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dict

Dictionaries are used to store data values in key:value pairs.

A dictionary is a collection which is ordered, changeable and do not allow duplicates.

As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

dictItem = {
  "name": "wangenius",
  "gender": "male",
  "year": 1999,
  "good_at":["run","eat","sleep"]
}
print(dictItem)

Dictionary items are presented in key:value pairs, and can be referred to by using the key name.

print(dictItem["name"])
print(type(dictItem))
output
wangenius
<class 'dict'>

duplicates are not allowed in dictionaries:

dictItem = {
  "name": "wangenius",
  "gender": "male",
  "year": 1999,
  "year": 2000 # ❌
}
print(dictItem)

To determine how many items a dictionary has, use the len() function:

print(len(dictItem))

constructor​

dictItem = dict(name = "wangenius", age = 18)
print(dictItem)

function​

access​

name = dictItem["name"]
gender = dictItem.get("gender")

The keys() method will return a list of all the keys in the dictionary.

ls = dictItem.keys()
print(ls)
output
dict_keys(['name', 'gender', 'year'])

The values() method will return a list of all the values in the dictionary.

ls = dictItem.values()
print(ls)
output
dict_values(['wangenius', 'male', 1999])

The items() method will return each item in a dictionary, as tuples in a list.

tup = dictItem.items()
print(tup)
output
dict_items([('name', 'wangenius'), ('gender', 'male'), ('year', 1999)])
notion

dictItem 改变后，相应的items(),values(),keys()生成的都会改变

x = dict_items.items()
print(x) #before the change
x["year"] = 2020
print(x) #after the change
if "name" in dictItem:
  print("Yes, 'name' is one of the keys in the dictItem dictionary")

change​

dictItem.update({"year": 2020})
dictItem["year"] = 2021

update​

dictItem.update({"year": 2020});
dictItem["year"] = 2021;

remove​

dictItem.pop("name")
dictItem.popitem() #removes the last inserted item

del dictItem["name"]
dictItem.clear()
print(dictItem)
del dictItem
print(dictItem)
output
{}
NameError: name 'dictItem' is not defined

loop​

You can loop through a dictionary by using a for loop.

When looping through a dictionary, the return value are the keys of the dictionary, but there are methods to return the values as well.

for x in dictItem:
  print(dictItem[x])

for x in dictItem.values():
  print(x)

for x, y in dictItem.items():
  print(x, y)

copy​

You cannot copy a dictionary simply by typing dict2 = dict1, because: dict2 will only be a reference to dict1, and changes made in dict1 will automatically also be made in dict2.

There are ways to make a copy, one way is to use the built-in Dictionary method copy().

dictItem_2 = dictItem.copy()
print(dictItem_2)

Another way to make a copy is to use the built-in function dict().

mydict = dict(dictItem)
print(mydict)

nested​

A dictionary can contain dictionaries, this is called nested dictionaries.

cats = {
  "cat1" : {
    "color" : "red",
    "year" : 2004
  },
  "cat2" : {
    "color" : "blue",
    "year" : 2007
  },
  "cat3" : {
    "color" : "yellow",
    "year" : 2011
  }
}
print(cats["cat2"]["year"])
MethodDescription
clear()Removes all the elements from the dictionary
copy()Returns a copy of the dictionary
fromkeys()Returns a dictionary with the specified keys and value
get()Returns the value of the specified key
items()Returns a list containing a tuple for each key value pair
keys()Returns a list containing the dictionary's keys
pop()Removes the element with the specified key
popitem()Removes the last inserted key-value pair
setdefault(keyname, value)返回该关键字的值，如果该关键字不存在, 则写入第二个参数value
update()Updates the dictionary with the specified key-value pairs
values()Returns a list of all the values in the dictionary
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