Add base.handlers.get_current_request()

This function allows to retrieve the current web request in async
contexts.

Retrieving the current context can be useful for example when
customizing Enterprise Gateway RemoteProcessProxy, to retrieve
authentication headers.

The implementation makes uses of Python 3.7's new contextvars module.
There are backports efforts for 3.6:

- https://pypi.org/project/aiocontextvars/ - does not have automatic
  asyncio support

- https://pypi.org/project/contextvars/ - which doesn't have it either.
  The tracking issue on this matter has been stale for 3 years now, and
  the conclusion of the discussion is "upgrade to a newer version of
  Python": MagicStack/contextvars#2

Since notebook has no control on the event loop it is running into, I
preferred to drop Python 3.6 support.

.github/workflows/docs.yml

@@ -11,7 +11,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu]
-        python-version: [ '3.6' ]
+        python-version: [ '3.7' ]
     steps:
     - name: Checkout
       uses: actions/checkout@v1

.github/workflows/python-nbconvert.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [ '3.6' , '3.7', '3.8', '3.9' ]
+        python-version: [ '3.7', '3.8', '3.9' ]
     steps:
     - name: Checkout
       uses: actions/checkout@v1

.github/workflows/python.yml

@@ -11,7 +11,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu, macos, windows]
-        python-version: [ '3.6' , '3.7', '3.8', '3.9' ] # Windows 3.9 fails due to the pywinpty dependency not working (Issue #5967)
+        python-version: [ '3.7', '3.8', '3.9' ] # Windows 3.9 fails due to the pywinpty dependency not working (Issue #5967)
     steps:
     - name: Checkout
       uses: actions/checkout@v1

.github/workflows/selenium.yml

@@ -12,7 +12,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu, macos]
-        python-version: [ '3.6', '3.7', '3.8', '3.9' ]
+        python-version: [ '3.7', '3.8', '3.9' ]
     steps:
     - name: Checkout
       uses: actions/checkout@v2

notebook/base/handlers.py

@@ -14,6 +14,7 @@
 import traceback
 import types
 import warnings
+import contextvars
 from http.client import responses
 from http.cookies import Morsel
 
@@ -35,6 +36,10 @@
 from notebook.utils import is_hidden, url_path_join, url_is_absolute, url_escape, urldecode_unix_socket_path
 from notebook.services.security import csp_report_uri
 
+
+_current_request_var: contextvars.ContextVar = contextvars.ContextVar("current_request")
+
+
 #-----------------------------------------------------------------------------
 # Top-level handlers
 #-----------------------------------------------------------------------------
@@ -56,6 +61,9 @@ def log():
 class AuthenticatedHandler(web.RequestHandler):
     """A RequestHandler with an authenticated user."""
 
+    def prepare(self):
+        _current_request_var.set(self.request)
+
     @property
     def content_security_policy(self):
         """The default Content-Security-Policy header
@@ -932,6 +940,13 @@ def get(self):
         self.write(prometheus_client.generate_latest(prometheus_client.REGISTRY))
 
 
+def get_current_request():
+    """
+    Get :class:`tornado.httputil.HTTPServerRequest` that is currently being processed.
+    """
+    return _current_request_var.get(None)
+
+
 #-----------------------------------------------------------------------------
 # URL pattern fragments for re-use
 #-----------------------------------------------------------------------------

setup.py

@@ -15,7 +15,7 @@
 
 name = "notebook"
 
-if sys.version_info < (3, 6):
+if sys.version_info < (3, 7):
     pip_message = 'This may be due to an out of date pip. Make sure you have pip >= 9.0.1.'
     try:
         import pip
@@ -31,7 +31,8 @@
 
 
     error = """
-Notebook 6.3+ supports Python 3.6 and above.
+Notebook 6.4+ supports Python 3.7 and above.
+When using Python 3.6, please install Notebook < 6.4.
 When using Python 3.5, please install Notebook <= 6.2.
 When using Python 3.4 or earlier (including 2.7), please install Notebook 5.x.
 
@@ -99,7 +100,6 @@
         'License :: OSI Approved :: BSD License',
         'Programming Language :: Python',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9'
@@ -131,7 +131,7 @@
         'test:sys_platform != "win32"': ['requests-unixsocket'],
         'json-logging': ['json-logging']
     },
-    python_requires = '>=3.6',
+    python_requires = '>=3.7',
     entry_points = {
         'console_scripts': [
             'jupyter-notebook = notebook.notebookapp:main',