[3.12] gh-108303: Move `double_const` to `test_import` where it belongs (GH-112108) (#112976)

gh-108303: Move `double_const` to `test_import` where it belongs (GH-112108)
(cherry picked from commit 0738b9a338fd27ff2d4456dd9c15801a8858ffd9)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>

2 files changed, 36 insertions, 3 deletions

diff --git a/Lib/test/test_import/__init__.py b/Lib/test/test_import/__init__.py
index 67904cf..0e38e48 100644
--- a/Lib/test/test_import/__init__.py
+++ b/Lib/test/test_import/__init__.py
@@ -406,9 +406,12 @@ class ImportTests(unittest.TestCase):
             import RAnDoM
 
     def test_double_const(self):
-        # Another brief digression to test the accuracy of manifest float
-        # constants.
-        from test import double_const  # don't blink -- that *was* the test
+        # Importing double_const checks that float constants
+        # serialiazed by marshal as PYC files don't lose precision
+        # (SF bug 422177).
+        from test.test_import.data import double_const
+        unload('test.test_import.data.double_const')
+        from test.test_import.data import double_const
 
     def test_import(self):
         def test_with_extension(ext):
diff --git a/Lib/test/test_import/data/double_const.py b/Lib/test/test_import/data/double_const.py
new file mode 100644
index 0000000..67852aa
--- /dev/null
+++ b/Lib/test/test_import/data/double_const.py
@@ -0,0 +1,30 @@
+from test.support import TestFailed
+
+# A test for SF bug 422177:  manifest float constants varied way too much in
+# precision depending on whether Python was loading a module for the first
+# time, or reloading it from a precompiled .pyc.  The "expected" failure
+# mode is that when test_import imports this after all .pyc files have been
+# erased, it passes, but when test_import imports this from
+# double_const.pyc, it fails.  This indicates a woeful loss of precision in
+# the marshal format for doubles.  It's also possible that repr() doesn't
+# produce enough digits to get reasonable precision for this box.
+
+PI    = 3.14159265358979324
+TWOPI = 6.28318530717958648
+
+PI_str    = "3.14159265358979324"
+TWOPI_str = "6.28318530717958648"
+
+# Verify that the double x is within a few bits of eval(x_str).
+def check_ok(x, x_str):
+    assert x > 0.0
+    x2 = eval(x_str)
+    assert x2 > 0.0
+    diff = abs(x - x2)
+    # If diff is no larger than 3 ULP (wrt x2), then diff/8 is no larger
+    # than 0.375 ULP, so adding diff/8 to x2 should have no effect.
+    if x2 + (diff / 8.) != x2:
+        raise TestFailed("Manifest const %s lost too much precision " % x_str)
+
+check_ok(PI, PI_str)
+check_ok(TWOPI, TWOPI_str)