Choosing Abstract Base Classes

Python
from collections.abc import Mapping, Sequence, Iterable

# Duck-typing check: does obj behave like a mapping?
if isinstance(obj, Mapping):
    for key in obj:
        print(key, obj[key])

# Custom container: implement minimum methods, get the rest free
class FrozenDict(Mapping):
    def __init__(self, data):
        self._data = dict(data)
    def __getitem__(self, key):
        return self._data[key]
    def __iter__(self):
        return iter(self._data)
    def __len__(self):
        return len(self._data)
    # __contains__, keys, items, values, get, __eq__, __ne__ come for free

Progress:

• Step 1: Identify what behavior is needed (iteration? indexing? mutation? hashing?)
• Step 2: Pick the narrowest ABC that matches (don't over-implement)
• Step 3: Check the ABC's "Abstract Methods" — implement only those
• Step 4: Check "Mixin Methods" — these come free from the abstract ones
• Step 5: Use isinstance()/issubclass() for duck-typing checks instead of concrete types
• Step 6: Register unrelated classes with .register() if virtual subclassing is needed

Reference table (most-used ABCs)

Example 1: Input: "I need a class that supports len(), in, and iteration, but I don't need indexing or mutation." Output: Inherit from Collection and implement __contains__, __iter__, __len__. Don't reach for Sequence — it forces __getitem__ and implies order/indexing you don't need.

Example 2: Input: "How do I check if a function argument is 'list-like' without requiring it to actually be a list?" Output:

Python
from collections.abc import Sequence

def process(items):
    if not isinstance(items, Sequence):
        raise TypeError("items must be a Sequence")
    return items[0], len(items)

This accepts tuple, custom Sequence subclasses, etc., while rejecting str only if that's explicitly desired (note str is a Sequence — guard separately if needed).

Example 3: Input: "Building a custom read-only set-like collection backed by a database query." Output: Inherit from Set, implement __contains__, __iter__, __len__. Get __le__, __lt__, __gt__, __ge__, __eq__, __and__, __or__, __sub__, __xor__, isdisjoint for free.