ci: Enable warnings-as-errors earlier

This catches import-time warnings in more modules, including a warning
about invalid escape sequences that is now an error on nightly python.

.travis.yml

@@ -43,7 +43,8 @@ install:
     #- if [[ $TRAVIS_PYTHON_VERSION != 'pypy'* ]]; then travis_retry pip install pycares; fi
     - if [[ $TRAVIS_PYTHON_VERSION != 'pypy'* ]]; then travis_retry pip install pycurl; fi
     # Twisted is flaky on pypy (TODO: still? this note is quite old)
-    - if [[ $TRAVIS_PYTHON_VERSION != 'pypy'* ]]; then travis_retry pip install Twisted; fi
+    # It also sometimes has problems on nightly.
+    - if [[ $TRAVIS_PYTHON_VERSION != 'pypy'* && $TRAVIS_PYTHON_VERSION != 'nightly' ]]; then travis_retry pip install Twisted; fi
     # Ideally we'd run the lint stuff on the latest Python
     # version supported. But Python 3.7 requires slower-to-start VMs,
     # so we run it on 3.6 to minimize total CI run time.
@@ -83,6 +84,8 @@ script:
     # run it with nightly cpython. Coverage is very slow on pypy.
     - if [[ $TRAVIS_PYTHON_VERSION != nightly && $TRAVIS_PYTHON_VERSION != 'pypy'* ]]; then export RUN_COVERAGE=1; fi
     - if [[ "$RUN_COVERAGE" == 1 ]]; then export TARGET="-m coverage run $TARGET"; fi
+    # See comments in tox.ini
+    - export PYTHONWARNINGS=error,ignore:::site
     - python -bb $TARGET
     - python -O $TARGET
     - LANG=C python $TARGET
@@ -92,6 +95,7 @@ script:
     # make coverage reports for Codecov to find
     - if [[ "$RUN_COVERAGE" == 1 ]]; then coverage xml; fi
     - export TORNADO_EXTENSION=0
+    - unset PYTHONWARNINGS
     - if [[ $TRAVIS_PYTHON_VERSION == 3.7 ]]; then (cd ../docs && mkdir sphinx-out && sphinx-build -E -n -W -b html . sphinx-out); fi
     - if [[ $TRAVIS_PYTHON_VERSION == 3.7 ]]; then (cd ../docs && mkdir sphinx-doctest-out && sphinx-build -E -n -b doctest . sphinx-out); fi
     - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then (cd .. && flake8); fi

tornado/httputil.py

@@ -100,7 +100,7 @@ def __missing__(self, key: str) -> str:
 _normalized_headers = _NormalizedHeaderCache(1000)
 
 
-class HTTPHeaders(collections.MutableMapping):
+class HTTPHeaders(collections.abc.MutableMapping):
     """A dictionary that maintains ``Http-Header-Case`` for all keys.
 
     Supports multiple values per key via a pair of new methods,

tornado/test/runtests.py

@@ -7,6 +7,7 @@
 import textwrap
 import sys
 import unittest
+import warnings
 
 from tornado.httpclient import AsyncHTTPClient
 from tornado.httpserver import HTTPServer
@@ -111,14 +112,11 @@ def flush(self):
 
 
 def main():
-    # The -W command-line option does not work in a virtualenv with
-    # python 3 (as of virtualenv 1.7), so configure warnings
-    # programmatically instead.
-    import warnings
-
-    # Be strict about most warnings.  This also turns on warnings that are
-    # ignored by default, including DeprecationWarnings and
-    # python 3.2's ResourceWarnings.
+    # Be strict about most warnings (This is set in our test running
+    # scripts to catch import-time warnings, but set it again here to
+    # be sure). This also turns on warnings that are ignored by
+    # default, including DeprecationWarnings and python 3.2's
+    # ResourceWarnings.
     warnings.filterwarnings("error")
     # setuptools sometimes gives ImportWarnings about things that are on
     # sys.path even if they're not being used.

tornado/test/web_test.py

@@ -427,7 +427,7 @@ def test_absolute_auth_redirect(self):
         self.assertEqual(response.code, 302)
         self.assertTrue(
             re.match(
-                "http://example.com/login\?next=http%3A%2F%2F127.0.0.1%3A[0-9]+%2Fabsolute",
+                r"http://example.com/login\?next=http%3A%2F%2F127.0.0.1%3A[0-9]+%2Fabsolute",
                 response.headers["Location"],
             ),
             response.headers["Location"],

tornado/util.py

@@ -217,7 +217,7 @@ def _re_unescape_replacement(match: Match[str]) -> str:
 
 
 def re_unescape(s: str) -> str:
-    """Unescape a string escaped by `re.escape`.
+    r"""Unescape a string escaped by `re.escape`.
 
     May raise ``ValueError`` for regular expressions which could not
     have been produced by `re.escape` (for example, strings containing

tornado/web.py

@@ -1956,7 +1956,7 @@ def get_target_delegate(
 
 
 class Application(ReversibleRouter):
-    """A collection of request handlers that make up a web application.
+    r"""A collection of request handlers that make up a web application.
 
     Instances of this class are callable and can be passed directly to
     HTTPServer to serve the application::

tox.ini

@@ -86,6 +86,18 @@ commands =
          # py3*: -b turns on an extra warning when calling
          # str(bytes), and -bb makes it an error.
          -bb \
+         # Treat warnings as errors by default. We have a whitelist of
+         # allowed warnings in runtests.py, but we want to be strict
+         # about any import-time warnings before that setup code is
+         # reached. Note that syntax warnings are only reported in
+         # -opt builds because regular builds reuse pycs created
+         # during sdist installation (and it doesn't seem to be
+         # possible to set environment variables during that phase of
+         # tox).
+         "-Werror" \
+         # Virtualenv hits a deprecation warning very early which we must
+         # suppress here.
+         "-Wignore:::site" \
          # Python's optimized mode disables the assert statement, so
          # run the tests in this mode to ensure we haven't fallen into
          # the trap of relying on an assertion's side effects or using