Merge pull request #94 from plotly/fix-start-error

Propagate start errors.

Author: T4rk1n

CHANGELOG.md

@@ -3,6 +3,9 @@ All notable changes to `jupyter-dash` will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [UNRELEASED]
+### Fixed
+- Propagate start error message. [#94](https://github.com/plotly/jupyter-dash/pull/94)
+
 ### Added
 
 - Support for `Dash.run` method added in Dash 2.4.0

jupyter_dash/_stoppable_thread.py

@@ -12,11 +12,12 @@ def get_id(self):
 
     def kill(self):
         thread_id = self.get_id()
-        res = ctypes.pythonapi.PyThreadState_SetAsyncExc(
-            ctypes.c_long(thread_id), ctypes.py_object(SystemExit)
-        )
-        if res == 0:
-            raise ValueError(f"Invalid thread id: {thread_id}")
-        if res > 1:
-            ctypes.pythonapi.PyThreadState_SetAsyncExc(ctypes.c_long(thread_id), None)
-            raise SystemExit("Stopping thread failure")
+        if thread_id:
+            res = ctypes.pythonapi.PyThreadState_SetAsyncExc(
+                ctypes.c_long(thread_id), ctypes.py_object(SystemExit)
+            )
+            if res == 0:
+                raise ValueError(f"Invalid thread id: {thread_id}")
+            if res > 1:
+                ctypes.pythonapi.PyThreadState_SetAsyncExc(ctypes.c_long(thread_id), None)
+                raise SystemExit("Stopping thread failure")

jupyter_dash/jupyter_app.py

@@ -2,13 +2,15 @@
 import os
 import requests
 import flask.cli
+from IPython.core.display import HTML
 from retrying import retry
 import io
 import re
 import sys
 import inspect
 import traceback
 import warnings
+import queue
 
 from IPython import get_ipython
 from IPython.display import IFrame, display
@@ -298,6 +300,8 @@ def run(
         except ImportError:
             pass
 
+        err_q = queue.Queue()
+
         @retry(
             stop_max_attempt_number=15,
             wait_exponential_multiplier=100,
@@ -308,6 +312,9 @@ def run():
                 super_run_server(**kwargs)
             except SystemExit:
                 pass
+            except Exception as error:
+                err_q.put(error)
+                raise error
 
         thread = StoppableThread(target=run)
         thread.setDaemon(True)
@@ -320,31 +327,55 @@ def run():
             host=host, port=port, token=JupyterDash._token
         )
 
+        def _get_error():
+            try:
+                err = err_q.get_nowait()
+                if err:
+                    raise err
+            except queue.Empty:
+                pass
+
         # Wait for app to respond to _alive endpoint
         @retry(
             stop_max_attempt_number=15,
             wait_exponential_multiplier=10,
             wait_exponential_max=1000
         )
         def wait_for_app():
-            res = requests.get(alive_url).content.decode()
-            if res != "Alive":
-                url = "http://{host}:{port}".format(
-                    host=host, port=port, token=JupyterDash._token
-                )
-                raise OSError(
-                    "Address '{url}' already in use.\n"
-                    "    Try passing a different port to run_server.".format(
-                        url=url
+            _get_error()
+            try:
+                req = requests.get(alive_url)
+                res = req.content.decode()
+                if req.status_code != 200:
+                    raise Exception(res)
+
+                if res != "Alive":
+                    url = "http://{host}:{port}".format(
+                        host=host, port=port, token=JupyterDash._token
                     )
-                )
+                    raise OSError(
+                        "Address '{url}' already in use.\n"
+                        "    Try passing a different port to run_server.".format(
+                            url=url
+                        )
+                    )
+            except requests.ConnectionError as err:
+                _get_error()
+                raise err
 
-        wait_for_app()
+        try:
+            wait_for_app()
 
-        if JupyterDash._in_colab:
-            self._display_in_colab(dashboard_url, port, mode, width, height)
-        else:
-            self._display_in_jupyter(dashboard_url, port, mode, width, height)
+            if JupyterDash._in_colab:
+                self._display_in_colab(dashboard_url, port, mode, width, height)
+            else:
+                self._display_in_jupyter(dashboard_url, port, mode, width, height)
+        except Exception as final_error:
+            msg = str(final_error)
+            if msg.startswith('<!'):
+                display(HTML(msg))
+            else:
+                raise final_error
 
     def _display_in_colab(self, dashboard_url, port, mode, width, height):
         from google.colab import output