详解Python中defaultdict的具体使用

# return true if the defaultdict is a subclass of dict (dictionary)
from collections import defaultdict
print(issubclass(defaultdict,dict))

# normal dictionary
ord_dict = {
    "x" :3,
    "y": 4
}

ord_dict["z"]   # --> KeyError

from collections import defaultdict

# the default value for the undefined key
def def_val():
    return "The searched Key Not Present"

dic = defaultdict(def_val)

dic["x"] = 3
dic["y"] = 4

# search 'z' in the dictionary 'dic'
print(dic["z"]) # instead of a KeyError, it has returned the default value
print(dic.items())

from collections import defaultdict

# the default value for the undefined key
def def_val():
    return "Not Present"

dic = defaultdict(def_val)

dic["x"] = 3
dic["y"] = 4

print(f'Before clearing the dic, values of x = {dic["x"]} and y = {dic["y"]}')
dic.clear() #dic.clear() -> None. it will remove all items from dic.
print(f'After clearing the dic, values of x = {dic["x"]} and y = {dic["y"]}')

from collections import defaultdict

# the default value for the undefined key
def def_val():
    return "Not Present"

dic = defaultdict(def_val)

dic["x"] = 3
dic["y"] = 4

dic_copy = dic.copy() # dic.copy will give you a shallow copy of dic.

print(f"dic = {dic.items()}")
print(f"dic_copy = {dic_copy.items()}")

from collections import defaultdict

# the default value for the undefined key
def def_val():
    return "Not present"

dic = defaultdict(def_val)

dic["x"] = 3
dic["y"] = 4

print(f"The value of z = {dic['Z']}")
print(dic.default_factory()) # default_factory returns the default value for defaultdict.

from collections import defaultdict

# the default value for the undefined key
def def_val():
    return "Not present"

dic = defaultdict(def_val)

dic["x"] = 3
dic["y"] = 4

# search the value of Z in the dictionary dic; if it exists, return the value; otherwise, display the message
print(dic.get("Z","Value doesn't exist")) # default value is None