Unroll calls to any #5062 (#5103)

Unroll calls to any #5062

changelog/5062.feature.rst

@@ -0,0 +1 @@
+Unroll calls to ``all`` to full for-loops for better failure messages, especially when using Generator Expressions.

src/_pytest/assertion/rewrite.py

@@ -949,6 +949,8 @@ warn_explicit(
         """
         visit `ast.Call` nodes on Python3.5 and after
         """
+        if isinstance(call.func, ast.Name) and call.func.id == "all":
+            return self._visit_all(call)
         new_func, func_expl = self.visit(call.func)
         arg_expls = []
         new_args = []
@@ -972,6 +974,27 @@ warn_explicit(
         outer_expl = "%s\n{%s = %s\n}" % (res_expl, res_expl, expl)
         return res, outer_expl
 
+    def _visit_all(self, call):
+        """Special rewrite for the builtin all function, see #5062"""
+        if not isinstance(call.args[0], (ast.GeneratorExp, ast.ListComp)):
+            return
+        gen_exp = call.args[0]
+        assertion_module = ast.Module(
+            body=[ast.Assert(test=gen_exp.elt, lineno=1, msg="", col_offset=1)]
+        )
+        AssertionRewriter(module_path=None, config=None).run(assertion_module)
+        for_loop = ast.For(
+            iter=gen_exp.generators[0].iter,
+            target=gen_exp.generators[0].target,
+            body=assertion_module.body,
+            orelse=[],
+        )
+        self.statements.append(for_loop)
+        return (
+            ast.Num(n=1),
+            "",
+        )  # Return an empty expression, all the asserts are in the for_loop
+
     def visit_Starred(self, starred):
         # From Python 3.5, a Starred node can appear in a function call
         res, expl = self.visit(starred.value)
@@ -982,6 +1005,8 @@ warn_explicit(
         """
         visit `ast.Call nodes on 3.4 and below`
         """
+        if isinstance(call.func, ast.Name) and call.func.id == "all":
+            return self._visit_all(call)
         new_func, func_expl = self.visit(call.func)
         arg_expls = []
         new_args = []

testing/test_assertrewrite.py

@@ -656,6 +656,12 @@ class TestAssertionRewrite(object):
         else:
             assert lines == ["assert 0 == 1\n +  where 1 = \\n{ \\n~ \\n}.a"]
 
+    def test_unroll_expression(self):
+        def f():
+            assert all(x == 1 for x in range(10))
+
+        assert "0 == 1" in getmsg(f)
+
     def test_custom_repr_non_ascii(self):
         def f():
             class A(object):
@@ -671,6 +677,53 @@ class TestAssertionRewrite(object):
         assert "UnicodeDecodeError" not in msg
         assert "UnicodeEncodeError" not in msg
 
+    def test_unroll_generator(self, testdir):
+        testdir.makepyfile(
+            """
+            def check_even(num):
+                if num % 2 == 0:
+                    return True
+                return False
+
+            def test_generator():
+                odd_list = list(range(1,9,2))
+                assert all(check_even(num) for num in odd_list)"""
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(["*assert False*", "*where False = check_even(1)*"])
+
+    def test_unroll_list_comprehension(self, testdir):
+        testdir.makepyfile(
+            """
+            def check_even(num):
+                if num % 2 == 0:
+                    return True
+                return False
+
+            def test_list_comprehension():
+                odd_list = list(range(1,9,2))
+                assert all([check_even(num) for num in odd_list])"""
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(["*assert False*", "*where False = check_even(1)*"])
+
+    def test_for_loop(self, testdir):
+        testdir.makepyfile(
+            """
+            def check_even(num):
+                if num % 2 == 0:
+                    return True
+                return False
+
+            def test_for_loop():
+                odd_list = list(range(1,9,2))
+                for num in odd_list:
+                    assert check_even(num)
+        """
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(["*assert False*", "*where False = check_even(1)*"])
+
 
 class TestRewriteOnImport(object):
     def test_pycache_is_a_file(self, testdir):