fix: 3.11.0b4 has 0-numbered lines. Fixes #1419

CPython added these lines in
python/cpython@1bfe83a

CHANGES.rst

@@ -20,11 +20,18 @@ development at the same time, such as 4.5.x and 5.0.
 Unreleased
 ----------
 
+- Updated for a small change in Python 3.11.0 beta 4: modules now start with a
+  line with line number 0, which is ignored.  This line cannnot be executed, so
+  coverage totals were thrown off.  This line is now ignored by coverage.py,
+  but this also means that truly empty modules (like ``__init__.py``) have no
+  lines in them, rather than one phantom line.  Fixes `issue 1419`_.
+
 - Internal debugging data added to sys.modules is now an actual module, to
   avoid confusing code that examines everything in sys.modules.  Thanks,
   Yilei Yang (`pull 1399`_).
 
 .. _pull 1399: https://github.com/nedbat/coveragepy/pull/1399
+.. _issue 1419: https://github.com/nedbat/coveragepy/issues/1419
 
 
 .. _changes_6-4-1:

coverage/env.py

@@ -110,6 +110,9 @@ class PYBEHAVIOR:
     # Some words are keywords in some places, identifiers in other places.
     soft_keywords = (PYVERSION >= (3, 10))
 
+    # Modules start with a line numbered zero. This means empty modules have
+    # only a 0-number line, which is ignored, giving a truly empty module.
+    empty_is_empty = (PYVERSION >= (3, 11, 0, 'beta', 4))
 
 # Coverage.py specifics.
 

coverage/parser.py

@@ -377,7 +377,7 @@ def _line_numbers(self):
         """
         if hasattr(self.code, "co_lines"):
             for _, _, line in self.code.co_lines():
-                if line is not None:
+                if line:
                     yield line
         else:
             # Adapted from dis.py in the standard library.

tests/test_arcs.py

@@ -158,6 +158,8 @@ def test_what_is_the_sound_of_no_lines_clapping(self):
         if env.JYTHON:
             # Jython reports no lines for an empty file.
             arcz_missing=".1 1."                    # pragma: only jython
+        elif env.PYBEHAVIOR.empty_is_empty:
+            arcz_missing=".1 1."
         else:
             # Other Pythons report one line.
             arcz_missing=""

tests/test_lcov.py

@@ -8,6 +8,7 @@
 from tests.coveragetest import CoverageTest
 
 import coverage
+from coverage import env
 
 
 class LcovTest(CoverageTest):
@@ -252,7 +253,8 @@ def test_empty_init_files(self):
         line. It will also note the lack of branches, and the checksum for
         the line.
 
-        Although there are no lines found, it will note one line as hit.
+        Although there are no lines found, it will note one line as hit in
+        old Pythons, and no lines hit in newer Pythons.
         """
 
         self.make_file("__init__.py", "")
@@ -261,15 +263,27 @@ def test_empty_init_files(self):
         self.start_import_stop(cov, "__init__")
         cov.lcov_report()
         self.assert_exists("coverage.lcov")
-        expected_result = textwrap.dedent("""\
-            TN:
-            SF:__init__.py
-            DA:1,1,1B2M2Y8AsgTpgAmY7PhCfg
-            LF:0
-            LH:1
-            BRF:0
-            BRH:0
-            end_of_record
-            """)
+        # Newer Pythons have truly empty empty files.
+        if env.PYBEHAVIOR.empty_is_empty:
+            expected_result = textwrap.dedent("""\
+                TN:
+                SF:__init__.py
+                LF:0
+                LH:0
+                BRF:0
+                BRH:0
+                end_of_record
+                """)
+        else:
+            expected_result = textwrap.dedent("""\
+                TN:
+                SF:__init__.py
+                DA:1,1,1B2M2Y8AsgTpgAmY7PhCfg
+                LF:0
+                LH:1
+                BRF:0
+                BRH:0
+                end_of_record
+                """)
         actual_result = self.get_lcov_report_content()
         assert actual_result == expected_result