Lec 18 Protocols

Interfaces using abstract classes are enforced
- Early error catching
Contracts are followed by convention, but not enforced
- Specifies what methods should do, rather than list the methods to be implemented
Len protocol:
- __len__() can be overridden, to replace len(obj)
- Can implement Sized interface, to force sizing
  - from collections.abc import Sized
  - class ClassName(Sized) -> must implement __len__()
Membership test protocol
- in calls __contains__()
- from collections.abc import Container
- class Document(Container[str]) -> must implement __contains__

Iterable protocol

for, in calls __iter__(self) -> Iterator[str] method
__next__(self) -> T: returns the next value in the sequence, or raises StopIteration if there are no more values left to iterate through

class AlternatingDayIterator(Iterator[str]):

    def init(self, days: list[str]) -> None:

        self.index = 0
<span class="token keyword">def</span> <span class="token function">__next__</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">></span> <span class="token builtin">str</span><span class="token punctuation">:</span>
    <span class="token comment">#if there are no days left, raise StopIteration</span>
    <span class="token keyword">if</span> self<span class="token punctuation">.</span>index <span class="token operator">>=</span> <span class="token builtin">len</span><span class="token punctuation">(</span>self<span class="token punctuation">.</span>days<span class="token punctuation">)</span><span class="token punctuation">:</span>
        <span class="token keyword">raise</span> StopIteration
    <span class="token comment"># if there are days left:</span>
        <span class="token comment"># figure out which day to return next</span>
        <span class="token comment"># increment index</span>
        <span class="token comment"># return the day</span>
    day_to_ret <span class="token operator">=</span> self<span class="token punctuation">.</span>days<span class="token punctuation">[</span>self<span class="token punctuation">.</span>index<span class="token punctuation">]</span>
    self<span class="token punctuation">.</span>index <span class="token operator">+=</span> <span class="token number">2</span>
    <span class="token keyword">return</span> day_to_ret</div>

class Interable(Iterable[int]):
    def __init__(self, number: int) -> None:
        self.number = number

    def __iter__(self) -> Iterator[int]:
        return Digiterator(self.number)

class Digiterator(Iterator[int]):
    def __init__(self, number: int) -> None:
        self.number = number

    def __next__(self) -> int:
        digit = self.num % 10 # rightmost
        rest = self.number // 10 # integer division (truncates)
        return digit

for digit in INTerable(34567):
print(f"{34567}")