An in-depth analysis of the underlying mechanism of the len function in Python

In-depth discussion of the implementation principle of the len function in Python

In Python, the len function is a very commonly used function, used to obtain strings, lists, and tuples , the length or number of elements of objects such as dictionaries. Although it is very simple to use, understanding its implementation principle can help us better understand the internal mechanism of Python. In this article, we will delve into the implementation principle of the len function in Python and give specific code examples.

As for the implementation principle of the len function, first of all, we need to make it clear that the len function is not an ordinary function, but a built-in function that is initialized and registered to Python's built-in function when the interpreter starts. in the namespace. This means that the implementation code of the len function cannot be viewed directly in Python, but we can understand its implementation principle through our own code analysis.

The implementation principle of len function is basically determined based on the object type. The following introduces the implementation principles of the len function of four common object types: string, list, tuple and dictionary.

1. Getting the string length
   A string is composed of several characters, so calculating the length of a string is to count the number of characters in the string. Strings in Python are immutable objects that are stored in a method called Unicode encoding, with each character occupying 1 to 4 bytes. Therefore, by looping through each character in the string, you can get the length of the string. The specific code examples are as follows:

def my_len(string):
    length = 0
    for char in string:
        length += 1
    return length

s = "Hello, World!"
print(len(s))      # 使用内建的len函数
print(my_len(s))   # 使用自定义的my_len函数

2. Getting the length of the list
   List is one of the most commonly used data structures in Python and can accommodate any type of elements. In order to obtain the length of the list efficiently, Python uses a variable to record the length of the list, and this variable is updated every time an element is added or deleted. Therefore, when getting the length of the list, you only need to return the value of the variable of this record. The specific code example is as follows:

def my_len(lst):
    length = 0
    for _ in lst:
        length += 1
    return length

lst = [1, 2, 3, 4, 5]
print(len(lst))     # 使用内建的len函数
print(my_len(lst))  # 使用自定义的my_len函数

3. Getting the tuple length
   Tuples are similar to lists and are also data structures that can accommodate elements of any type. Like lists, in order to efficiently obtain the length of a tuple, Python uses a variable to record the length of the tuple. Therefore, the method of obtaining the tuple length is the same as the list, and you only need to return the value of the variable of this record. The specific code examples are as follows:

def my_len(tpl):
    length = 0
    for _ in tpl:
        length += 1
    return length

tpl = (1, 2, 3, 4, 5)
print(len(tpl))     # 使用内建的len函数
print(my_len(tpl))  # 使用自定义的my_len函数

4. Getting the dictionary length
   The dictionary is an unnecessary data structure consisting of key-value pairs. Unlike lists and tuples, the length of a dictionary is not simply stored in a variable. In order to get the length of the dictionary, Python needs to traverse the key-value pairs in the dictionary and count their number. The specific code examples are as follows:

def my_len(dct):
    length = 0
    for _ in dct:
        length += 1
    return length

dct = {1: 'one', 2: 'two', 3: 'three', 4: 'four', 5: 'five'}
print(len(dct))     # 使用内建的len函数
print(my_len(dct))  # 使用自定义的my_len函数

In summary, the implementation principle of the len function is determined based on the object type. For string types, the length is obtained by traversing the characters in the string; for list and tuple types, the length is obtained by recording the length variable; for dictionary types, it is necessary to traverse the key-value pairs in the dictionary to calculate the number. Through these examples, we can better understand the implementation principle of the len function and customize similar functions when needed.

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