Make HTTPException truthy (#3098)

Author: jcjf

CHANGES/3096.feature

@@ -0,0 +1 @@
+Preserve the cause when `HTTPException` is raised from another exception.

CONTRIBUTORS.txt

@@ -112,6 +112,7 @@ Julien Duponchelle
 Jungkook Park
 Junjie Tao
 Justas Trimailovas
+Justin Foo
 Justin Turner Arthur
 Kay Zheng
 Kimmo Parviainen-Jalanko

aiohttp/web_exceptions.py

@@ -83,6 +83,9 @@ def __init__(self, *, headers=None, reason=None,
         if self.body is None and not self.empty_body:
             self.text = "{}: {}".format(self.status, self.reason)
 
+    def __bool__(self):
+        return True
+
 
 class HTTPError(HTTPException):
     """Base class for exceptions with status codes in the 400s and 500s."""

tests/test_web_exceptions.py

@@ -1,5 +1,6 @@
 import collections
 import re
+from traceback import format_exception
 from unittest import mock
 
 import pytest
@@ -172,3 +173,14 @@ def test_link_header_451(buf, request):
 
     assert 'http://warning.or.kr/' == resp.link
     assert '<http://warning.or.kr/>; rel="blocked-by"' == resp.headers['Link']
+
+
+def test_HTTPException_retains_cause():
+    with pytest.raises(web.HTTPException) as ei:
+        try:
+            raise Exception('CustomException')
+        except Exception as exc:
+            raise web.HTTPException() from exc
+    tb = ''.join(format_exception(ei.type, ei.value, ei.tb))
+    assert 'CustomException' in tb
+    assert 'direct cause' in tb