Bug #132313 error message confusing for assignment in lambda.

They're actually complaining about something more specific, an assignment
in a lambda as an actual argument, so that Python parses the
lambda as if it were a keyword argument.  Like f(lambda x: x[0]=42).
The "lambda x: x[0]" part gets parsed as if it were a keyword, being
bound to 42, and the resulting error msg didn't make much sense.

Python/compile.c

@@ -1522,7 +1522,14 @@ com_argument(struct compiling *c, node *n, PyObject **pkeywords)
 		m = CHILD(m, 0);
 	} while (NCH(m) == 1);
 	if (TYPE(m) != NAME) {
+		/* f(lambda x: x[0] = 3) ends up getting parsed with
+		 * LHS test = lambda x: x[0], and RHS test = 3.
+		 * SF bug 132313 points out that complaining about a keyword
+		 * then is very confusing.
+		 */
 		com_error(c, PyExc_SyntaxError,
+			  TYPE(m) == lambdef ?
+				  "lambda cannot contain assignment" :
 				  "keyword can't be an expression");
 	}
 	else {