Make sure replaced streams have fileno.

This is important for example for subprocess that will peak at the
filedescriptor.

Author: Carreau

ipykernel/iostream.py

@@ -286,6 +286,16 @@ class OutStream(TextIOBase):
     topic = None
     encoding = 'UTF-8'
 
+
+    def fileno(self):
+        """
+        Things like subprocess will peak and write to the fileno() of stderr/stdout.
+        """
+        if getattr(self, '_original_stdstream_copy', None) is not None:
+            return self._original_stdstream_copy
+        else:
+            raise UnsupportedOperation('fileno')
+
     def _watch_pipe_fd(self):
         """
         We've redirected standards steams 0 and 1 into a pipe.

ipykernel/kernelapp.py

@@ -419,7 +419,6 @@ def init_io(self):
             sys.stderr = outstream_factory(
                 self.session, self.iopub_thread, "stderr", echo=e_stderr
             )
-            self.log.error("this %s", hasattr(sys.stderr, "_original_stdstream_copy"))
             if hasattr(sys.stderr, "_original_stdstream_copy"):
 
                 for handler in self.log.handlers:
@@ -433,7 +432,6 @@ def init_io(self):
                         handler.stream = TextIOWrapper(
                             FileIO(sys.stderr._original_stdstream_copy, "w")
                         )
-                        self.log.error("Redirected to raw FD.")
         if self.displayhook_class:
             displayhook_factory = import_item(str(self.displayhook_class))
             self.displayhook = displayhook_factory(self.session, self.iopub_socket)

ipykernel/tests/test_kernel.py

@@ -45,7 +45,7 @@ def test_simple_print():
     """simple print statement in kernel"""
     with kernel() as kc:
         iopub = kc.iopub_channel
-        msg_id, content = execute(kc=kc, code="print ('hi')")
+        msg_id, content = execute(kc=kc, code="print('hi')")
         stdout, stderr = assemble_output(iopub)
         assert stdout == 'hi\n'
         assert stderr == ''
@@ -63,6 +63,19 @@ def test_capture_fd():
         _check_master(kc, expected=True)
 
 
+@pytest.mark.skipif(sys.platform=='win32', reason='Not meant to work on windows')
+def test_subprocess_peek_at_stream_fileno():
+    """"""
+    with kernel() as kc:
+        iopub = kc.iopub_channel
+        msg_id, content = execute(kc=kc, code="import subprocess, sys; subprocess.run(['python', '-c', 'import os; os.system(\"echo CAP1\"); print(\"CAP2\")'], stderr=sys.stderr)")
+        stdout, stderr = assemble_output(iopub)
+        assert stdout == 'CAP1\nCAP2\n'
+        assert stderr == ''
+        _check_master(kc, expected=True)
+
+
+
 
 def test_sys_path():
     """test that sys.path doesn't get messed up by default"""