Lib/test/double_const.py

   1 from test.test_support import TestFailed
   2
   3 # A test for SF bug 422177:  manifest float constants varied way too much in
   4 # precision depending on whether Python was loading a module for the first
   5 # time, or reloading it from a precompiled .pyc.  The "expected" failure
   6 # mode is that when test_import imports this after all .pyc files have been
   7 # erased, it passes, but when test_import imports this from
   8 # double_const.pyc, it fails.  This indicates a woeful loss of precision in
   9 # the marshal format for doubles.  It's also possible that repr() doesn't
  10 # produce enough digits to get reasonable precision for this box.
  11
  12 PI    = 3.14159265358979324
  13 TWOPI = 6.28318530717958648
  14
  15 PI_str    = "3.14159265358979324"
  16 TWOPI_str = "6.28318530717958648"
  17
  18 # Verify that the double x is within a few bits of eval(x_str).
  19 def check_ok(x, x_str):
  20     assert x > 0.0
  21     x2 = eval(x_str)
  22     assert x2 > 0.0
  23     diff = abs(x - x2)
  24     # If diff is no larger than 3 ULP (wrt x2), then diff/8 is no larger
  25     # than 0.375 ULP, so adding diff/8 to x2 should have no effect.
  26     if x2 + (diff / 8.) != x2:
  27         raise TestFailed("Manifest const %s lost too much precision " % x_str)
  28
  29 check_ok(PI, PI_str)
  30 check_ok(TWOPI, TWOPI_str)