Add pylint checker for docstring quotes. Fix #296

we want all docstring to be enclosed in triple double quotes

kiwi_lint/__init__.py

@@ -5,8 +5,10 @@
 # NOTE: import order matches the numeric ID of the checker
 from .dunder_attributes import DunderClassAttributeChecker
 from .list_comprehension import ListComprehensionChecker
+from .docstring import DocstringChecker
 
 
 def register(linter):
     linter.register_checker(DunderClassAttributeChecker(linter))
     linter.register_checker(ListComprehensionChecker(linter))
+    linter.register_checker(DocstringChecker(linter))

kiwi_lint/docstring.py

@@ -0,0 +1,57 @@
+# Copyright (c) 2018 Alexander Todorov <atodorov@MrSenko.com>
+
+# Licensed under the GPL 2.0: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+
+import tokenize
+
+from pylint import interfaces
+from pylint import checkers
+from pylint.checkers import utils
+
+
+class DocstringChecker(checkers.BaseTokenChecker):
+    __implements__ = (interfaces.ITokenChecker, interfaces.IAstroidChecker,)
+
+    name = 'docstring-checker'
+
+    msgs = {'R4421': ('Use """ for docstrings',
+                      'use-triple-double-quotes',
+                      'Use """ for docstrings!')}
+
+    _string_tokens = {}
+
+    def process_tokens(self, tokens):
+        """
+            Note: this method is executed before all of the visit_ methods
+            and self._string_tokens will contain all possible strings
+            before we try linting docstrings!
+        """
+        for (tok_type, token, (_start_row, _), _, _) in tokens:
+            if tok_type == tokenize.STRING:
+                # 'token' is the whole un-parsed token; we can look at the start
+                # of it to see whether it's a raw or unicode string etc.
+                # 'text_token' is the text without the quites
+                text_token = token.replace('"', '').replace("'", "")
+                # NOTE: this is sub-optimal b/c same tokens will override
+                # any previous ones which may not be complacent to the
+                # triple double quotes policy. However this isn't a big deal
+                # because it's unlikely to have exactly the same docstrings
+                # and it is more likely for them to diverge over time and
+                # expose the problem with quotes!
+                self._string_tokens[text_token] = token
+
+    def visit_module(self, node):
+        self._check_docstring(node)
+
+    def visit_classdef(self, node):
+        self._check_docstring(node)
+
+    def visit_functiondef(self, node):
+        self._check_docstring(node)
+
+    @utils.check_messages('use-triple-double-quotes')
+    def _check_docstring(self, node):
+        if node.doc in self._string_tokens:
+            token = self._string_tokens[node.doc]
+            if not token.startswith('"""'):
+                self.add_message('use-triple-double-quotes', node=node)