defaultdict(dict)
时间: 2023-09-02 12:05:45 浏览: 38
defaultdict is a subclass of the built-in dict class in Python. It is a container that provides a default value for nonexistent keys.
When a defaultdict object is created, you specify a default factory function that will be used to create a default value for a nonexistent key. The default factory function is called whenever a nonexistent key is accessed, and its return value is used as the default value for that key.
For example, if you create a defaultdict with the default factory function set to int, then any nonexistent key will have a default value of 0. If you create a defaultdict with the default factory function set to list, then any nonexistent key will have a default value of an empty list.
Here is an example of creating a defaultdict with the default factory function set to int:
```
from collections import defaultdict
d = defaultdict(int)
d['a'] += 1
d['b'] += 2
d['c'] += 3
print(d)
# Output: {'a': 1, 'b': 2, 'c': 3,}
```
In this example, when we accessed the keys 'a', 'b', and 'c', they didn't exist in the dictionary yet, but the default factory function (int) was called to create a default value of 0 for each of them. We then incremented the values for each key, and the resulting dictionary contains the expected values.
Note that if you try to access a nonexistent key with a non-callable default factory function (such as None), a KeyError will still be raised.