python 字典/列表理解 : why is it slower than for loop?

Question

考虑 events 这里有大约 48,000 个字典对象:

keyed_events = { gid: [ r for r in events if r['gid'] == gid ] for gid in gidlist }

大约比以下慢 4 倍:

keyed_events = {}
for event in events:
    gid = event['gid']
    if gid not in keyed_events:
        keyed_events[gid] = []
    keyed_events[gid].append(event)

虽然前者看起来更有效率。为什么比较慢？遍历 events 每次通过字典理解？

Answer 1

keyed_events = { gid: [ r for r in events if r['gid'] == gid ] for gid in gidlist }

列表/字典理解运行 len(gidlist)*len(events) 次，因为它在 gidlist 的循环内循环遍历 events 。

另一方面，for 循环在 events 上只有一个 for 循环，gid not in keyed_events 这是一个 O(1) 操作