Defense in depth

It seems we must rely on heuristics, so let's go all the way.

Needs more testing.

src/incremental/__init__.py

@@ -374,14 +374,13 @@ def _findPath(path, package):  # type: (str, str) -> str
         )
 
 
-def _existing_version(path):  # type: (str) -> Version
+def _existing_version(version_path):  # type: (str) -> Version
     """
-    Load the current version from {path}/_version.py.
+    Load the current version from a ``_version.py`` file.
     """
     version_info = {}  # type: Dict[str, Version]
 
-    versionpath = os.path.join(path, "_version.py")
-    with open(versionpath, "r") as f:
+    with open(version_path, "r") as f:
         exec(f.read(), version_info)
 
     return version_info["__version__"]
@@ -414,7 +413,19 @@ def _get_setuptools_version(dist):  # type: (_Distribution) -> None
     if not config or not config.opt_in:
         return
 
-    dist.metadata.version = _existing_version(config.path).public()
+    try:
+        version = _existing_version(config.version_path)
+    except Exception:
+        return
+
+    # It's possible we're in the working directory of a *different* package!
+    # We do a case-insensitive comparison because the CLI tool historically
+    # required you to type the package name and not all maintainers
+    # capitalize the same.
+    if config.package.lower() != version.package.lower():
+        return
+
+    dist.metadata.version = version.public()
 
 
 def _get_distutils_version(dist, keyword, value):  # type: (_Distribution, object, object) -> None
@@ -435,8 +446,9 @@ def _get_distutils_version(dist, keyword, value):  # type: (_Distribution, objec
 
     for item in sp_command.find_all_modules():
         if item[1] == "_version":
-            package_path = os.path.dirname(item[2])
-            dist.metadata.version = _existing_version(package_path).public()
+            version_path = os.path.join(
+                os.path.dirname(item[2]), "_version.py")
+            dist.metadata.version = _existing_version(version_path).public()
             return
 
     raise Exception("No _version.py found.")  # pragma: no cover
@@ -475,8 +487,13 @@ class _IncrementalConfig:
     path: str
     """Path to the package root"""
 
+    @property
+    def version_path(self): # type: () => str
+        """Path of the ``_version.py`` file. May not exist."""
+        return os.path.join(self.path, "_version.py")
+
 
-def _load_pyproject_toml(toml_path):  # type: (str) -> Optional[_IncrementalConfig]
+def _load_pyproject_toml(toml_path):  # type: (str) -> _IncrementalConfig
     """
     Load Incremental configuration from a ``pyproject.toml``
 

src/incremental/newsfragments/106.bugfix

@@ -1,3 +1,3 @@
 Incremental could mis-identify that a project had opted in to version management.
 
-If a ``pyproject.toml`` in the current directory contained a ``[project]`` table with a ``name`` key, but did not contain the opt-in ``[tool.incremental]`` table, Incremental would still treat the file as if the opt-in were present and attempt to validate the configuration. This could happen in contexts outside of packaging, such as when creating a virtualenv. When operating as a setuptools plugin Incremental now always requires the ``[tool.incremental]`` opt-in.  Additionally, it suppresses any exceptions that occur while attempting to read ``pyproject.toml`` until it finds a valid ``[tool.incremental]`` table.
+If a ``pyproject.toml`` in the current directory contained a ``[project]`` table with a ``name`` key, but did not contain the opt-in ``[tool.incremental]`` table, Incremental would still treat the file as if the opt-in were present and attempt to validate the configuration. This could happen in contexts outside of packaging, such as when creating a virtualenv. When operating as a setuptools plugin Incremental now always ignores invalid configuration, such as configuration that doesn't match the content of the working directory.

src/incremental/update.py

@@ -77,10 +77,11 @@ def _run(
     ):
         raise ValueError("Only give --create")
 
+    versionpath = os.path.join(path, "_version.py")
     if newversion:
         from pkg_resources import parse_version
 
-        existing = _existing_version(path)
+        existing = _existing_version(versionpath)
         st_version = parse_version(newversion)._version  # type: ignore[attr-defined]
 
         release = list(st_version.release)
@@ -212,7 +213,6 @@ def _run(
                 with open(filepath, "wb") as f:
                     f.write(content)
 
-    versionpath = os.path.join(path, "_version.py")
     _print("Updating %s" % (versionpath,))
     with open(versionpath, "wb") as f:
         f.write(