Add the statistics module

src/flake8/statistics.py

@@ -0,0 +1,101 @@
+"""Statistic collection logic for Flake8."""
+import collections
+
+
+class Statistics(object):
+    """Manager of aggregated statistics for a run of Flake8."""
+
+    def __init__(self):
+        """Initialize the underlying dictionary for our statistics."""
+        self._store = {}
+
+    def record(self, error):
+        """Add the fact that the error was seen in the file.
+
+        :param error:
+            The Error instance containing the information about the violation.
+        :type error:
+            flake8.style_guide.Error
+        """
+        key = Key.create_from(error)
+        if key in self._store:
+            statistic = self._store[key]
+        else:
+            statistic = Statistic.create_from(error)
+        self._store[key] = statistic.increment()
+
+    def statistics_for(self, prefix, filename=None):
+        """Generate statistics for the prefix and filename.
+
+        If you have a :class:`Statistics` object that has recorded errors,
+        you can generate the statistics for a prefix (e.g., ``E``, ``E1``,
+        ``W50``, ``W503``) with the optional filter of a filename as well.
+
+        .. code-block:: python
+
+            >>> stats = Statistics()
+            >>> stats.statistics_for('E12',
+                                     filename='src/flake8/statistics.py')
+            <generator ...>
+            >>> stats.statistics_for('W')
+            <generator ...>
+
+        :param str prefix:
+            The error class or specific error code to find statistics for.
+        :param str filename:
+            (Optional) The filename to further filter results by.
+        :returns:
+            Generator of instances of :class:`Statistic`
+        """
+        matching_errors = sorted(key for key in self._store.keys()
+                                 if key.matches(prefix, filename))
+        for error_code in matching_errors:
+            yield self._store[error_code]
+
+
+class Key(collections.namedtuple('Key', ['filename', 'code'])):
+    __slots__ = ()
+
+    @classmethod
+    def create_from(cls, error):
+        return cls(
+            filename=error.filename,
+            code=error.code,
+        )
+
+    def matches(self, prefix, filename):
+        return (self.code.startswith(prefix) and
+                (filename is None or
+                    self.filename == filename))
+
+
+_Statistic = collections.namedtuple('Statistic', [
+    'error_code',
+    'filename',
+    'message',
+    'count',
+])
+
+
+class Statistic(_Statistic):
+    __slots__ = ()
+
+    @classmethod
+    def create_from(cls, error):
+        return cls(
+            error_code=error.code,
+            filename=error.filename,
+            message=error.message,
+            count=0,
+        )
+
+    def increment(self):
+        return Statistic(
+            error_code=self.error_code,
+            filename=self.filename,
+            message=self.message,
+            count=self.count + 1,
+        )
+
+
+del _Statistic