How to use bisect.insort_left with a key?

You could wrap your iterable in a class that implements __getitem__ and __len__. This allows you the opportunity to use a key with bisect_left. If you set up your class to take the iterable and a key function as arguments.

To extend this to be usable with insort_left it’s required to implement the insert method. The problem here is that if you do that is that insort_left will try to insert your key argument into the list containing the objects of which the the key is a member.

An example is clearer

from bisect import bisect_left, insort_left


class KeyWrapper:
    def __init__(self, iterable, key):
        self.it = iterable
        self.key = key

    def __getitem__(self, i):
        return self.key(self.it[i])

    def __len__(self):
        return len(self.it)

    def insert(self, index, item):
        print('asked to insert %s at index%d' % (item, index))
        self.it.insert(index, {"time":item})

timetable = [{"time": "0150"}, {"time": "0250"}, {"time": "0350"}, {"time": "0450"}, {"time": "0550"}, {"time": "0650"}, {"time": "0750"}]

bslindex = bisect_left(KeyWrapper(timetable, key=lambda t: t["time"]), "0359")

islindex = insort_left(KeyWrapper(timetable, key=lambda t: t["time"]), "0359")

See how in my insert method I had to make it specific to the timetable dictionary otherwise insort_left would try insert "0359" where it should insert {"time": "0359"}?

Ways round this could be to construct a dummy object for the comparison, inherit from KeyWrapper and override insert or pass some sort of factory function to create the object. None of these ways are particularly desirable from an idiomatic python point of view.

So the easiest way is to just use the KeyWrapper with bisect_left, which returns you the insert index and then do the insert yourself. You could easily wrap this in a dedicated function.

e.g.

bslindex = bisect_left(KeyWrapper(timetable, key=lambda t: t["time"]), "0359")
timetable.insert(bslindex, {"time":"0359"})

In this case ensure you don’t implement insert, so you will be immediately aware if you accidentally pass a KeyWrapper to a mutating function like insort_left which probably wouldn’t do the right thing.

To use your example data

from bisect import bisect_left


class KeyWrapper:
    def __init__(self, iterable, key):
        self.it = iterable
        self.key = key

    def __getitem__(self, i):
        return self.key(self.it[i])

    def __len__(self):
        return len(self.it)

data = [('red', 5), ('blue', 1), ('yellow', 8), ('black', 0)]
data.sort(key=lambda c: c[1])

newcol = ('brown', 7)

bslindex = bisect_left(KeyWrapper(data, key=lambda c: c[1]), newcol[1])
data.insert(bslindex, newcol)

print(data)

Here is the class with proper typing:

from typing import TypeVar, Generic, Sequence, Callable


T = TypeVar('T')
V = TypeVar('V')


class KeyWrapper(Generic[T, V]):
    def __init__(self, iterable: Sequence[T], key: Callable[[T], V]):
        self.it = iterable
        self.key = key

    def __getitem__(self, i: int) -> V:
        return self.key(self.it[i])

    def __len__(self) -> int:
        return len(self.it)