fix WriteString standard output

Author: mmatera

mathics/builtin/files_io/files.py

@@ -1696,13 +1696,15 @@ class Write(Builtin):
     def apply(self, channel, expr, evaluation):
         "Write[channel_, expr___]"
 
-        strm = channel_to_stream(channel)
+        stream = None
+        if isinstance(channel, String):
+            stream = {"stdout": 1, "stderr": 2}.get(channel.value, None)
 
-        if strm is None:
-            return
-
-        n = strm.elements[1].get_int_value()
-        stream = stream_manager.lookup_stream(n)
+        if stream is None:
+            strm = channel_to_stream(channel, "w")
+            if strm is None:
+                return
+            stream = stream_manager.lookup_stream(strm.elements[1].get_int_value())
 
         if stream is None or stream.io is None or stream.io.closed:
             evaluation.message("General", "openx", channel)
@@ -1763,6 +1765,9 @@ class WriteString(Builtin):
     #> DeleteFile[pathname];
     #> Clear[pathname];
 
+
+    If stream is the string "stdout" or "stderr", writes to the system standard output/ standard error channel:
+    >> WriteString["stdout", "Hola"]
     """
 
     summary_text = "write a sequence of strings to a stream, with no extra newlines"
@@ -1773,12 +1778,18 @@ class WriteString(Builtin):
 
     def apply(self, channel, expr, evaluation):
         "WriteString[channel_, expr___]"
-        strm = channel_to_stream(channel, "w")
-
-        if strm is None:
-            return
-
-        stream = stream_manager.lookup_stream(strm.elements[1].get_int_value())
+        stream = None
+        if isinstance(channel, String):
+            if channel.value == "stdout":
+                stream = stream_manager.lookup_stream(1)
+            elif channel.value == "stderr":
+                stream = stream_manager.lookup_stream(2)
+
+        if stream is None:
+            strm = channel_to_stream(channel, "w")
+            if strm is None:
+                return
+            stream = stream_manager.lookup_stream(strm.elements[1].get_int_value())
 
         if stream is None or stream.io is None or stream.io.closed:
             return None