diff --git a/lldb/packages/Python/lldbsuite/test/lldbtest.py b/lldb/packages/Python/lldbsuite/test/lldbtest.py
--- a/lldb/packages/Python/lldbsuite/test/lldbtest.py
+++ b/lldb/packages/Python/lldbsuite/test/lldbtest.py
@@ -2388,7 +2388,35 @@
 
         self.assertTrue(cmd_status == 0)
 
-    def expect(
+
+    def expect(self, str, *args, **kwargs):
+        "Validate `expect` arguments before calling `_expect_impl`"
+        # Support success/fail only checks like:
+        #     self.expect("lldb command")
+        if not args and not kwargs:
+            return self._expect(str)
+
+        # Prevent this ambiguous mistake:
+        #     self.expect("lldb command", "some string")
+        # Instead, this invocation must be changed to either:
+        #     self.expect("lldb command", msg="some string")
+        # or use a matcher of some kind, such as:
+        #     self.expect("lldb command", substrs=["some string"])
+        if args:
+            needed = ("patterns", "startstr", "endstr", "substrs", "error")
+            assert any(arg in kwargs for arg in needed), \
+                "expect() requires a keyword argument"
+
+        # Check `patterns` and `substrs` are not accidentally given as strings.
+        assert not isinstance(kwargs.get("patterns"), six.string_types), \
+            "patterns must be a collection of strings"
+        assert not isinstance(kwargs.get("substrs"), six.string_types), \
+            "substrs must be a collection of strings"
+
+        return self._expect(str, **kwargs)
+
+
+    def _expect_impl(
             self,
             str,
             msg=None,
@@ -2429,22 +2457,6 @@
         set to False, the 'str' is treated as a string to be matched/not-matched
         against the golden input.
         """
-        # Catch cases where `expect` has been miscalled. Specifically, prevent
-        # this easy to make mistake:
-        #     self.expect("lldb command", "some substr")
-        # The `msg` parameter is used only when a failed match occurs. A failed
-        # match can only occur when one of `patterns`, `startstr`, `endstr`, or
-        # `substrs` has been given. Thus, if a `msg` is given, it's an error to
-        # not also provide one of the matcher parameters.
-        if msg and not (patterns or startstr or endstr or substrs or error):
-            assert False, "expect() missing a matcher argument"
-
-        # Check `patterns` and `substrs` are not accidentally given as strings.
-        assert not isinstance(patterns, six.string_types), \
-            "patterns must be a collection of strings"
-        assert not isinstance(substrs, six.string_types), \
-            "substrs must be a collection of strings"
-
         trace = (True if traceAlways else trace)
 
         if exe:
diff --git a/lldb/test/API/assert_messages_test/TestAssertMessages.py b/lldb/test/API/assert_messages_test/TestAssertMessages.py
--- a/lldb/test/API/assert_messages_test/TestAssertMessages.py
+++ b/lldb/test/API/assert_messages_test/TestAssertMessages.py
@@ -122,10 +122,6 @@
         self.assert_expect_fails_with("any command",
             dict(substrs="some substring"),
             "substrs must be a collection of strings")
-        # Prevent `self.expect("cmd", "substr")`
-        self.assert_expect_fails_with("any command",
-            dict(msg="some substring"),
-            "expect() missing a matcher argument")
         # Prevent `self.expect("cmd", "msg", "substr")`
         self.assert_expect_fails_with("any command",
             dict(msg="a message", patterns="some substring"),