Tests for explicit exception chaining

This also breaks out the PY3 only tests into their own file.  We need to do this because raise from is a syntax error in PY2, so we can't rely on the previous hack of catching a HyCompileError - it would compile fine through Hy and then be a syntax error in Python.

tests/__init__.py

@@ -1,7 +1,6 @@
-#
-
 
 import hy  # noqa
+from hy._compat import PY3
 from .native_tests.cons import *  # noqa
 from .native_tests.defclass import *  # noqa
 from .native_tests.math import *  # noqa
@@ -20,3 +19,6 @@ from .native_tests.contrib.meth import *  # noqa
 from .native_tests.contrib.walk import *  # noqa
 from .native_tests.contrib.multi import *  # noqa
 from .native_tests.contrib.curry import *  # noqa
+
+if PY3:
+    from .native_tests.py3_only_tests import *  # noqa

tests/compilers/test_ast.py

@@ -27,6 +27,7 @@ from hy.compiler import hy_compile
 from hy.errors import HyCompileError, HyTypeError
 from hy.lex.exceptions import LexException
 from hy.lex import tokenize
+from hy._compat import PY3
 
 import ast
 
@@ -111,23 +112,29 @@ def test_ast_good_do():
 def test_ast_good_throw():
     "Make sure AST can compile valid throw"
     can_compile("(throw)")
-    can_compile("(throw 1)")
+    can_compile("(throw Exception)")
 
 
 def test_ast_bad_throw():
     "Make sure AST can't compile invalid throw"
-    cant_compile("(raise 1 2 3)")
+    cant_compile("(throw Exception Exception)")
 
 
 def test_ast_good_raise():
     "Make sure AST can compile valid raise"
     can_compile("(raise)")
-    can_compile("(raise 1)")
+    can_compile("(raise Exception)")
+    can_compile("(raise e)")
+
+
+if PY3:
+    def test_ast_raise_from():
+        can_compile("(raise Exception :from NameError)")
 
 
 def test_ast_bad_raise():
     "Make sure AST can't compile invalid raise"
-    cant_compile("(raise 1 2 3)")
+    cant_compile("(raise Exception Exception)")
 
 
 def test_ast_good_try():

tests/native_tests/native_macros.hy

@@ -100,21 +100,6 @@
 (assert initialized)
 (assert (test-initialized))
 
-(defn test-yield-from []
-  "NATIVE: testing yield from"
-  
-  (try
-   (eval
-    '(do (defn yield-from-test []
-           (for* [i (range 3)]
-             (yield i))
-           (yield-from [1 2 3]))
-         (assert (= (list (yield-from-test)) [0 1 2 1 2 3]))))
-   (catch [e HyCompileError]
-     ;; Yup, this should happen on non-Python3.3 thingies
-     (assert (not PY33)))
-   (else (assert PY33))))
-
 (defn test-if-python2 []
   (import sys)
   (assert (= (get sys.version_info 0)

tests/native_tests/py3_only_tests.hy

@@ -0,0 +1,15 @@
+;; Tests where the emited code relies on Python 3.
+;; Conditionally included in nosetests runs.
+
+(defn test-yield-from []
+  "NATIVE: testing yield from"
+  (do (defn yield-from-test []
+              (for* [i (range 3)]
+                (yield i))
+              (yield-from [1 2 3]))
+            (assert (= (list (yield-from-test)) [0 1 2 1 2 3]))))
+
+(defn test-exception-cause []
+  (try (raise ValueError :from NameError)
+  (except [e [ValueError]]
+    (assert (= (type (. e __cause__)) NameError)))))

tests/test_bin.py

@@ -23,6 +23,7 @@
 # DEALINGS IN THE SOFTWARE.
 import os
 import subprocess
+from hy._compat import PY3
 
 
 def run_cmd(cmd, stdin_data=None):
@@ -128,12 +129,15 @@ def test_hy2py():
     for dirpath, dirnames, filenames in os.walk("tests/native_tests"):
         for f in filenames:
             if f.endswith(".hy"):
-                i += 1
-                ret = run_cmd("hy2py -s -a "
-                              + os.path.join(dirpath, f))
-                assert ret[0] == 0, f
-                assert len(ret[1]) > 1, f
-                assert len(ret[2]) == 0, f
+                if f == "py3_only_tests.hy" and not PY3:
+                    continue
+                else:
+                    i += 1
+                    ret = run_cmd("hy2py -s -a "
+                                  + os.path.join(dirpath, f))
+                    assert ret[0] == 0, f
+                    assert len(ret[1]) > 1, f
+                    assert len(ret[2]) == 0, f
     assert i