Attempt at addressing review comment https://github.com/jupyter/notebook/pull/5799#issuecomment-705263500

notebook-5799-json-logging.diff

diff --git a/notebook/notebookapp.py b/notebook/notebookapp.py
index 45c1cb5eb..a443125fd 100755
--- a/notebook/notebookapp.py
+++ b/notebook/notebookapp.py
@@ -139,19 +139,6 @@ try:
 except ImportError:
     terminado_available = False
 
-# Tolerate missing json_logging package.
-_enable_json_logs = enable_json_logs()
-try:
-    import json_logging
-except ImportError:
-    # If configured for json logs and we can't do it, log a hint.
-    if _enable_json_logs:
-        logging.getLogger(__name__).exception(
-            'Unable to use json logging due to missing packages. '
-            'Run "pip install notebook[json-logging]" to fix.'
-        )
-    _enable_json_logs = False
-
 #-----------------------------------------------------------------------------
 # Module globals
 #-----------------------------------------------------------------------------
@@ -716,9 +703,42 @@ class NotebookApp(JupyterApp):
         password=(NotebookPasswordApp, NotebookPasswordApp.description.splitlines()[0]),
     )
 
-    # Unless there is a way to re-initialize the log formatter (like with _log_format_changed?)
-    # we need to load the json logging formatter early here otherwise traitlets complains.
-    _log_formatter_cls = json_logging.JSONLogFormatter if _enable_json_logs else LogFormatter
+    _log_formatter_cls = LogFormatter
+
+    log_format_json = Bool(False, config=True,
+        help=_('Set to True to enable JSON formatted logs. Run '
+               '"pip install notebook[json-logging]" to install the required '
+               'dependent packages. Can also be set using the '
+               'environment variable ENABLE_JSON_LOGGING=true.')
+    )
+
+    @default('log_format_json')
+    def _default_log_format_json(self):
+        """Get the log_format_json value from the environment."""
+        return enable_json_logs()
+
+    @validate('log_format_json')
+    def _validate_log_format_json(self, proposal):
+        # See if the `json_logging` package can be imported and, if so,
+        # "tickle" the current `log_format` value after setting the
+        # `_log_formatter_cls`.
+        logger = logging.getLogger(__name__)
+        logger.info('in _validate_log_format_json with proposal: %s', proposal)
+        if proposal['value']:  # log_format_json == True
+            try:
+                import json_logging
+                logger.info('changing to JSONLogFormatter')
+                _log_formatter_cls = json_logging.JSONLogFormatter
+                # TODO: should we call json_logging.init_non_web(enable_json=True) here?
+                self.log_format = self.log_format  # trigger _log_format_changed
+            except ImportError:
+                # If configured for json logs and we can't do it, log a hint.
+                logger.exception(
+                    'Unable to use json logging due to missing packages. '
+                    'Run "pip install notebook[json-logging]" to fix.'
+                )
+        # Don't actually change the log_format_json value.
+        return proposal['value']
 
     @default('log_level')
     def _default_log_level(self):