今天来梳理一下Python dict 类型的知识点，特此申明下面信息均参考自公司培训课PPT Nagiza F. Samatova, NC State Univ. All rights，主要涉及到下面几个点：
- Characteristics of the keys and dictionaries
- How to createa dictionary object
- How to retrieve dictionary items
- How to updatea dictionary: add, delete, change items
- How to iterateover dictionary items
- How to query dictionary items
Characteristics of the keys and dictionaries
Dictionary is a mutable map of keys to values
Key is an immutable and unique object
Key is an immutable
key 不可能是list， set，因为他们是mutable，不可hash的
d1 = {
['script']:'java'} # error unhashable type: 'list'
d2 = {
{
'A', 'B', 'C'}:'all grades'} # error unhashable type: 'list'
key可以为bool， str， tuple，int， float
d_bool_key = {
True : 5}
d_str_key = {
'name': 'kelly'}
d_int_float_key = {
9 : 'score', 3.7 : 'gpa'}
d_tuple_key = {
('first', 'last') : ('kelly', 'xia')}
Keys are UNIQUE取最后一个值
d1 = {
'name':'kelly','name':'peter','name':'bob' }
d2 = {
'first':'Smith', 'last':'Smith'}
print(d1)
print(d2)
# output：
{
'name': 'bob'}
{
'first': 'Smith', 'last': 'Smith'}
Keys can be of HETEROGENEOUS Types
d中的key有str和tuple
d = {
'ssn: 555-55-5555':['jane', 'doe'], (16,17,18):'age group'}
print(d)
# output：
{
'ssn: 555-55-5555': ['jane', 'doe'], (16, 17, 18): 'age group'}
How to Create a Dictionary Object?
empty
d = dict()
d = {}
d = {key : value}
d1 = {
'key1':'value1', 'key2':'value2'}
d1
# output:
{
'key1': 'value1', 'key2': 'value2'}
Not support: dict(‘key’:‘value’)
d = dict('key':'value')
d
# output:
File "<ipython-input-640-efaed8bcc2a7>", line 1
d = dict('key':'value')
^
SyntaxError: invalid syntax
d=dict(key_name=value_object)
d2 = dict(key1='value1', key2='value2')
d2
# output:
{
'key1': 'value1', 'key2': 'value2'}
d = dict([tuple, tuple])------via tuple list
d_2_tuple_list = dict([('script', 'python'), ('version', 3.8)])
d_1_tuple_list = dict([('script', 'python')])
print('d_2_tuple_list: {}'.format(d_2_tuple_list))
print('d_1_tuple_list: {}'.format(d_1_tuple_list))
# output:
d_2_tuple_list: {
'script': 'python', 'version': 3.8}
d_1_tuple_list: {
'script': 'python'}
Not support: d = dict(tuple)
d = dict(('a', 'b'))
d
# output:
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-633-423a7c460802> in <module>
----> 1 d = dict(('a', 'b'))
2 d
ValueError: dictionary update sequence element #0 has length 1; 2 is required
{tuple}----set
d_2_tuple = {
('script', 'python'), ('version', 3.8)}
d_1_tuple = {
('script', 'python')}
print('d_2_tuple: {}'.format(d_2_tuple))
print('d_1_tuple: {}'.format(d_1_tuple))
# output:
d_2_tuple: {
('script', 'python'), ('version', 3.8)}
d_1_tuple: {
('script', 'python')}
Not support: d = {[tuple, tuple]}
d = {
[('script', 'python'), ('version', 3.8)]}
d
# output:
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-612-db86a05588ef> in <module>
----> 1 d = {
[('script', 'python'), ('version', 3.8)]}
2 d
TypeError: unhashable type: 'list'
d=dict.fromkeys(key sequence, value)
keys = {
'peter', 'bob', 'bruce'}
d1 = dict.fromkeys(keys, ['male', 'smoking', 30])
d2 = dict.fromkeys(keys, 30)
print('d1: {}'.format(d1))
print('d2: {}'.format(d2))
# output:
d1: {
'bob': ['male', 'smoking', 30], 'peter': ['male', 'smoking', 30], 'bruce': ['male', 'smoking', 30]}
d2: {
'bob': 30, 'peter': 30, 'bruce': 30}
d = dict(zip(collection1, collection2)
zip_3_3 = zip(['one', 'two', 'three'], [1,2,3])
zip_4_3 = zip(['one', 'two', 'three', 'four'], [1,2,3])
zip_3_4 = zip(['one', 'two', 'three'], [1,2,3,4])
d33 = dict(zip_3_3)
d43 = dict(zip_4_3)
d34 = dict(zip_3_4)
print('d33: {}'.format(d33))
print('d43: {}'.format(d43))
print('d34: {}'.format(d34))
只输出两个collection匹配成功的对数
# output:
d33: {
'one': 1, 'two': 2, 'three': 3}
d43: {
'one': 1, 'two': 2, 'three': 3}
d34: {
'one': 1, 'two': 2, 'three': 3}
字典的推导式 创建
dic = {
str(i) : i*2 for i in range(4)}
print(dic)
# output：
{
'0': 0, '1': 2, '2': 4, '3': 6}
作业： 哈哈，巩固成果的时候到了。
ACCESS, RETRIEVE, UPDATE ITEMS
Access
key不存在会unknown key error
info_dict = {
} # do not miss this statement, or name 'info_dict' is not defined
key = 'name'
info_dict [key] = 'kelly'
print(info_dict [key])
print(info_dict ['unkonwnKey'])
# output:
kelly
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
<ipython-input-644-de1581161396> in <module>
3 info_dict [key] = 'kelly'
4 print(info_dict [key])
----> 5 print(info_dict ['unkonwnKey'])
KeyError: 'unkonwnKey'
d = {
'first': 'kelly', 'last':'xia'}
print('d.keys:\n{} '.format(d.keys()))
print('d.values:\n{} '.format(d.values()))
print('d.items:\n{} '.format(d.items()))
print('d.get("first"):\n{} '.format(d.get('first')))
print('d.get("phone"):\n{} '.format(d.get('phone')))
print('d.get("phone", "111-222-333-444"):\n{} '.format(d.get('phone', '111-222-333-444')))
print('d content:\n{}'.format(d))
注意：get(key, [default]) key：value是不是add到dict的
# output:
d.keys:
dict_keys(['first', 'last'])
d.values:
dict_values(['kelly', 'xia'])
d.items:
dict_items([('first', 'kelly'), ('last', 'xia')])
d.get("first"):
kelly
d.get("phone"):
None
d.get("phone", "111-222-333-444"):
111-222-333-444
d content:
{
'first': 'kelly', 'last': 'xia'}
setdefault(key, default=None)
d = {
'first': 'kelly', 'last':'xia'}
print('d.setdefault("first", "lydia"):\n{}'.format(d.setdefault('first', 'lydia')))
print('d.setdefault("age", 18):\n{}'.format(d.setdefault('age', 18)))
print('d.setdefault("cellphone"):\n{}'.format(d.setdefault('cellphone')))
print('afer setting:\n{}'.format(d))
如果key存在则返回value，如果key不存在，则设置默认None或value
d.setdefault("first", "lydia"):
kelly
d.setdefault("age", 18):
18
d.setdefault("cellphone"):
None
afer setting:
{
'first': 'kelly', 'last': 'xia', 'age': 18, 'cellphone': None}
作业：
QUERYING BY-KEY: MEMBERSHIP CHECK
in operator
● to quickly check if a key is in a dictionary
d = {
'script':'python'}
print('Is the key "script" in d:\n{}'.format('script' in d))
print('Is the key "version" in d:\n{}'.format('version' in d))
print('Is the value "python" in d:\n{}'.format('python' in d))
只应用到key上，对value无效
# output:
Is the key "script" in d:
True
Is the key "version" in d:
False
Is the value "python" in d:
False
ITERATING OVER DICTIONARY ITEMS
for key, item in d.items()
d = {
'first':'kelly', 'last':'xia'}
for key, item in d.items():
print('{}:{}'.format(key, item))
# output:
first:kelly
last:xia
for key in d.keys()
d = {
'first':'kelly', 'last':'xia'}
for key in d.keys():
print('{}:{}'.format(key, d[key]))
# output:
first:kelly
last:xia
for key in d
d = {
'first':'kelly', 'last':'xia'}
for key in d:
print('{}:{}'.format(key, d[key]))
# output:
first:kelly
last:xia
for value in d.values(), 但是不能访问key
d = {
'first':'kelly', 'last':'xia'}
for value in d.values():
print('{}'.format(value))
# output:
kelly
xia
Dictionaries: Other Methods
d = dict (name='John', age=20)
| Method | Description | Result |
|---|---|---|
| d.copy () | Return copy, without modifying the original dictionary | d.copy (name=‘John’, age=20) |
| d.clear () | Remove all items | {} |
| del d[key] | Remove an items with a given key del | d[name] |
| d.pop (key, [default]) | Remove and return an element from a dictionary | d.pop (‘age’) |
| d.update () | Update the dictionary with elements from dictionary object | d.update (d1) |
| d.fromkeys (sequence, [value]) | Create a dictionary from sequence as keys w/ value provided keys = {‘1’, ‘2’, ‘3’} | d.fromkeys (keys, 0) |
dict.copy(d)
d = dict (name='John', age=20)
print('original dict:\n id:{}\t content:{}'.format(id(d), d))
d_copy = dict.copy(d)
print('copy dict:\n id:{}\t content:{}'.format(id(d_copy), d_copy))
生成一个新的dict
# output:
original dict:
id:2239127189312 content:{
'name': 'John', 'age': 20}
copy dict:
id:2239126915072 content:{
'name': 'John', 'age': 20}
del d[key]—没有返回值
d = {
'first':'kelly', 'last':'xia'}
del d['first']
d
# output:
{
'last': 'xia'}
pop(key)-移除和返回value
d = {
'first':'kelly', 'last':'xia'}
print('return d.pop("first"):{}'.format(d.pop('first')))
print('d content afer pop:{}'.format(d))
# output：
return d.pop("first"):kelly
d content afer pop:{
'last': 'xia'}
d1.update(d2): key相同时会将d2的value替换d1的value，key不同时，会将d2的key value加入d1中
d1 = {
'first':'kelly', 'last':'xia'}
d2 = {
'first':'kelly', 'last':'wang', 'age':'18'}
d1.update(d2)
d1
# output：
{
'first': 'kelly', 'last': 'wang', 'age': '18'}
作业：
