1, dictionary

Create dictionary

• A dictionary is another variable container model and can store any type of object
• Each key value pair of the dictionary is separated by a colon (:), each pair is separated by a comma (,), and the whole dictionary is included in curly braces ({}).
• Key must be unique, but value is not required
• Value can take any data type, but the key must be immutable, such as string, number or tuple

#format
d = {key1 : value1, key2 : value2 }
#Example
dict = {'Alice': '2341', 'Beth': '9102', 'Cecil': '3258'}
dict1 = { 'abc': 456 }
dict2 = { 'abc': 123, 98.6: 37 }

ACCESS Dictionary

dict = {'Name': 'python', 'Age': 7, 'Class': 'First'}
 
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
'''
dict['Name']:  python
dict['Age']:  7
'''

• If you use a key that is not in the dictionary to access the data, the output error is as follows

dict = {'Name': 'python', 'Age': 7, 'Class': 'First'}
print ("dict['Alice']: ", dict['Alice'])
Traceback (most recent call last):
  File "test.py", line 5, in <module>
    print ("dict['Alice']: ", dict['Alice'])
KeyError: 'Alice'

Modify dictionary

dict = {'Name': 'python', 'Age': 7, 'Class': 'First'}
dict['Age'] = 8               # Update Age
dict['School'] = "LG"  # Add information
print ("dict['Age']: ", dict['Age'])#dict['Age']:  8
print ("dict['School']: ", dict['School'])#dict['School']: LG

Delete dictionary element

dict = {'Name': 'Runoob', 'Age': 7, 'Class': 'First'}
del dict['Name'] # Delete key 'Name'
dict.clear()     # Empty dictionary
del dict         # Delete dictionary
print ("dict['Age']: ", dict['Age'])
print ("dict['School']: ", dict['School'])
Traceback (most recent call last):
  File "test.py", line 9, in <module>
    print ("dict['Age']: ", dict['Age'])
TypeError: 'type' object is not subscriptable
#An exception was thrown. The dictionary no longer exists after del operation.

• Two occurrences of the same key are not allowed. If the same key is assigned twice during creation, the latter value will be remembered.

dict = {'Name': 'python', 'Age': 7, 'Name': 'LG'}
print ("dict['Name']: ", dict['Name'])#dict['Name']:  LG

• Keys must be immutable, so they can be numbers, strings, or tuples, not lists

dict = {['Name']: 'Runoob', 'Age': 7}
print ("dict['Name']: ", dict['Name'])
Traceback (most recent call last):
  File "test.py", line 3, in <module>
    dict = {['Name']: 'Runoob', 'Age': 7}
TypeError: unhashable type: 'list'

Dictionary built-in functions
Dictionary built in method

Assignment and copying of dictionaries

• Assignment reference - b = A: both a and b point to the same object
• Shallow copy - b = a.copy(): A and b parent objects are independent objects, but their child objects still point to unified objects (references).
• Deep copy - b = copy.deep copy (a): completely copies the parent object and its children, and the parent object and its children of a and b, both of which are completely independent.

import copy
a = [1, 2, 3, 4, ['a', 'b']] #Original object 
b = a                        #Assignment, reference to passed object
c = copy.copy(a)             #Object copy
d = copy.deepcopy(a)         #Object copy  
a.append(5)                  #Modify object a
a[4].append('c')             #Modify the array object of ['a ','b'] in object a
print( 'a = ', a )           #a =  [1, 2, 3, 4, ['a', 'b', 'c'], 5]
print( 'b = ', b )           #b =  [1, 2, 3, 4, ['a', 'b', 'c'], 5]
print( 'c = ', c )           #c =  [1, 2, 3, 4, ['a', 'b', 'c']]
print( 'd = ', d )           #d =  [1, 2, 3, 4, ['a', 'b']]

Dictionary sorting

• Sorting by key

d = {'d1':2, 'd2':4, 'd4':1,'d3':3,}
for k in sorted(d):
    print(k,d[k])
'''
d1 2
d2 4
d3 3
d4 1
'''

• Sorting by value: getitem

d = {'d1':2, 'd2':4, 'd4':1,'d3':3,}
for k in sorted(d,key=d.__getitem__):
    print(k,d[k])
'''
d4 1
d1 2
d3 3
d2 4
'''

• Reverse: reverse=True

d = {'d1':2, 'd2':4, 'd4':1,'d3':3,}
for k in sorted(d,key=d.__getitem__,reverse=True):
    print(k,d[k])
'''d2 4
d3 3
d1 2
d4 1
'''

• Sort dict items

d = {'d1':2, 'd2':4, 'd4':1,'d3':3,}
res = sorted(d.items(),key=lambda d:d[1],reverse=True)
print(res)
# [('d2', 4), ('d3', 3), ('d1', 2), ('d4', 1)]

2, set

• A set is an unordered sequence of distinct elements
• You can create a collection using curly braces {} or the set() function
• To create an empty set, you must use set() instead of {}; {} is used to create an empty dictionary

basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
print(basket)     # De duplication function
{'orange', 'banana', 'pear', 'apple'}

• Quickly determine whether an element is in a set

>> 'orange' in basket        
True
>>> 'crabgrass' in basket
False

• Operation between two sets

>>> a = set('abracadabra')
>>> b = set('alacazam')
>>> a                                  
{'a', 'r', 'b', 'c', 'd'}
>>> a - b                              # Elements contained in set a but not contained in set b
{'r', 'd', 'b'}
>>> a | b                              # All elements contained in set a or b
{'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'}
>>> a & b                              # Elements contained in sets a and b
{'a', 'c'}
>>> a ^ b                              # Elements different from a and b
{'r', 'd', 'b', 'm', 'z', 'l'}

Additive elements

• S.add (x) --- add element X to set s, and do nothing if the element already exists

>>>thisset = set(("Google", "Runoob", "Taobao"))
>>> thisset.add("Facebook")
>>> print(thisset)
{'Taobao', 'Facebook', 'Google', 'Runoob'}

• S.update (x) -- add elements, and parameters can be lists, tuples, dictionaries, etc.

>>>thisset = set(("Google", "Runoob", "Taobao"))
>>> thisset.update({1,3})
>>> print(thisset)
{1, 3, 'Google', 'Taobao', 'Runoob'}
>>> thisset.update([1,4],[5,6])  
>>> print(thisset)
{1, 3, 4, 5, 6, 'Google', 'Taobao', 'Runoob'}

Removing Elements

• s.remove (x) --- remove element x from set s. if the element does not exist, an error will occur.

>>>thisset = set(("Google", "Runoob", "Taobao"))
>>> thisset.remove("Taobao")
>>> print(thisset)
{'Google', 'Runoob'}
>>> thisset.remove("Facebook")   # No error will occur
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Facebook'

• S.discard (x) -- another method is to remove the elements from the collection, and if the elements do not exist, no errors will occur.

>>>thisset = set(("Google", "Runoob", "Taobao"))
>>> thisset.discard("Facebook")  # No no error
>>> print(thisset)
{'Taobao', 'Google', 'Runoob'}

• s.pop() -- set to randomly delete an element in the collection

thisset = set(("Google", "Runoob", "Taobao", "Facebook"))
x = thisset.pop()
print(x)
'''
$ python3 test.py 
Runoob
 The results of multiple tests are different
'''

• In interactive mode, pop is the first element of the deleted collection (the first element of the sorted collection)

>>>thisset = set(("Google", "Runoob", "Taobao", "Facebook"))
>>> thisset.pop()
'Facebook'
>>> print(thisset)
{'Google', 'Taobao', 'Runoob'}

• len(s) --- count the number of set elements

>>>thisset = set(("Google", "Runoob", "Taobao"))
>>> len(thisset)
3

• s.clear() -- empty set

>>>thisset = set(("Google", "Runoob", "Taobao"))
>>> thisset.clear()
>>> print(thisset)
set()

• x in s -- judge whether an element exists in a set

>>>thisset = set(("Google", "Runoob", "Taobao"))
>>> "Runoob" in thisset
True
>>> "Facebook" in thisset
False

Collection built-in method

Reference

• https://www.runoob.com/python3/python3-set.html
• https://www.runoob.com/python3/python3-dictionary.html
• https://www.cnblogs.com/kilometerwine/p/9712329.html
• https://www.cnblogs.com/faithfu/p/10825965.html