在List子类的方法中对“ self”进行切片的正确方法是什么？

Question

I intend to create a method to slice a list , however I don't have the correct syntax here, and I haven't been able to come up with the correct search query to find out how to do this:

class BarList(list):
    """Items in the list are always in ascending order of .date"""
    def trim(self, start:dt.datetime=None, end:dt.datetime=None):
        """Removes all bars before `start` and after `end`"""
        if start:
            for i, bar in enumerate(self):
                if bar.date >= start:
                    self = self[i:]
                    break

What's the proper way to do what I'm trying to express in the above pseudocode?

Answer 1

The reason why self = self[i:] is incorrect is because all it does is slice the list and assign that slice to a local variable named self . It doesn't actually modify the list.

In order to change the content of the list, you can use a slice assignment :

self[:] = self[i:]

This basically means "replace the entire content of the list with the value on the right side of the = symbol".

---

The other part of the problem is making your trim method work correctly. There are a few cases that your code doesn't handle. Here's the updated code:

import datetime as dt

class BarList(list):
    """Items in the list are always in ascending order of .date"""
    def trim(self, start:dt.datetime=None, end:dt.datetime=None):
        """Removes all bars before `start` and after `end`"""
        if start:
            for i, bar in enumerate(self):
                if bar >= start:
                    # if this date is later than `start`, remove
                    # everything up to here
                    self[:] = self[i:]
                    break
            else:
                # if no date greater than `start` was
                # found, delete everything
                del self[:]

Test run:

l = BarList([dt.datetime.now()])
print(l)  # output: [datetime.datetime(2018, 4, 19, 0, 18, 14, 23474)]
l.trim(dt.datetime.now())
print(l)  # output: []