Exempt str.format from redundant-keyword-arg when self is passed as a keyword

It's perfectly valid to pass self as a keyword to str.format when accessing
attributes using format's dot syntax, as in the following example:

   def __str__(self):
        return "{self.value}{self.other_value}...".format(self=self)

We're exempting only str.format with this commit, because for any other
function or method it's either impossible to do so or it's weird and
a potential sign of misdesign. Closes issue #642.

2 files changed, 18 insertions, 2 deletions

diff --git a/pylint/checkers/typecheck.py b/pylint/checkers/typecheck.py
index fb1c5fb..b971503 100644
--- a/pylint/checkers/typecheck.py
+++ b/pylint/checkers/typecheck.py
@@ -42,6 +42,7 @@ _ZOPE_DEPRECATED = (
     "This option is deprecated. Use generated-members instead."
 )
 BUILTINS = six.moves.builtins.__name__
+STR_FORMAT = "%s.str.format" % BUILTINS
 
 
 def _unflatten(iterable):
@@ -537,8 +538,15 @@ accessed. Python regular expressions are accepted.'}
                 i = parameter_name_to_index[keyword]
                 if parameters[i][1]:
                     # Duplicate definition of function parameter.
-                    self.add_message('redundant-keyword-arg',
-                                     node=node, args=(keyword, callable_name))
+                   
+                    # Might be too hardcoded, but this can actually
+                    # happen when using str.format and `self` is passed
+                    # by keyword argument, as in `.format(self=self)`.
+                    # It's perfectly valid to so, so we're just skipping
+                    # it if that's the case.
+                    if not (keyword == 'self' and called.qname() == STR_FORMAT):
+                        self.add_message('redundant-keyword-arg',
+                                         node=node, args=(keyword, callable_name))
                 else:
                     parameters[i][1] = True
             elif keyword in kwparams:
diff --git a/pylint/test/functional/arguments.py b/pylint/test/functional/arguments.py
index a589af4..f8db22c 100644
--- a/pylint/test/functional/arguments.py
+++ b/pylint/test/functional/arguments.py
@@ -134,3 +134,11 @@ class Test(object):
         self.lam(1, 2, 3) # [too-many-function-args]
 
 Test().lam() # [no-value-for-parameter]
+
+# Don't emit a redundant-keyword-arg for this example,
+# it's perfectly valid
+
+class Issue642(object):
+    attr = 0
+    def __str__(self):
+        return "{self.attr}".format(self=self)