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