Catalog
2. Dictionary-related operations
a. Direct creation: (simple and rough)
c. Create dictionary objects from zip()
d. fromkeys creates a dictionary from a list of one-dimensional keys
b. It is better to get the value by get().
d. List the number of key-value pairs to check if a key exists
3. Dictionary Additions and Deletions
a. a['key']='value'==>direct dispatch
c. Detect dictionary keys or increase key-value pairs by setdefault
a. del(a['key']) delete directly
b. pop(a['key']) Deletes key-value pairs and returns a value;
c. clear() Delete all key-value pairs
Activity address: CSDN 21-day Learning Challenge
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1. Dictionary Concepts
1. Concepts:
In Python, a dictionary is simply something enclosed in curly braces with key-value pairs.
2. The format is:
{'Key 1':'Value 1','Key 2':'Value 2','Key 3':'Value 3'}
Explanation citing the novice bird tutorial:
Dictionaries are another variable container model and can store any type of object.
Each key:value pair of the dictionary is colon: split, comma between each key-value pair, split, and the entire dictionary is enclosed in curly brackets {}.
3. Simple examples:
a = { 'School_class':2 ,'name':'SYW','age':20,'School':'SZschool','Youlike':'Basketball'} #This is a dictionary that prints directly: print(a) # Here is the output >>> {'School_class': 2, 'name': 'SYW', 'age': 20, 'School': 'SZschool', 'Youlike': 'Basketball'}
2. Dictionary-related operations
1. Dictionary Creation
There are many ways to create dictionaries. Here's an introduction.
a. Direct creation: (simple and rough)
dd = {'name':'SYW','age':21,'job':'student'} cc = {} #Empty Dictionary Object print(dd,cc) Output: >>> {'name': 'SYW', 'age': 21, 'job': 'student'} {}
b. Borrowing dict to create
z = dict(name='SYW',age=21,job='student') v = dict([("name","SYW"),("age",21)]) dit0 = dict() #Empty Dictionary Object print(z,v,dit0) Output: >>> {'name': 'SYW', 'age': 21, 'job': 'student'} {'name': 'SYW', 'age': 21} {}
c. Create dictionary objects from zip()
Advantage: Separate key-value pairs, create lists, and create flexible formats.
Suggestion: Keys and values should be imported one-to-one, or keys will be discarded after they are created.
f = ['name','age','job'] s = ['SYW',21,'Student'] dit0 = dict(zip(f,s)) print(dit0) Output: >>> {'name': 'SYW', 'age': 21, 'job': 'Student'}
d. fromkeys creates a dictionary from a list of one-dimensional keys
# 1. Dictionaries with an empty default value (None) list1 = ['name','age','job'] a = dict.fromkeys(list1) print(a) Output: >>> {'name': None, 'age': None, 'job': None} # 2. Change the default to any value and create a dictionary: list1 = ['name','age','job'] b = dict.fromkeys(list1,"Han Dynasty" ) print(b) Output: >>> {'name': 'Han Dynasty', 'age': 'Han Dynasty', 'job': 'Han Dynasty'}
2. Dictionary Access
Create a root dictionary to provide access to the corresponding method functions.
a = {'name':'SYW','age':18,'job':'student'}
a. Get'value'by [key]
print(a['name']) print(a['age']) # Keys that do not exist throw exceptions. # The exception part was replaced by try...except...so no exception was thrown and the output "Error! The key'sex'does not exist!" when an exception occurs. try: print(a['sex']) except Exception as e: print("Report errors! key'sex'Non-existent!") Output: >>> SYW >>> 18 >>> Report errors! key'sex'Non-existent!
b. It is better to get the value by get().
Function: key exists, return value; Key does not exist, returns None or default object.
print(a.get('name')) print(a.get('sex')) print(a.get('sex','One Man')) # If the key does not exist, go back to'a man' # List all key-value pairs print(a.items()) Output: >>> SYW >>> None >>> One Man >>> dict_items([('name', 'SYW'), ('age', 18), ('job', 'student')])
List all keys and values
# List all keys print(a.keys()) # List all values print(a.values()) Output: >>> dict_keys(['name', 'age', 'job']) >>> dict_values(['SYW', 18, 'student'])
d. List the number of key-value pairs to check if a key exists
# Number of len() key-value pairs print('The number of key-value pairs is:',len(a)) # Detect if a key is in a dictionary a = {"name":"SYW","age":18} print("name" in a) Output: >>> The number of key-value pairs is: 3 >>> True
3. Dictionary Additions and Deletions
Additions and changes:
a. a['key']='value'==>direct dispatch
Keys exist ==>Overwrite old key values;
Keys do not exist =>Add a new key-value pair. (
a = {'name':'SYW','age':21,'job':'SZstudent'} a['address']='Shantou Professional Street' a['age']=19 print(a) Output: >>> {'name': 'SYW', 'age': 19, 'job': 'SZstudent', 'address': 'Shantou Professional Street'}
b. a.update(b)
Plug all the key values of b into a and override the values of the same key of a directly.
dict0={'name':'SYW','salary':'18w'} dict1={'name':'xiaolin','city':'guangdong'} dict1.update(dict0) print(dict1) Output: >>> {'name': 'SYW', 'city': 'guangdong', 'salary': '18w'}
c. Detect dictionary keys or increase key-value pairs by setdefault
A=['name','age'] B=['SYW',20] dict0 = dict(zip(A,B)) print(dict0) dict0.setdefault('name','XL') # Since name exists, the value of name does not change. print(dict0) dict0.setdefault('city','guangdong') # Because ciy does not exist, a new key-value pair is added. print(dict0) Output: >>> {'name': 'SYW', 'age': 20} >>> {'name': 'SYW', 'age': 20} >>> {'name': 'SYW', 'age': 20, 'city': 'guangdong'}
Delete:
a. del(a['key']) delete directly
b. pop(a['key']) Deletes key-value pairs and returns a value;
c. clear() Delete all key-value pairs
a = {'name':'SYW','age':20,'job':'Student'} del(a['name']) print(a) b = a.pop('age') print(b) a.clear() try: print(a['job']) except Exception as e: print("Dictionaries a Has been cleared~") Output: >>> {'age': 20, 'job': 'Student'} >>> 20 >>> Dictionaries a Has been cleared~
d. popitem() :
Delete and return the last key-value pair.
a = {'name':'SYW','age':21,'job':'Student'} print(a) print(a.popitem()) print(a) print(a.popitem()) print(a) Output: >>> {'name': 'SYW', 'age': 21, 'job': 'Student'} >>> ('job', 'Student') >>> {'name': 'SYW', 'age': 21} >>> ('age', 21) >>> {'name': 'SYW'}
4. Dictionary traversal:
Key traversal, value traversal, and key-value traversal:
A=['name','age'] B=['SYW',20] dict0 = dict(zip(A,B)) for key in dict0: print("The dictionary keys are:",key) print("-------------") for value in dict0: print("Dictionary value traversal:",value) print("-------------") for key,value in dict0.items(): print("The key of the dictionary is{0},Value is{1}. ".format(key,value)) print("-------------") Output: >>> The dictionary keys are: name >>> The dictionary keys are: age >>> ------------- >>> Dictionary value traversal: name >>> Dictionary value traversal: age >>> ------------- >>> The key of the dictionary is name,Value is SYW. >>> The key of the dictionary is age,The value is 20. >>> -------------
Ending:
That's all the more basic operations of the dictionary. Please read the official documents for more detailed operations.