Add Override The Boolean Context Of A Class as a Python TIL

python/override-the-boolean-context-of-a-class.md

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+# Override The Boolean Context Of A Class
+
+Everything in Python has a truthiness that can be checked with `bool()`. An
+empty list (`[]`) is falsy. A non-empty list (`[1,2,3]`) is truthy. Similar
+with numbers:
+
+```python
+>>> bool(0)
+False
+>>> bool(1)
+True
+```
+
+Any instance of an object is going to be truthy by default. If you want to
+control in what context an instance is considered truthy or falsy, you can
+override
+[`__bool__()`](https://docs.python.org/3/reference/datamodel.html#object.__bool__).
+If that's not implemented, but
+[`__len__()`](https://docs.python.org/3/reference/datamodel.html#object.__len__)
+is, then it will fallback to that.
+
+Let's look at a few example classes:
+
+```python
+class CartZero:
+    def __init__(self, items=[]):
+        self.items = items or []
+
+class CartBool:
+    def __init__(self, items=[]):
+        self.items = items or []
+
+    def __bool__(self):
+        print("__bool__() override")
+        return bool(self.items)
+
+class CartLen:
+    def __init__(self, items=[]):
+        self.items = items or []
+
+    def __len__(self):
+        print("__len__() override")
+        return len(self.items)
+
+class CartBoolAndLen:
+    def __init__(self, items=[]):
+        self.items = items or []
+
+    def __len__(self):
+        print("__len__() override")
+        return len(self.items)
+
+    def __bool__(self):
+        print("__bool__() override")
+        return bool(self.items)
+
+cart1 = CartZero()
+cart2 = CartBool()
+cart3 = CartLen()
+cart4 = CartBoolAndLen()
+
+print("CartZero() -> %s" %(bool(cart1)))
+print('')
+print("CartBool() -> %s" %(bool(cart2)))
+print('')
+print("CartLen() -> %s" %(bool(cart3)))
+print('')
+print("CartBoolAndLen() -> %s" %(bool(cart4)))
+```
+
+An 'empty' `Cart` be default is truthy. However, we can override some
+combination of `__bool__()` or `__len__()` to give it a boolean context that
+goes `false` when "empty".
+
+```
+CartZero() -> True
+
+__bool__() override
+CartBool() -> False
+
+__len__() override
+CartLen() -> False
+
+__bool__() override
+CartBoolAndLen() -> False
+```