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.cursor/skills/tdd/mocking.md

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+
+# When to Mock
+
+Mock at **system boundaries** only:
+
+* External APIs (payment, email, etc.)
+* Databases (sometimes - prefer test DB)
+* Time/randomness
+* File system (sometimes)
+
+Don't mock:
+
+* Your own classes/modules
+* Internal collaborators
+* Anything you control
+
+## Designing for Mockability
+
+At system boundaries, design interfaces that are easy to mock:
+
+**1. Use dependency injection**
+
+Pass external dependencies in rather than creating them internally:
+
+```python
+import os
+
+# Easy to mock
+def process_payment(order, payment_client):
+    return payment_client.charge(order.total)
+
+# Hard to mock
+def process_payment(order):
+    client = StripeClient(os.getenv("STRIPE_KEY"))
+    return client.charge(order.total)
+
+```
+
+**2. Prefer SDK-style interfaces over generic fetchers**
+
+Create specific functions for each external operation instead of one generic function with conditional logic:
+
+```python
+import requests
+
+# GOOD: Each function is independently mockable
+class UserAPI:
+    def get_user(self, user_id):
+        return requests.get(f"/users/{user_id}")
+
+    def get_orders(self, user_id):
+        return requests.get(f"/users/{user_id}/orders")
+
+    def create_order(self, data):
+        return requests.post("/orders", json=data)
+
+# BAD: Mocking requires conditional logic inside the mock
+class GenericAPI:
+    def fetch(self, endpoint, method="GET", data=None):
+        return requests.request(method, endpoint, json=data)
+
+```
+
+The SDK approach means:
+
+* Each mock returns one specific shape
+* No conditional logic in test setup
+* Easier to see which endpoints a test exercises
+* Type safety per endpoint