How do dictionaries work in Python?

How do dictionaries work in Python?

Dictionary is a very important data structure in Python. It stores data in the form of key-value pairs and can quickly obtain the corresponding value based on the key. This article will introduce in detail how to use the dictionary and the underlying implementation mechanism to help readers understand the dictionary in depth.

1. Create a dictionary

In Python, we can use curly braces {} or the dict() function to create a dictionary. For example:

# 创建一个空字典
empty_dict = {}
empty_dict2 = dict()

# 创建一个包含键值对的字典
person = {"name": "Tom", "age": 21, "country": "USA"}

In the above example, empty_dict and empty_dict2 are respectively an empty dictionary, while person is a dictionary containing three A dictionary of key-value pairs.

2. Accessing and modifying dictionary values

To access the values ​​in the dictionary, we need to use the key to extract the corresponding value. For example:

# 访问字典中的值
print(person["name"])  # 输出：Tom
print(person["age"])  # 输出：21

If we want to modify the value in the dictionary, we can assign the new value through the key. For example:

# 修改字典中的值
person["age"] = 22
print(person["age"])  # 输出：22

3. Dictionary addition and deletion operations

To add new key-value pairs to the dictionary, we can use the assignment operator. For example:

# 增加新的键值对
person["gender"] = "male"
print(person)  # 输出：{"name": "Tom", "age": 22, "country": "USA", "gender": "male"}

To delete key-value pairs in the dictionary, we can use the del keyword. For example:

# 删除键值对
del person["country"]
print(person)  # 输出：{"name": "Tom", "age": 22, "gender": "male"}

4. Dictionary traversal operation

We can use the for loop to traverse the key-value pairs in the dictionary. For example:

# 遍历字典中的键值对
for key, value in person.items():
    print(key, value)

The above code will output all key-value pairs in the dictionary. If we only need to iterate over the keys or values ​​in the dictionary, we can use the keys() or values() method. For example:

# 遍历字典中的键
for key in person.keys():
    print(key)

# 遍历字典中的值
for value in person.values():
    print(value)

5. Dictionary search operation

To find whether a certain key exists in the dictionary, we can use the in keyword. For example:

# 检查键是否存在
if "name" in person:
    print("name键存在")

6. The internal implementation mechanism of the dictionary

The dictionary in Python is implemented using the data structure of a hash table, which makes the dictionary lookup operation very efficient. s efficiency. Each key in the dictionary will be calculated by a hash function to obtain a hash value, and then placed into the corresponding slot based on the hash value. When we want to find the value of a key, Python will first locate the corresponding slot based on the hash value of the key, and then search in that slot.

Summary:

The dictionary in Python is a very practical data structure that stores data in the form of key-value pairs and has efficient search speed. Through the introduction of this article, we have learned about the creation, access, modification, addition and deletion operations of the dictionary, as well as the methods of traversal and search operations. In actual programming, reasonable use of dictionaries will make our code more efficient and readable.

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