duration: 5 sec
Tests result: SUCCESS