Merge pull request #1338 from Kodiologist/with-tempvar

Initialize the return variable of `with`

NEWS

@@ -27,6 +27,8 @@ Changes from 0.13.0
    * `exec` now works under Python 2
    * No TypeError from multi-arity defn returning values evaluating to None
    * try form now possible in defmacro/deftag
+   * Fixed a crash when `with` suppresses an exception. `with` now returns
+     `None` in this case.
 
    [ Misc. Improvements ]
    * `read`, `read_str`, and `eval` are exposed and documented as top-level

docs/language/api.rst

@@ -1678,6 +1678,13 @@ screen. The file is automatically closed after it has been processed.
 
     (with [f (open "NEWS")] (print (.read f)))
 
+``with`` returns the value of its last form, unless it suppresses an exception
+(because the context manager's ``__exit__`` method returned true), in which
+case it returns ``None``. So, the previous example could also be written
+
+.. code-block:: clj
+
+    (print (with [f (open "NEWS")] (.read f)))
 
 with-decorator
 --------------

hy/compiler.py

@@ -1452,29 +1452,36 @@ class HyASTCompiler(object):
             raise HyTypeError(expr,
                               "with expects a list, received `{0}'".format(
                                   type(args).__name__))
-        if len(args) < 1:
-            raise HyTypeError(expr, "with needs [[arg (expr)]] or [[(expr)]]]")
-
-        args.reverse()
-        ctx = self.compile(args.pop(0))
+        if len(args) not in (1, 2):
+            raise HyTypeError(expr, "with needs [arg (expr)] or [(expr)]")
 
         thing = None
-        if args != []:
+        if len(args) == 2:
             thing = self._storeize(args[0], self.compile(args.pop(0)))
+        ctx = self.compile(args.pop(0))
 
         body = self._compile_branch(expr)
 
+        # Store the result of the body in a tempvar
         var = self.get_anon_var()
         name = ast.Name(id=ast_str(var), arg=ast_str(var),
                         ctx=ast.Store(),
                         lineno=expr.start_line,
                         col_offset=expr.start_column)
-
-        # Store the result of the body in a tempvar
         body += ast.Assign(targets=[name],
                            value=body.force_expr,
                            lineno=expr.start_line,
                            col_offset=expr.start_column)
+        # Initialize the tempvar to None in case the `with` exits
+        # early with an exception.
+        initial_assign = ast.Assign(targets=[name],
+                                    value=ast.Name(
+                                        id=ast_str("None"),
+                                        ctx=ast.Load(),
+                                        lineno=expr.start_line,
+                                        col_offset=expr.start_column),
+                                    lineno=expr.start_line,
+                                    col_offset=expr.start_column)
 
         the_with = ast.With(context_expr=ctx.force_expr,
                             lineno=expr.start_line,
@@ -1486,7 +1493,7 @@ class HyASTCompiler(object):
             the_with.items = [ast.withitem(context_expr=ctx.force_expr,
                                            optional_vars=thing)]
 
-        ret = ctx + the_with
+        ret = Result(stmts = [initial_assign]) + ctx + the_with
         ret.contains_yield = ret.contains_yield or body.contains_yield
         # And make our expression context our temp variable
         expr_name = ast.Name(id=ast_str(var), arg=ast_str(var),
@@ -1494,7 +1501,10 @@ class HyASTCompiler(object):
                              lineno=expr.start_line,
                              col_offset=expr.start_column)
 
-        ret += Result(expr=expr_name, temp_variables=[expr_name, name])
+        ret += Result(expr=expr_name)
+        # We don't give the Result any temp_vars because we don't want
+        # Result.rename to touch `name`. Otherwise, initial_assign will
+        # clobber any preexisting value of the renamed-to variable.
 
         return ret
 

tests/native_tests/with_test.hy

@@ -34,12 +34,37 @@
         (assert (= t2 2))
         (assert (= t3 3))))
 
-  (defn test-quince-with []
-    "NATIVE: test four withs, one with no args"
-    (with [t1 (WithTest 1)
-           t2 (WithTest 2)
-           t3 (WithTest 3)
-           _ (WithTest 4)]
-          (assert (= t1 1))
-          (assert (= t2 2))
-          (assert (= t3 3))))
+(defn test-quince-with []
+  "NATIVE: test four withs, one with no args"
+  (with [t1 (WithTest 1)
+         t2 (WithTest 2)
+         t3 (WithTest 3)
+         _ (WithTest 4)]
+        (assert (= t1 1))
+        (assert (= t2 2))
+        (assert (= t3 3))))
+
+(defclass SuppressZDE [object]
+  (defn --enter-- [self])
+  (defn --exit-- [self exc-type exc-value traceback]
+    (and (not (none? exc-type)) (issubclass exc-type ZeroDivisionError))))
+
+(defn test-exception-suppressing-with []
+  ; https://github.com/hylang/hy/issues/1320
+
+  (setv x (with [(SuppressZDE)] 5))
+  (assert (= x 5))
+
+  (setv y (with [(SuppressZDE)] (/ 1 0)))
+  (assert (none? y))
+
+  (setv z (with [(SuppressZDE)] (/ 1 0) 5))
+  (assert (none? z))
+
+  (defn f [] (with [(SuppressZDE)] (/ 1 0)))
+  (assert (none? (f)))
+
+  (setv w 7  l [])
+  (setv w (with [(SuppressZDE)] (.append l w) (/ 1 0) 5))
+  (assert (none? w))
+  (assert (= l [7])))