Add a Counter class.

tests/helpers/counter.py

@@ -0,0 +1,60 @@
+
+class Counter(object):
+    """An introspectable, dynamic alternative to itertools.count()."""
+
+    def __init__(self, start=0, step=1):
+        self._start = int(start)
+        self._step = int(step)
+
+    def __repr__(self):
+        if self._last is None:
+            start = self._start
+        else:
+            start = self._last + self._step
+        return '{}(start={}, step={})'.format(
+            type(self).__name__,
+            start,
+            self._step,
+        )
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        try:
+            self._last += self._step
+        except AttributeError:
+            self._last = self._start
+        return self._last
+
+    @property
+    def start(self):
+        return self._start
+
+    @property
+    def step(self):
+        return self._step
+
+    @property
+    def last(self):
+        try:
+            return self._last
+        except AttributeError:
+            return None
+
+    def peek(self, iterations=1):
+        """Return the value that will be used next."""
+        try:
+            last = self._last
+        except AttributeError:
+            last = self._start - self._step
+        return last + self._step * iterations
+
+    def reset(self, start=None):
+        """Set the next value to the given one.
+
+        If no value is provided then the previous start value is used.
+        """
+        if start is not None:
+            self._start = int(start)
+        self._next = self._start