gh-51524: Fix bug when calling trace.CoverageResults with valid infile (#99629)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

2 files changed, 11 insertions, 1 deletions

diff --git a/Lib/test/test_trace.py b/Lib/test/test_trace.py
index 5f71211..fad2b3b 100644
--- a/Lib/test/test_trace.py
+++ b/Lib/test/test_trace.py
@@ -1,4 +1,5 @@
 import os
+from pickle import dump
 import sys
 from test.support import captured_stdout
 from test.support.os_helper import (TESTFN, rmtree, unlink)
@@ -412,6 +413,15 @@ class TestCoverage(unittest.TestCase):
         self.assertIn(modname, coverage)
         self.assertEqual(coverage[modname], (5, 100))
 
+    def test_coverageresults_update(self):
+        # Update empty CoverageResults with a non-empty infile.
+        infile = TESTFN + '-infile'
+        with open(infile, 'wb') as f:
+            dump(({}, {}, {'caller': 1}), f, protocol=1)
+        self.addCleanup(unlink, infile)
+        results = trace.CoverageResults({}, {}, infile, {})
+        self.assertEqual(results.callers, {'caller': 1})
+
 ### Tests that don't mess with sys.settrace and can be traced
 ### themselves TODO: Skip tests that do mess with sys.settrace when
 ### regrtest is invoked with -T option.
diff --git a/Lib/trace.py b/Lib/trace.py
index 2cf3643..213e465 100755
--- a/Lib/trace.py
+++ b/Lib/trace.py
@@ -172,7 +172,7 @@ class CoverageResults:
             try:
                 with open(self.infile, 'rb') as f:
                     counts, calledfuncs, callers = pickle.load(f)
-                self.update(self.__class__(counts, calledfuncs, callers))
+                self.update(self.__class__(counts, calledfuncs, callers=callers))
             except (OSError, EOFError, ValueError) as err:
                 print(("Skipping counts file %r: %s"
                                       % (self.infile, err)), file=sys.stderr)