Add actual tests around statistics module

Also refactor our statistics module to be a bit smarter and less
namedtuple happy. The Statistic class had no reason to be a tuple,
I have no clue why I wrote it that way last night.

src/flake8/statistics.py

@@ -18,11 +18,9 @@ class Statistics(object):
             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()
+        if key not in self._store:
+            self._store[key] = Statistic.create_from(error)
+        self._store[key].increment()
 
     def statistics_for(self, prefix, filename=None):
         """Generate statistics for the prefix and filename.
@@ -54,48 +52,67 @@ class Statistics(object):
 
 
 class Key(collections.namedtuple('Key', ['filename', 'code'])):
+    """Simple key structure for the Statistics dictionary.
+
+    To make things clearer, easier to read, and more understandable, we use a
+    namedtuple here for all Keys in the underlying dictionary for the
+    Statistics object.
+    """
+
     __slots__ = ()
 
     @classmethod
     def create_from(cls, error):
+        """Create a Key from :class:`flake8.style_guide.Error`."""
         return cls(
             filename=error.filename,
             code=error.code,
         )
 
     def matches(self, prefix, filename):
+        """Determine if this key matches some constraints.
+
+        :param str prefix:
+            The error code prefix that this key's error code should start with.
+        :param str filename:
+            The filename that we potentially want to match on. This can be
+            None to only match on error prefix.
+        :returns:
+            True if the Key's code starts with the prefix and either filename
+            is None, or the Key's filename matches the value passed in.
+        :rtype:
+            bool
+        """
         return (self.code.startswith(prefix) and
                 (filename is None or
                     self.filename == filename))
 
 
-_Statistic = collections.namedtuple('Statistic', [
-    'error_code',
-    'filename',
-    'message',
-    'count',
-])
+class Statistic(object):
+    """Simple wrapper around the logic of each statistic.
 
+    Instead of maintaining a simple but potentially hard to reason about
+    tuple, we create a namedtuple which has attributes and a couple
+    convenience methods on it.
+    """
 
-class Statistic(_Statistic):
-    __slots__ = ()
+    def __init__(self, error_code, filename, message, count):
+        """Initialize our Statistic."""
+        self.error_code = error_code
+        self.filename = filename
+        self.message = message
+        self.count = count
 
     @classmethod
     def create_from(cls, error):
+        """Create a Statistic from a :class:`flake8.style_guide.Error`."""
         return cls(
             error_code=error.code,
             filename=error.filename,
-            message=error.message,
+            message=error.text,
             count=0,
         )
 
     def increment(self):
-        return Statistic(
-            error_code=self.error_code,
-            filename=self.filename,
-            message=self.message,
-            count=self.count + 1,
-        )
-
-
-del _Statistic
+        """Increment the number of times we've seen this error in this file."""
+        self.count += 1

tests/unit/test_statistics.py

@@ -1,5 +1,72 @@
+"""Tests for the statistics module in Flake8."""
+import pytest
+
 from flake8 import statistics as stats
+from flake8 import style_guide
+
+DEFAULT_ERROR_CODE = 'E100'
+DEFAULT_FILENAME = 'file.py'
+DEFAULT_TEXT = 'Default text'
 
 
-def test_nothing():
-    pass
+def make_error(**kwargs):
+    """Create errors with a bunch of default values."""
+    return style_guide.Error(
+        code=kwargs.pop('code', DEFAULT_ERROR_CODE),
+        filename=kwargs.pop('filename', DEFAULT_FILENAME),
+        line_number=kwargs.pop('line_number', 1),
+        column_number=kwargs.pop('column_number', 1),
+        text=kwargs.pop('text', DEFAULT_TEXT),
+        physical_line=None,
+    )
+
+
+def test_key_creation():
+    """Verify how we create Keys from Errors."""
+    key = stats.Key.create_from(make_error())
+    assert key == (DEFAULT_FILENAME, DEFAULT_ERROR_CODE)
+    assert key.filename == DEFAULT_FILENAME
+    assert key.code == DEFAULT_ERROR_CODE
+
+
+@pytest.mark.parametrize('code, filename, args, expected_result', [
+    # Error prefix matches
+    ('E123', 'file000.py', ('E', None), True),
+    ('E123', 'file000.py', ('E1', None), True),
+    ('E123', 'file000.py', ('E12', None), True),
+    ('E123', 'file000.py', ('E123', None), True),
+    # Error prefix and filename match
+    ('E123', 'file000.py', ('E', 'file000.py'), True),
+    ('E123', 'file000.py', ('E1', 'file000.py'), True),
+    ('E123', 'file000.py', ('E12', 'file000.py'), True),
+    ('E123', 'file000.py', ('E123', 'file000.py'), True),
+    # Error prefix does not match
+    ('E123', 'file000.py', ('W', None), False),
+    # Error prefix matches but filename does not
+    ('E123', 'file000.py', ('E', 'file001.py'), False),
+    # Error prefix does not match but filename does
+    ('E123', 'file000.py', ('W', 'file000.py'), False),
+    # Neither error prefix match nor filename
+    ('E123', 'file000.py', ('W', 'file001.py'), False),
+])
+def test_key_matching(code, filename, args, expected_result):
+    """Verify Key#matches behaves as we expect with fthe above input."""
+    key = stats.Key.create_from(make_error(code=code, filename=filename))
+    assert key.matches(*args) is expected_result
+
+
+def test_statistic_creation():
+    """Verify how we create Statistic objects from Errors."""
+    stat = stats.Statistic.create_from(make_error())
+    assert stat.error_code == DEFAULT_ERROR_CODE
+    assert stat.message == DEFAULT_TEXT
+    assert stat.filename == DEFAULT_FILENAME
+    assert stat.count == 0
+
+
+def test_statistic_increment():
+    """Verify we update the count."""
+    stat = stats.Statistic.create_from(make_error())
+    assert stat.count == 0
+    stat.increment()
+    assert stat.count == 1