Support deletion of Variables via patch.

This supports some up-coming test changes in the fix for pex-tool#1422.

pex/variables.py

@@ -292,11 +292,20 @@ def strip(self):
 
     @contextmanager
     def patch(self, **kw):
-        # type: (**str) -> Iterator[Dict[str, str]]
-        """Update the environment for the duration of a context."""
+        # type: (**Optional[str]) -> Iterator[Dict[str, str]]
+        """Update the environment for the duration of a context.
+
+        Any environment variable with a value of `None` will be removed from the environment if
+        present. The rest will be added to the environment or else updated if already present in
+        the environment.
+        """
         old_environ = self._environ
         self._environ = self._environ.copy()
-        self._environ.update(kw)
+        for k, v in kw.items():
+            if v is None:
+                self._environ.pop(k, None)
+            else:
+                self._environ[k] = v
         yield self._environ
         self._environ = old_environ
 

tests/test_variables.py

@@ -194,3 +194,25 @@ def test_pex_vars_value_or(tmpdir):
         "Expected the fallback to be validated, and in the case of PEX_ROOT, replaced with a "
         "writeable tmp dir"
     )
+
+
+def test_patch():
+    v = Variables(environ=dict(PEX_VERBOSE="3", PEX_PYTHON="jython", PEX_EMIT_WARNINGS="True"))
+    assert v.PEX_VERBOSE == 3
+    assert v.PEX_PYTHON == "jython"
+    assert v.PEX_EMIT_WARNINGS is True
+    assert v.PEX_FORCE_LOCAL is False
+
+    with v.patch(PEX_VERBOSE="1", PEX_EMIT_WARNINGS=None, PEX_FORCE_LOCAL="True") as env:
+        assert env["PEX_VERBOSE"] == "1"
+        assert env["PEX_PYTHON"] == "jython"
+        assert "PEX_EMIT_WARNINGS" not in env
+        assert env["PEX_FORCE_LOCAL"] == "True"
+
+        assert v.PEX_VERBOSE == 1
+        assert v.PEX_PYTHON == "jython"
+        assert v.PEX_EMIT_WARNINGS is None
+
+        # If the assertion is flipped from `is True` to `is False` this test fails; so MyPy is just
+        # confused here about the statement being unreachable.
+        assert v.PEX_FORCE_LOCAL is True  # type: ignore[unreachable]