Add a new convention message, 'consider-using-enumerate'

The message is emitted when code that uses `range` and `len` for iterating is
encountered, which can be easily simplified by using `enumerate` instead.
This makes the code a bit faster and cleaner. Closes issue #684.

4 files changed, 117 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 5e21556..7f44f14 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -365,6 +365,10 @@ ChangeLog for Pylint
       encountered in languages with variabile assignments in conditional
       statements.
 
+    * Add a new convention message, 'consider-using-enumerate', which is
+      emitted when code that uses `range` and `len` for iterating is encountered.
+      Closes issue #684.
+
 
 
 2015-03-14 -- 1.4.3
diff --git a/pylint/checkers/base.py b/pylint/checkers/base.py
index f368a65..c6f6ac5 100644
--- a/pylint/checkers/base.py
+++ b/pylint/checkers/base.py
@@ -1463,6 +1463,74 @@ class LambdaForComprehensionChecker(_BasicChecker):
             self.add_message('deprecated-lambda', node=node)
 
 
+class RecommandationChecker(_BasicChecker):
+    msgs = {'C0200': ('Consider using enumerate instead of iterating with range and len',
+                      'consider-using-enumerate',
+                      'Emitted when code that iterates with range and len is '
+                      'encountered. Such code can be simplified by using the '
+                      'enumerate builtin.')
+           }
+
+    @staticmethod
+    def _is_builtin(node, function):
+        inferred = helpers.safe_infer(node)
+        if not inferred:
+             return False
+        return is_builtin_object(inferred) and inferred.name == function
+
+    @check_messages('consider-using-enumerate')
+    def visit_for(self, node):
+        """Emit a convention whenever range and len are used for indexing."""
+        # Verify that we have a `range(len(...))` call and that the object
+        # which is iterated is used as a subscript in the body of the for.
+
+        # Is it a proper range call?
+        if not isinstance(node.iter, astroid.Call):
+            return
+        if not self._is_builtin(node.iter.func, 'range'):
+            return
+        if len(node.iter.args) != 1:
+            return
+
+        # Is it a proper len call?
+        if not isinstance(node.iter.args[0], astroid.Call):
+            return
+        second_func = node.iter.args[0].func
+        if not self._is_builtin(second_func, 'len'):
+            return
+        len_args = node.iter.args[0].args
+        if not len_args or len(len_args) != 1:
+            return
+        iterating_object = len_args[0]
+        if not isinstance(iterating_object, astroid.Name):
+            return
+
+        # Verify that the body of the for loop uses a subscript
+        # with the object that was iterated. This uses some heuristics
+        # in order to make sure that the same object is used in the
+        # for body.
+        for child in node.body:
+            for subscript in child.nodes_of_class(astroid.Subscript):
+                if not isinstance(subscript.value, astroid.Name):
+                    continue
+                if not isinstance(subscript.slice, astroid.Index):
+                    continue
+                if not isinstance(subscript.slice.value, astroid.Name):
+                    continue
+                if subscript.slice.value.name != node.target.name:
+                    continue
+                if iterating_object.name != subscript.value.name:
+                    continue                
+                if subscript.value.scope() != node.scope():
+                    # Ignore this subscript if it's not in the same
+                    # scope. This means that in the body of the for
+                    # loop, another scope was created, where the same
+                    # name for the iterating object was used.
+                    continue    
+                self.add_message('consider-using-enumerate', node=node)
+                return            
+
+
 def _is_one_arg_pos_call(call):
     """Is this a call with exactly 1 argument,
     where that argument is positional?
@@ -1576,3 +1644,4 @@ def register(linter):
     linter.register_checker(PassChecker(linter))
     linter.register_checker(LambdaForComprehensionChecker(linter))
     linter.register_checker(ComparisonChecker(linter))
+    linter.register_checker(RecommandationChecker(linter))
diff --git a/pylint/test/functional/consider_using_enumerate.py b/pylint/test/functional/consider_using_enumerate.py
new file mode 100644
index 0000000..f906da0
--- /dev/null
+++ b/pylint/test/functional/consider_using_enumerate.py
@@ -0,0 +1,43 @@
+"""Emit a message for iteration through range and len is encountered."""

+

+# pylint: disable=missing-docstring, import-error

+

+def bad():

+    iterable = [1, 2, 3]

+    for obj in range(len(iterable)): # [consider-using-enumerate]

+        yield iterable[obj]

+

+

+def good():

+    iterable = other_obj = [1, 2, 3]

+    total = 0

+    for obj in range(len(iterable)):

+        total += obj

+        yield total

+        yield iterable[obj + 1: 2]

+        yield iterable[len(obj)]

+    for obj in iterable:

+        yield iterable[obj - 1]

+

+    for index, obj in enumerate(iterable):

+        yield iterable[index]

+    for index in range(0, 10):

+        yield iterable[index + 1]

+    for index in range(10):

+        yield iterable[index]

+    for index in range(len([1, 2, 3, 4])):

+        yield index

+    for index in range(len(iterable)):

+        yield [1, 2, 3][index]

+        yield len([1, 2, 3])

+    for index in range(len(iterable)):

+        yield other_obj[index]

+

+    from unknown import unknown

+    for index in range(unknown(iterable)):

+        yield iterable[index]

+

+    for index in range(len(iterable)):

+        def test(iterable):

+            return iterable[index]

+        yield test([1, 2, 3])

diff --git a/pylint/test/functional/consider_using_enumerate.txt b/pylint/test/functional/consider_using_enumerate.txt
new file mode 100644
index 0000000..36cd55d
--- /dev/null
+++ b/pylint/test/functional/consider_using_enumerate.txt
@@ -0,0 +1 @@
+consider-using-enumerate:7:bad:Consider using enumerate instead of iterating with range and len