defaultdict has all the functions of a dict, to be precise, it is a subclass of dict. except that it does not raise KeyError but returns a default value.
When we use dict, we can first make sure that the key is in the dict, and then take it out.
d = {"val": 10, "val2": 20}
if "val" in d:
print(d["val"])
else:
d["val"] = 10
print(d["val"])We can use dict's built-in function get() to get the value and define a default value to prevent the KeyError.
d = {"val": 10, "val2": 20}
d.get("val", 10) # Instead of d["val"]The first parameter of the defaultdict must be the default value (factory). After that, we can put the initial dict object into the parameters.
from collections import defaultdict
d = defaultdict(lambda: -1, {"val": 10, "val2": 20})
d["val"] # 10
d["val3"] # -1
d # defaultdict(<function __main__.<lambda>()>,
# {'val': 10, 'val2': 20, 'val3': -1})Default factory must be a function to determine the default value of the corresponding key when the KeyError occurred.
def def_value():
return "No value"
d = defaultdict(def_value)
# same as
d = defaultdict(lambda: "No value")
d["a"] = 1
d["b"]
d # {'a': 1, 'b': 'No value'}When the int class is passed as the default_factory, then a defaultdict is created with the default value as 0.
d = defaultdict(int)
d["a"] = 1
d["b"]
d # {'a': 1, 'b': 0}When the list class is passed as the default_factory, then a defaultdict is created with the values that are list.
d = defaultdict(list)
d["a"] = [1, 2, 3]
d["b"].append(4)
d["c"].extend([5, 6])
d # {'a': [1, 2, 3], 'b': [4], 'c': [5, 6]}| Article | Link |
|---|---|
| 7 More Tricks to Write Better Python Code | https://www.youtube.com/watch?list=LL&t=1319 |
| Defaultdict in Python | https://www.geeksforgeeks.org/defaultdict-in-python/ |