[pylons] ensure the middleware code is Python 3 compatible (#475)

* [compat] add Py2 and Py3 `reraise` compatible function
* [pylons] use Py2 and Py3 compatible reraise

Author: Emanuele Palazzetti

ddtrace/compat.py

@@ -79,6 +79,25 @@ def to_unicode(s):
     msgpack_type = bytes
     numeric_types = (int, float)
 
+if PY2:
+    # avoids Python 3 `SyntaxError`
+    # this block will be replaced with the `six` library
+    from .utils.reraise import _reraise as reraise
+else:
+    def reraise(tp, value, tb=None):
+        """Python 3 re-raise function. This function is internal and
+        will be replaced entirely with the `six` library.
+        """
+        try:
+            if value is None:
+                value = tp()
+            if value.__traceback__ is not tb:
+                raise value.with_traceback(tb)
+            raise value
+        finally:
+            value = None
+            tb = None
+
 
 __all__ = [
     'httplib',
@@ -89,4 +108,5 @@ def to_unicode(s):
     'StringIO',
     'urlencode',
     'parse',
+    'reraise',
 ]

ddtrace/contrib/pylons/middleware.py

@@ -7,6 +7,7 @@
 from .renderer import trace_rendering
 from .constants import CONFIG_MIDDLEWARE
 
+from ...compat import reraise
 from ...ext import http, AppTypes
 from ...propagation.http import HTTPPropagator
 
@@ -78,7 +79,7 @@ def _start_response(status, *args, **kwargs):
                 span.error = 1
 
                 # re-raise the original exception with its original traceback
-                raise typ, val, tb
+                reraise(typ, val, tb=tb)
             except SystemExit:
                 span.set_tag(http.STATUS_CODE, 500)
                 span.error = 1

ddtrace/utils/reraise.py

@@ -0,0 +1,5 @@
+def _reraise(tp, value, tb=None):
+    """Python 2 re-raise function. This function is internal and
+    will be replaced entirely with the `six` library.
+    """
+    raise tp, value, tb

tests/test_compat.py

@@ -1,11 +1,12 @@
 # -*- coding: utf-8 -*-
 # Define source file encoding to support raw unicode characters in Python 2
+import sys
 
 # Third party
-from nose.tools import eq_
+from nose.tools import eq_, assert_raises
 
 # Project
-from ddtrace.compat import to_unicode, PY2
+from ddtrace.compat import to_unicode, PY2, reraise
 
 
 # Use different test suites for each Python version, this allows us to test the expected
@@ -92,3 +93,26 @@ def test_to_unicode_non_string(self):
             eq_(to_unicode(True), 'True')
             eq_(to_unicode(None), 'None')
             eq_(to_unicode(dict(key='value')), '{\'key\': \'value\'}')
+
+
+class TestPy2Py3Compat(object):
+    """Common tests to ensure functions are both Python 2 and
+    Python 3 compatible.
+    """
+    def test_reraise(self):
+        # ensure the `raise` function is Python 2/3 compatible
+        with assert_raises(Exception) as ex:
+            try:
+                raise Exception('Ouch!')
+            except Exception as e:
+                # original exception we want to re-raise
+                (typ, val, tb) = sys.exc_info()
+                try:
+                    # this exception doesn't allow a re-raise, and we need
+                    # to use the previous one collected via `exc_info()`
+                    raise Exception('Obfuscate!')
+                except Exception:
+                    pass
+                # this call must be Python 2 and 3 compatible
+                raise reraise(typ, val, tb)
+        eq_(ex.exception.args[0], 'Ouch!')