improve comments

Author: picnixz

Lib/_pyrepl/console.py

@@ -214,8 +214,15 @@ def runsource(self, source, filename="<input>", symbol="single"):
             self.showsyntaxerror(filename, source=source)
             return False
 
-        # validate stuff that cannot be validated with AST parsing only
-        flags = self.compile.compiler.flags
+        # Validate stuff that cannot be validated with AST parsing only,
+        # such as assigning to a variable before a global declaration,
+        #
+        # While runsource("x = 1; global x") would fail, runsource("x = 1")
+        # followed by runsource("global x") would still work since preventing
+        # this requires the REPL to remember the global names whose number
+        # grows faster than in a regular program, which then becomes less
+        # efficient or relevant for the user.
+        flags = self.compile.compiler.flags  # may contain active futures
         flags &= ~codeop.PyCF_DONT_IMPLY_DEDENT
         flags &= ~codeop.PyCF_ALLOW_INCOMPLETE_INPUT
         try: