in 是测试 dict.

d = {"key1": 10, "key2": 23}

if "key1" in d:
    print("this will execute")

if "nonexistent key" in d:
    print("this will not")

如果你想要一个默认值，你总是可以使用 dict.get():

If you wanted a default, you can always use dict.get():

d = dict()

for i in range(100):
    key = i % 10
    d[key] = d.get(key, 0) + 1

如果您想始终确保任何键的默认值，您可以使用 dict.setdefault() 重复或 defaultdict 来自 collections 模块，像这样:

and if you wanted to always ensure a default value for any key you can either use dict.setdefault() repeatedly or defaultdict from the collections module, like so:

from collections import defaultdict

d = defaultdict(int)

for i in range(100):
    d[i % 10] += 1

但总的来说，in 关键字是***的方法.

but in general, the in keyword is the best way to do it.