1  from 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)