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      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)
     31