Add warning for assertions on tuples #1562

CHANGELOG.rst

@@ -79,6 +79,10 @@
   finalizer and has access to the fixture's result cache.
   Thanks `@d6e`_, `@sallner`_
 
+* Issue a warning for asserts whose test is a tuple literal. Such asserts will
+  never fail because tuples are always truthy and are usually a mistake
+  (see `#1562`_). Thanks `@kvas-it`_, for the PR.
+
 * New cli flag ``--override-ini`` or ``-o`` that overrides values from the ini file.
   Example '-o xfail_strict=True'. A complete ini-options can be viewed
   by py.test --help. Thanks `@blueyed`_ and `@fengxx`_ for the PR
@@ -207,6 +211,7 @@
 .. _#1619: https://github.com/pytest-dev/pytest/issues/1619
 .. _#372: https://github.com/pytest-dev/pytest/issues/372
 .. _#1544: https://github.com/pytest-dev/pytest/issues/1544
+.. _#1562: https://github.com/pytest-dev/pytest/issues/1562
 .. _#1616: https://github.com/pytest-dev/pytest/pull/1616
 .. _#1628: https://github.com/pytest-dev/pytest/pull/1628
 .. _#1629: https://github.com/pytest-dev/pytest/issues/1629

_pytest/assertion/rewrite.py

@@ -125,7 +125,7 @@ def find_module(self, name, path=None):
         co = _read_pyc(fn_pypath, pyc, state.trace)
         if co is None:
             state.trace("rewriting %r" % (fn,))
-            source_stat, co = _rewrite_test(state, fn_pypath)
+            source_stat, co = _rewrite_test(self.config, fn_pypath)
             if co is None:
                 # Probably a SyntaxError in the test.
                 return None
@@ -252,8 +252,9 @@ def _write_pyc(state, co, source_stat, pyc):
 cookie_re = re.compile(r"^[ \t\f]*#.*coding[:=][ \t]*[-\w.]+")
 BOM_UTF8 = '\xef\xbb\xbf'
 
-def _rewrite_test(state, fn):
+def _rewrite_test(config, fn):
     """Try to read and rewrite *fn* and return the code object."""
+    state = config._assertstate
     try:
         stat = fn.stat()
         source = fn.read("rb")
@@ -298,7 +299,7 @@ def _rewrite_test(state, fn):
         # Let this pop up again in the real import.
         state.trace("failed to parse: %r" % (fn,))
         return None, None
-    rewrite_asserts(tree)
+    rewrite_asserts(tree, fn, config)
     try:
         co = compile(tree, fn.strpath, "exec")
     except SyntaxError:
@@ -354,9 +355,9 @@ def _read_pyc(source, pyc, trace=lambda x: None):
         return co
 
 
-def rewrite_asserts(mod):
+def rewrite_asserts(mod, module_path=None, config=None):
     """Rewrite the assert statements in mod."""
-    AssertionRewriter().run(mod)
+    AssertionRewriter(module_path, config).run(mod)
 
 
 def _saferepr(obj):
@@ -543,6 +544,11 @@ class AssertionRewriter(ast.NodeVisitor):
 
     """
 
+    def __init__(self, module_path, config):
+        super(AssertionRewriter, self).__init__()
+        self.module_path = module_path
+        self.config = config
+
     def run(self, mod):
         """Find all assert statements in *mod* and rewrite them."""
         if not mod.body:
@@ -683,6 +689,10 @@ def visit_Assert(self, assert_):
         the expression is false.
 
         """
+        if isinstance(assert_.test, ast.Tuple) and self.config is not None:
+            fslocation = (self.module_path, assert_.lineno)
+            self.config.warn('R1', 'assertion is always true, perhaps '
+                              'remove parentheses?', fslocation=fslocation)
         self.statements = []
         self.variables = []
         self.variable_counter = itertools.count()

testing/test_assertion.py

@@ -640,3 +640,21 @@ def test_diff():
         *  + asdf
         *  ?     +
     """)
+
+def test_assert_tuple_warning(testdir):
+    testdir.makepyfile("""
+        def test_tuple():
+            assert(False, 'you shall not pass')
+    """)
+    result = testdir.runpytest('-rw')
+    result.stdout.fnmatch_lines('WR1*:2 assertion is always true*')
+
+def test_assert_indirect_tuple_no_warning(testdir):
+    testdir.makepyfile("""
+        def test_tuple():
+            tpl = ('foo', 'bar')
+            assert tpl
+    """)
+    result = testdir.runpytest('-rw')
+    output = '\n'.join(result.stdout.lines)
+    assert 'WR1' not in output