Add no locals() for string formatting test

Using locals() for formatting strings is not as clear as explicit
dictionaries and can produce hidden errors during refactoring. locals() are
already being partially removed in nova and cinder. Related to
bug 1171936

Change-Id: I6ef790ebf50d640af62c535952c7a98053869e43

HACKING.rst

@@ -152,6 +152,8 @@ Example::
       },
   }
 
+Do not use ``locals()`` for formatting strings, it is not clear as using
+explicit dictionaries and can hide errors during refactoring.
 
 Calling Methods
 ---------------

hacking/core.py

@@ -586,6 +586,28 @@ def hacking_docstring_multiline_start(physical_line, previous_logical, tokens):
                         "should start with a summary")
 
 
+@flake8ext
+def hacking_no_locals(logical_line, physical_line, tokens):
+    """Do not use locals() for string formatting.
+
+    Okay: 'locals()'
+    Okay: 'locals'
+    Okay: locals()
+    Okay: print(locals())
+    H501: print("%(something)" % locals())
+    Okay: print("%(something)" % locals())  # noqa
+    """
+    if pep8.noqa(physical_line):
+        return
+    for_formatting = False
+    for token_type, text, start, _, _ in tokens:
+        if text == "%" and token_type == tokenize.OP:
+            for_formatting = True
+        if (for_formatting and token_type == tokenize.NAME and text ==
+                "locals" and "locals()" in logical_line):
+            yield (start[1], "H501: Do not use locals() for string formatting")
+
+
 @flake8ext
 def hacking_no_cr(physical_line):
     r"""Check that we only use newlines not carriage returns.

setup.cfg

@@ -41,6 +41,7 @@ flake8.extension =
     H402 = hacking.core:hacking_docstring_one_line
     H403 = hacking.core:hacking_docstring_multiline_end
     H404 = hacking.core:hacking_docstring_multiline_start
+    H501 = hacking.core:hacking_no_locals
     H601 = hacking.core:hacking_no_cr
     H700 = hacking.core:hacking_localization_strings
     H801 = hacking.core:OnceGitCheckCommitTitleBug