I was experimenting with python sorting using key. I'm interested in the inner working of the algorithm. Is it roughly equivalent to Schwartzian transform (Decorate-Sort-Undecorate)?
Specifically:
- The spec keys are extracted only once. Can it be asssumed that this happens before any comparisions occur?
- How are the extracted keys maintained in memory? As tuples
(key, orginal_value)or does it use some other method.
I used the following test program
class Isbn:
def __init__(self, isbn_num):
self.isbn_num = isbn_num
def __lt__(self, other):
print(f"__lt__ {self.isbn_num} {other.isbn_num}")
return self.isbn_num < other.isbn_num
def __repr__(self) -> str:
return f'Isbn({self.isbn_num})'
class Book:
def __init__(self, isbn):
self.isbn = Isbn(isbn)
def __repr__(self) -> str:
return f'Book({self.isbn})'
@property
def key(self):
print(f"key {self.isbn}")
return self.isbn
books = [Book(5), Book(10), Book(6), Book(2)]
books.sort(key=lambda b: b.key)
print(books)
Which gives the following output:
key Isbn(5)
key Isbn(10)
key Isbn(6)
key Isbn(2)
__lt__ 10 5
__lt__ 6 10
__lt__ 6 10
__lt__ 6 5
__lt__ 2 6
__lt__ 2 5
[Book(Isbn(2)), Book(Isbn(5)), Book(Isbn(6)), Book(Isbn(10))]
