Python · The collections Module

Building a list of 10000 items by inserting each at the front (l.insert(0, x)) is far slower than appending to the end. Why, and what collections type fixes it?

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More in The collections Module

When should you prefer dict.setdefault(k, []).append(x) over a defaultdict(list)?

You need to look up scores by team name when the data is split across several separate dicts (group_a, group_g, ...). You don't want to merge them into a new dict. Which collections type fits?

What does this print? ``` from collections import ChainMap a = {"x": 1} b = {"x": 2, "y": 9} cm = ChainMap(a, b) print(cm["x"], cm["y"]) ```

With cm = ChainMap(a, b), you do cm["x"] = 100. Which underlying dict gets modified?

You want to count how often each character appears in a string, with the least code. Which collections type, and how in one line?

What does this print? ``` from collections import Counter c = Counter("aabbbc") print(c["z"]) print("z" in c) ```

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