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Chapter 12 — Special Methods for Sequences

Theme: To make a custom class feel like a native Python sequence, you do not need to inherit from anything. You just need to implement the sequence protocol (__len__ and __getitem__) and handle slice objects correctly.

What You'll Learn

Topic Key insight
Sequence Protocol Python asks "does it have __len__ and __getitem__?" instead of "is it a Sequence?"
slice Objects my_obj[1:4] passes a slice(1, 4, None) object to __getitem__.
Slice Awareness A slice of a custom class should return a new instance of that class, not a native list.
__getattr__ Intercepts attribute lookups only when the attribute is not found in __dict__.
Shadowing If you don't restrict __setattr__, users can assign obj.x = 10, silencing your __getattr__ logic permanently.
Hashing N-Dimensions Combine map(hash, components) with functools.reduce(operator.xor, ...) for fast, reliable hashing.
Short-circuit __eq__ Avoid converting large arrays to tuples just to compare them. Use zip() and all().

Key Files

  • examples.py — The multidimensional Vector implementing sequence protocols and dynamic properties.
  • exercises.py — Proving duck-typing fallbacks (like reversed()) and catching the __getattr__ recursion bug.
  • mini_project.py — A TimeSeries sequence mapping .max and .min via __getattr__.
  • benchmarks.py — Demonstrates that short-circuit __eq__ is significantly faster than tuple conversion.
  • notes.md — Reference on duck typing, slicing, and hashing multidimensional objects.
  • pitfalls.md — Infinite recursion in __getattr__ and returning native arrays from slices.
  • interview_questions.md — L3 to L6 interview prep.
  • architecture_notes.md — How Python implements slice objects internally.

30-Second Rules

# Rule 1: Always check for slice objects in __getitem__
def __getitem__(self, index):
    if isinstance(index, slice):
        return type(self)(self._data[index])  # Return a new instance!
    return self._data[index]

# Rule 2: If you implement __getattr__, heavily restrict __setattr__
def __setattr__(self, name, value):
    if name in self.dynamic_fields:
        raise AttributeError(f"Read-only attribute {name}")
    super().__setattr__(name, value)

# Rule 3: Use zip() and all() for fast sequence equality
def __eq__(self, other):
    return len(self) == len(other) and all(a == b for a, b in zip(self, other))

Reference: Fluent Python 2nd ed., Chapter 12 — pages 425–461