Basic knowledge of dictionaries and collections

Dictionary creation

A dictionary stores key value pairs
Format ：info = {"":"","":""}
sign ： Curly braces 、 Each key value pair is separated by a comma .
In the dictionary key Value cannot be modified , And only ,value You can modify .

Access to dictionaries

Not accessed by subscript , Instead, fill in the brackets directly key Value .

Access to nonexistent keys

Direct access to nonexistent keys will report an error

Can pass get Method to determine what to look for key Not in the dictionary , If not , Default return None.

You can also set the value returned when it cannot be found .

Operation of the dictionary

increase

Set a dictionary object directly , Assign a value to it .

Delete

Use del Method to delete an object , Will delete the whole object , When using this object again , The system will report an error saying that this object has not been defined .

You can also delete the entire dictionary , All dictionary data can be output without brackets after the dictionary name .

Empty

use clear Method , Clear all the data in the dictionary, but the dictionary still exists .

modify

You can directly access and modify it .

lookup

When printing out , You can choose to output all key、 Output all value、 Output all key value pairs . But the output is followed by a list of all these values .

If you need to continue to visit the content inside , You can traverse the list

When traversing key value pairs ,for A loop can assign multiple variables at a time

Because it is stored in the list , If you need to operate the subscript of the list , You can use enumeration functions enumerate（）.

aggregate SET

Only... Is stored in the collection key Do not store value, And key Is the only one. , If there is duplication , The assembly will automatically de weight , Just one .

list 、 Tuples 、 Dictionaries 、 The difference between sets

Lists and tuples are ordered , Can be accessed by subscript .
Dictionaries and collections are out of order , Access by writing the variable name in brackets .
The values in the list can be changed .
Values in tuples cannot be changed .
In the dictionary key The value cannot be changed ,value You can change .
In the collection key It can be changed , And not repeated .