10-1 Shape family

class Shape:
    def area(self) -> float: ...
    def name(self) -> str: ...
class Circle(Shape):
    def __init__(self, r: float) -> None: ...
class Rectangle(Shape):
    def __init__(self, w: float, h: float) -> None: ...
class Triangle(Shape):
    def __init__(self, a: float, b: float, c: float) -> None: ...
def total_area(shapes: list[Shape]) -> float: ...

Shape can be constructed; its area raises NotImplementedError. Name reports the actual class name. Dimensions are finite and positive; triangles satisfy strict inequalities. Invalid construction raises ValueError. Use math.pi and Heron’s formula. Empty total_area returns0; neither the list nor objects are changed.

Circle(1) has areaπ; Rectangle(2,3) and Triangle(3,4,5) have area6. Reject Triangle(1,1,3). Explain the degeneracy rejection, rather than selecting a different None/error policy. Code: w10-shapes,4 automatic.

10-2 Account inheritance

Submit all three definitions, not subclasses assuming L09 was run first.

class BankAccount:
    def __init__(self, owner: str, balance: int = 0) -> None: ...
    def deposit(self, amount: int) -> None: ...
    def withdraw(self, amount: int) -> bool: ...
    def statement(self) -> str: ...
class SavingsAccount(BankAccount):
    def __init__(self, owner: str, balance: int = 0,
                 interest_rate: float = 0.0) -> None: ...
    def add_interest(self) -> None: ...
class CheckingAccount(BankAccount):
    def __init__(self, owner: str, balance: int = 0,
                 overdraft_limit: int = 0) -> None: ...

Money uses integer cents. Public owner/balance/history follow L09. A nonnegative integer starting balance is required, otherwise ValueError. Nonpositive/noninteger deposit and withdrawal amounts are rejected: deposit returns None, withdrawal False, with unchanged state. Accepted history entries are (“deposit”,amount) or (“withdraw”,amount) using ordinary ASCII string quotes in code. Statement starts owner: balance cents, followed by action: amount cents, newline-joined without a trailing newline.

Both subclass initializers use super. Savings takes a finite rate0≤rate≤1; invalid rates raise ValueError. Quantize Decimal(str(rate))×balance to integer cents using ROUND_HALF_UP. Positive interest adds (“interest”,cents), zero adds no history, and add_interest returns None. This is an educational rounding contract, not a financial production system.

Checking starts nonnegative but supports later balances down to−overdraft_limit. The limit is a nonnegative integer. Overridden withdrawal rejects a value below that bound without changing history/balance. The base’s nonnegative-balance guarantee is not a universal family contract. Discuss whether directly calling base withdrawal meets this policy.

Code: w10-accounts,4 automatic.

10-3 Money

class Money:
    def __init__(self, yuan: float) -> None: ...
    def __repr__(self) -> str: ...
    def __add__(self, other: "Money") -> "Money": ...
    def __sub__(self, other: "Money") -> "Money": ...
    def __eq__(self, other: object) -> bool: ...
    def __lt__(self, other: "Money") -> bool: ...

Accept finite int/float yuan with magnitude≤10^9; otherwise ValueError. Store integer cents. Convert str(value) to Decimal, multiply by100 and quantize using ROUND_HALF_UP:1.005→101 cents,−1.005→−101. This operates on the supplied value’s decimal display, not an imagined original decimal intent. Read the decimal interface first; int(float*100) truncation is not this contract.

Repr has exactly two decimals, including Money(12.50), Money(-1.01), Money(0.00). Addition/subtraction return new objects and preserve inputs; comparisons use cents. Unsupported operand methods return NotImplemented. Equality to an unrelated object is False; unsupported arithmetic/ordering generally raises TypeError. Results may exceed the constructor-input bound without passing through float storage.

Check Money(0.1)+Money(0.2)==Money(0.3), sorting, negatives, ties and input preservation. Code: w10-money,4 automatic.

10-4 Deck

class Deck:
    def __init__(self) -> None: ...
    def __len__(self) -> int: ...
    def __getitem__(self, i: int) -> str: ...
    def __contains__(self, card: str) -> bool: ...
    def shuffle(self) -> None: ...
    def deal(self, n: int) -> list[str]: ...

Use ASCII rank+suit. Suits are C/D/H/S; ranks are A/2/…/10/J/Q/K. Initial ordering groups by suit then rank: AC at0, KC at12, AD at13, KS last. There are52 unique cards, no jokers, and independent decks.

Length reports remaining cards. Integer positive/negative indexing works; invalid indices raise IndexError. Membership returns bool. Sequence-style iteration must stop at an invalid index, not return None forever.

Deal takes n cards from the front, removes them and returns a new list. n is a nonnegative integer no larger than the remainder; invalid values raise ValueError without mutation. Zero deals nothing. Shuffle mutates and returns None while retaining length and card multiplicities. No particular shuffled sequence or mandatory change on every call is required; do not reseed internally to fake randomness.

Code: w10-deck,4 automatic.

A design record

w10-a-notes: four staff responses worth2 each: shape rejection/polymorphism; account and overdraft contracts/super; Money rounding/sentinel/preservation; Deck representation, edges and isolation. A separate zero-point declaration records actual assistance.

Homework 4

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