nyergler · September 11, 2013 23:11 · Sep 11, 2013
diff --git a/methodinspector.py b/methodinspector.py
@@ -0,0 +1,73 @@
+import collections
+from mock import patch
+
+
+MethodCall = collections.namedtuple(
+    'MethodCall',
+    ('args',
+     'kwargs',
+     'return_value',
+     ),
+)
+
+
+class MethodInspector(object):
+    """Wrap a method and track calls to allow making assertions about them.
+
+    In some cases mocking a method isn't sufficient: we need to
+    inspect calls and their return values to determine that our code
+    is operating as expected. For example, the payment service doesn't
+    lend itself to mocking, but we can inspect the return value of
+    Order.charge_via_payment_service to assert that the right amount
+    was charged.
+
+    MethodInspector is a context manager that instruments a method and
+    tracks its arguments and return values to allow this sort of testing.
+
+    >>> with MethodInspector(Order, 'charge_via_payment_service') as inspector:
+    ...     # do something
+
+    The calls are stored as a sequence of named tuples. Each has three values:
+
+    # args
+    # kwargs
+    # return_value
+
+
+    """
+
+    def __init__(self, klass, method_name):
+
+        self.klass = klass
+        self.method_name = method_name
+        self.orig = getattr(klass, method_name)
+
+    def __enter__(self):
+
+        self.calls = []
+
+        def wrapper(*a, **kw):
+
+            result = self.orig(*a, **kw)
+            self.calls.append(
+                MethodCall(
+                    args=a,
+                    kwargs=kw,
+                    return_value=result,
+                )
+            )
+            return result
+
+        self._patch = patch.object(self.klass, self.method_name, autospec=True)
+        self._patch.start().side_effect = wrapper
+
+        return self
+
+    def __exit__(self, *a):
+
+        self._patch.stop()
+
+    def assertCalledOnce(self, msg=None):
+
+        if len(self.calls) != 1:
+            raise AssertionError(msg or '')
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