feat: unrecognized options are now a warning rather than error. #1035 (#1206)

Because they are warnings issued while parsing the configuration file, it's not
possible to suppress them with the coverage configuration.

4 files changed, 35 insertions, 24 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 3566eb52..f48b8ad7 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -24,9 +24,15 @@ want to know what's different in 5.0 since 4.5.x, see :ref:`whatsnew5x`.
 Unreleased
 ----------
 
+- Unrecognized options in the configuration file are no longer errors. They are
+  now warnings, to ease the use of coverage across versions.  Fixes `issue
+  1035`_.
+
 - Fix another rarer instance of "Error binding parameter 0 - probably
   unsupported type." (`issue 1010`_).
 
+.. _issue 1035: https://github.com/nedbat/coveragepy/issues/1035
+
 
 .. _changes_60b1:
 
diff --git a/coverage/config.py b/coverage/config.py
index 7287e963..3b873579 100644
--- a/coverage/config.py
+++ b/coverage/config.py
@@ -248,7 +248,7 @@ class CoverageConfig:
                 setattr(self, k, v)
 
     @contract(filename=str)
-    def from_file(self, filename, our_file):
+    def from_file(self, filename, warn, our_file):
         """Read configuration from a .rc file.
 
         `filename` is a file name to read.
@@ -297,7 +297,7 @@ class CoverageConfig:
             real_section = cp.has_section(section)
             if real_section:
                 for unknown in set(cp.options(section)) - options:
-                    raise CoverageException(
+                    warn(
                         "Unrecognized option '[{}] {}=' in config file {}".format(
                             real_section, unknown, filename
                         )
@@ -517,12 +517,13 @@ def config_files_to_try(config_file):
     return files_to_try
 
 
-def read_coverage_config(config_file, **kwargs):
+def read_coverage_config(config_file, warn, **kwargs):
     """Read the coverage.py configuration.
 
     Arguments:
         config_file: a boolean or string, see the `Coverage` class for the
             tricky details.
+        warn: a function to issue warnings.
         all others: keyword arguments from the `Coverage` class, used for
             setting values in the configuration.
 
@@ -541,7 +542,7 @@ def read_coverage_config(config_file, **kwargs):
         files_to_try = config_files_to_try(config_file)
 
         for fname, our_file, specified_file in files_to_try:
-            config_read = config.from_file(fname, our_file=our_file)
+            config_read = config.from_file(fname, warn, our_file=our_file)
             if config_read:
                 break
             if specified_file:
diff --git a/coverage/control.py b/coverage/control.py
index e1eb9add..c45b1245 100644
--- a/coverage/control.py
+++ b/coverage/control.py
@@ -192,15 +192,7 @@ class Coverage:
         if data_file is _DEFAULT_DATAFILE:
             data_file = None
 
-        # Build our configuration from a number of sources.
-        self.config = read_coverage_config(
-            config_file=config_file,
-            data_file=data_file, cover_pylib=cover_pylib, timid=timid,
-            branch=branch, parallel=bool_or_none(data_suffix),
-            source=source, source_pkgs=source_pkgs, run_omit=omit, run_include=include, debug=debug,
-            report_omit=omit, report_include=include,
-            concurrency=concurrency, context=context,
-            )
+        self.config = None
 
         # This is injectable by tests.
         self._debug_file = None
@@ -235,6 +227,16 @@ class Coverage:
         # Should we write the debug output?
         self._should_write_debug = True
 
+        # Build our configuration from a number of sources.
+        self.config = read_coverage_config(
+            config_file=config_file, warn=self._warn,
+            data_file=data_file, cover_pylib=cover_pylib, timid=timid,
+            branch=branch, parallel=bool_or_none(data_suffix),
+            source=source, source_pkgs=source_pkgs, run_omit=omit, run_include=include, debug=debug,
+            report_omit=omit, report_include=include,
+            concurrency=concurrency, context=context,
+            )
+
         # If we have sub-process measurement happening automatically, then we
         # want any explicit creation of a Coverage object to mean, this process
         # is already coverage-aware, so don't auto-measure it.  By now, the
@@ -352,16 +354,18 @@ class Coverage:
 
         """
         if self._no_warn_slugs is None:
-            self._no_warn_slugs = list(self.config.disable_warnings)
+            if self.config is not None:
+                self._no_warn_slugs = list(self.config.disable_warnings)
 
-        if slug in self._no_warn_slugs:
-            # Don't issue the warning
-            return
+        if self._no_warn_slugs is not None:
+            if slug in self._no_warn_slugs:
+                # Don't issue the warning
+                return
 
         self._warnings.append(msg)
         if slug:
             msg = f"{msg} ({slug})"
-        if self._debug.should('pid'):
+        if self._debug is not None and self._debug.should('pid'):
             msg = f"[{os.getpid()}] {msg}"
         warnings.warn(msg, category=CoverageWarning, stacklevel=2)
 
diff --git a/tests/test_config.py b/tests/test_config.py
index bf9cb4a5..9e126827 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -10,7 +10,7 @@ import pytest
 
 import coverage
 from coverage.config import HandyConfigParser
-from coverage.exceptions import CoverageException
+from coverage.exceptions import CoverageException, CoverageWarning
 
 from tests.coveragetest import CoverageTest, UsingModulesMixin
 from tests.helpers import without_module
@@ -392,7 +392,7 @@ class ConfigTest(CoverageTest):
             xyzzy = 17
             """)
         msg = r"Unrecognized option '\[run\] xyzzy=' in config file .coveragerc"
-        with pytest.raises(CoverageException, match=msg):
+        with pytest.warns(CoverageWarning, match=msg):
             _ = coverage.Coverage()
 
     def test_unknown_option_toml(self):
@@ -401,7 +401,7 @@ class ConfigTest(CoverageTest):
             xyzzy = 17
             """)
         msg = r"Unrecognized option '\[tool.coverage.run\] xyzzy=' in config file pyproject.toml"
-        with pytest.raises(CoverageException, match=msg):
+        with pytest.warns(CoverageWarning, match=msg):
             _ = coverage.Coverage()
 
     def test_misplaced_option(self):
@@ -410,7 +410,7 @@ class ConfigTest(CoverageTest):
             branch = True
             """)
         msg = r"Unrecognized option '\[report\] branch=' in config file .coveragerc"
-        with pytest.raises(CoverageException, match=msg):
+        with pytest.warns(CoverageWarning, match=msg):
             _ = coverage.Coverage()
 
     def test_unknown_option_in_other_ini_file(self):
@@ -418,8 +418,8 @@ class ConfigTest(CoverageTest):
             [coverage:run]
             huh = what?
             """)
-        msg = (r"Unrecognized option '\[coverage:run\] huh=' in config file setup.cfg")
-        with pytest.raises(CoverageException, match=msg):
+        msg = r"Unrecognized option '\[coverage:run\] huh=' in config file setup.cfg"
+        with pytest.warns(CoverageWarning, match=msg):
             _ = coverage.Coverage()
 
     def test_exceptions_from_missing_things(self):