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Chapter 12 — Pitfalls: Sequences and Dynamic Attributes

Pitfall 1: The __getattr__ Infinite Recursion Bug

__getattr__ is called when an attribute lookup fails. If your __getattr__ implementation references an attribute that also doesn't exist on self, it triggers another call to __getattr__. This loops infinitely until Python crashes with a RecursionError.

class BrokenRoute:
    def __getattr__(self, name):
        # BUG: If self.target isn't defined in __init__, this 
        # triggers __getattr__('target'), which triggers 
        # __getattr__('target')... infinitely!
        return self.target.get(name)

Fix: Ensure any attribute referenced inside __getattr__ is definitively created in __init__, or use super().__getattribute__ to bypass fallback lookup.

Pitfall 2: The Shadowing Vulnerability

Because __getattr__ only fires when an attribute is missing from the instance __dict__, it is highly vulnerable to "shadowing." If a user assigns a value to a dynamic attribute, it gets written to __dict__. From then on, __getattr__ is silenced for that name.

class Vector:
    def __init__(self):
        self._data = [10, 20]
        
    def __getattr__(self, name):
        if name == 'x': return self._data[0]

v = Vector()
print(v.x)  # 10 (via __getattr__)

v.x = 99    # Puts 'x' in v.__dict__
print(v.x)  # 99 (Reads from dict, bypassing __getattr__)

Fix: If you implement __getattr__ for dynamic properties, you must implement __setattr__ to block assignment to those specific names.

Pitfall 3: Returning Native Types from Slices

If you build a custom sequence (e.g., a TimeSeries or a custom Vector), developers expect a slice of that sequence to return a smaller instance of the same type. A common mistake is to return a native list or array instead.

class Deck:
    def __init__(self, cards):
        self.cards = cards
        
    def __getitem__(self, index):
        # BUG: If index is a slice, this returns a native list!
        return self.cards[index]

d = Deck(['Ace', 'King', 'Queen'])
hand = d[0:2]
print(type(hand))  # <class 'list'> — We lost the Deck type!

Fix: Check if index is a slice, and wrap the result back in type(self).

    def __getitem__(self, index):
        cls = type(self)
        if isinstance(index, slice):
            return cls(self.cards[index])  # Returns a new Deck!
        return self.cards[index]