put the explanation generating code in the conditional fail body (fixes #79)

CHANGELOG

@@ -1,6 +1,7 @@
 Changes between 2.1.2 and [next version]
 ----------------------------------------
 
+- fix issue79: rewriting failed on some comparisons in boolops
 - correctly handle zero length arguments (a la pytest '')
 - fix issue67 / junitxml now contains correct test durations, thanks ronny
 - fix issue75 / skipping test failure on jython

_pytest/assertion/rewrite.py

@@ -491,13 +491,13 @@ class AssertionRewriter(ast.NodeVisitor):
         self.push_format_context()
         # Process each operand, short-circuting if needed.
         for i, v in enumerate(boolop.values):
-            self.push_format_context()
-            res, expl = self.visit(v)
-            body.append(ast.Assign([ast.Name(res_var, ast.Store())], res))
             if i:
                 fail_inner = []
                 self.on_failure.append(ast.If(cond, fail_inner, []))
                 self.on_failure = fail_inner
+            self.push_format_context()
+            res, expl = self.visit(v)
+            body.append(ast.Assign([ast.Name(res_var, ast.Store())], res))
             expl_format = self.pop_format_context(ast.Str(expl))
             call = ast.Call(app, [expl_format], [], None, None)
             self.on_failure.append(ast.Expr(call))

testing/test_assertrewrite.py

@@ -144,6 +144,14 @@ class TestAssertionRewrite:
         def f():
             assert False or x()
         assert getmsg(f, {"x" : x}) == "assert (False or x())"
+        def f():
+            assert 1 in {} and 2 in {}
+        assert getmsg(f) == "assert (1 in {})"
+        def f():
+            x = 1
+            y = 2
+            assert x in {1 : None} and y in {}
+        assert getmsg(f) == "assert (1 in {1: None} and 2 in {})"
         def f():
             f = True
             g = False