Honor --cov as an override of source_pkgs in the coverage config

src/pytest_cov/engine.py

@@ -55,6 +55,10 @@ class CovController:
     def __init__(self, options: argparse.Namespace, config: Union[None, object], nodeid: Union[None, str]):
         """Get some common config used by multiple derived classes."""
         self.cov_source = options.cov_source
+        # An explicit --cov=... must narrow measurement to exactly what was asked for.
+        # Passing source_pkgs=[] overrides any source_pkgs in the coverage config, which
+        # would otherwise be added to the sources and widen the report again.
+        self.cov_source_pkgs = None if options.cov_source is None else []
         self.cov_report = options.cov_report
         self.cov_config = options.cov_config
         self.cov_append = options.cov_append
@@ -236,6 +240,7 @@ class Central(CovController):
     def start(self):
         self.cov = coverage.Coverage(
             source=self.cov_source,
+            source_pkgs=self.cov_source_pkgs,
             branch=self.cov_branch,
             data_suffix=True,
             config_file=self.cov_config,
@@ -249,6 +254,7 @@ def start(self):
 
         self.combining_cov = coverage.Coverage(
             source=self.cov_source,
+            source_pkgs=self.cov_source_pkgs,
             branch=self.cov_branch,
             data_suffix=f'{filename_suffix(True)}.combine',
             data_file=os.path.abspath(self.cov.config.data_file),  # noqa: PTH100
@@ -286,6 +292,7 @@ class DistMaster(CovController):
     def start(self):
         self.cov = coverage.Coverage(
             source=self.cov_source,
+            source_pkgs=self.cov_source_pkgs,
             branch=self.cov_branch,
             data_suffix=True,
             config_file=self.cov_config,
@@ -301,6 +308,7 @@ def start(self):
         self.cov._warn_preimported_source = False
         self.combining_cov = coverage.Coverage(
             source=self.cov_source,
+            source_pkgs=self.cov_source_pkgs,
             branch=self.cov_branch,
             data_suffix=f'{filename_suffix(True)}.combine',
             data_file=os.path.abspath(self.cov.config.data_file),  # noqa: PTH100
@@ -389,6 +397,7 @@ def start(self):
         # Erase any previous data and start coverage.
         self.cov = coverage.Coverage(
             source=self.cov_source,
+            source_pkgs=self.cov_source_pkgs,
             branch=self.cov_branch,
             data_suffix=True,
             config_file=self.cov_config,

tests/test_pytest_cov.py

@@ -206,6 +206,31 @@ def test_central(pytester, testdir, prop):
     assert result.ret == 0
 
 
+def test_central_explicit_cov_overrides_source_pkgs(testdir):
+    testdir.tmpdir.join('pkga').ensure_dir().join('__init__.py').write('def fa():\n    return 1\n')
+    testdir.tmpdir.join('pkgb').ensure_dir().join('__init__.py').write('def fb():\n    return 2\n')
+    script = testdir.makepyfile("""
+        from pkga import fa
+        from pkgb import fb
+
+        def test_both():
+            assert fa() == 1
+            assert fb() == 2
+    """)
+    testdir.tmpdir.join('.coveragerc').write("""
+[run]
+source_pkgs =
+    pkga
+    pkgb
+""")
+
+    result = testdir.runpytest('-v', '--cov=pkga', '--cov-report=term-missing', script)
+
+    result.stdout.fnmatch_lines(['*_ coverage: platform *, python * _*', 'pkga*100%*'])
+    assert not any(line.startswith('pkgb') for line in result.stdout.lines)
+    assert result.ret == 0
+
+
 def test_annotate(testdir):
     script = testdir.makepyfile(SCRIPT)