Add a check for delayed string interpolation

This is stolen from the Neutron checks, since this seems useful for all
OpenStack projects.

See the oslo i18n guideline:
http://docs.openstack.org/developer/oslo.i18n/guidelines.html

Change-Id: Ibeaa82b295e011d87db2a0c44186a106f8d4254d

HACKING.rst

@@ -32,6 +32,7 @@ General
   it easier to find out who the author of the comment was.
 - [H105] Don't use author tags. We use version control instead.
 - [H106] Don't put vim configuration in source files (off by default).
+- [H904] Delay string interpolations at logging calls.
 - Do not shadow a built-in or reserved word. Shadowing built -in or reserved
   words makes the code harder to understand. Example::
 

hacking/checks/other.py

@@ -10,9 +10,16 @@
 #  License for the specific language governing permissions and limitations
 #  under the License.
 
+import re
+
 from hacking import core
 
 
+log_string_interpolation = re.compile(r".*LOG\.(?:error|warn|warning|info"
+                                      r"|critical|exception|debug)"
+                                      r"\([^,]*%[^,]*[,)]")
+
+
 @core.flake8ext
 def hacking_no_cr(physical_line):
     r"""Check that we only use newlines not carriage returns.
@@ -25,3 +32,23 @@ def hacking_no_cr(physical_line):
     pos = physical_line.find('\r')
     if pos != -1 and pos == (len(physical_line) - 2):
         return (pos, "H903: Windows style line endings not allowed in code")
+
+
+@core.flake8ext
+@core.off_by_default
+def hacking_delayed_string_interpolation(logical_line, noqa):
+    r"""String interpolation should be delayed at logging calls.
+
+    H904: LOG.debug('Example: %s' % 'bad')
+    Okay: LOG.debug('Example: %s', 'good')
+    """
+    msg = ("H904: String interpolation should be delayed to be "
+           "handled by the logging code, rather than being done "
+           "at the point of the logging call. "
+           "Use ',' instead of '%'.")
+
+    if noqa:
+        return
+
+    if log_string_interpolation.match(logical_line):
+        yield 0, msg

hacking/tests/test_doctest.py

@@ -39,7 +39,7 @@ class HackingTestCase(hacking.tests.TestCase):
     def test_pep8(self):
 
         # NOTE(jecarey): Add tests marked as off_by_default to enable testing
-        turn_on = set(['H106'])
+        turn_on = set(['H106', 'H904'])
         if self.options.select:
             turn_on.update(self.options.select)
         self.options.select = tuple(turn_on)

setup.cfg

@@ -54,6 +54,7 @@ flake8.extension =
     H501 = hacking.checks.dictlist:hacking_no_locals
     H700 = hacking.checks.localization:hacking_localization_strings
     H903 = hacking.checks.other:hacking_no_cr
+    H904 = hacking.checks.other:hacking_delayed_string_interpolation
 
 [extras]
 pep257 =