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Chapter 1 — Notes: The Python Data Model

Core Concept: The Data Model as a Framework

Python's data model is not a feature — it's the meta-architecture of the entire language. It defines how objects integrate with:

  • Built-in functions (len, abs, bool, repr)
  • Operators (+, *, in, [])
  • Language statements (for, if, with, del)

Key Special Method Families

Sequence Protocol

Method Triggers
__len__ len(obj)
__getitem__ obj[key], obj[start:stop], for x in obj (fallback)
__setitem__ obj[key] = value
__delitem__ del obj[key]
__contains__ item in obj (falls back to __getitem__ scan)
__iter__ for x in obj, iter(obj)
__reversed__ reversed(obj)

Numeric Protocol

Method Triggers
__add__ a + b
__radd__ b + a (when b doesn't know how to add a)
__iadd__ a += b
__mul__ a * b
__abs__ abs(a)
__neg__ -a
__pos__ +a

Representation Protocol

Method Triggers
__repr__ repr(obj), REPL output, f"{obj!r}"
__str__ str(obj), print(obj), f"{obj}"
__format__ format(obj, spec), f"{obj:spec}"
__bytes__ bytes(obj)

Comparison & Hashing

Method Triggers
__eq__ a == b
__lt__ a < b
__hash__ hash(obj), using as dict key
__bool__ bool(obj), if obj:, while obj:

Critical Rules

1. __repr__ is mandatory; __str__ is optional

If __str__ is missing, Python falls back to __repr__. So always implement __repr__. __repr__ should ideally return a string that could reconstruct the object: Vector(3, 4) not "vector at 0x...".

2. __eq__ implies __hash__ rules

  • Define __eq__ without __hash__ → object becomes unhashable (can't use in sets/dicts)
  • If __eq__ compares by value, __hash__ must hash the same fields: hash((self.x, self.y))
  • Mutable objects should NOT be hashable (no __hash__)

3. Return NotImplemented not NotImplementedError

When a binary operator can't handle the other type:

def __add__(self, other):
    if not isinstance(other, MyClass):
        return NotImplemented  # ← correct: a sentinel value
    ...

This lets Python try the reflected operator on the other object. NotImplementedError is an exception for abstract methods.

4. __bool__ fallback chain

bool(obj)
  → __bool__?  → return bool result
  → __len__?   → return len(obj) != 0
  → True        (default: all objects are truthy)

Vocabulary

Term Meaning
Special method / dunder Methods like __len__ called by Python internally
Protocol Informal interface defined by which special methods you implement
Duck typing Python checks what an object can do, not what it is
Structural typing Type compatibility via structure (methods), not inheritance
Nominal typing Type compatibility via explicit type declaration (Java, C#)