Size_t new_allocated, num_allocated_bytes List_resize(PyListObject *self, Py_ssize_t newsize) Here’s a snippet of the C++ implementation of the list data structure: static int The method then returns the value self.length. For example, say, the variable length stores the current length of the list. The method is implemented in C++ and it’s just a counter that’s increased each time you add an element to the list and decreased if you remove an element from the list.You pass a list object as an argument to the len() function so the Python virtual machine checks the _len_ method of the list object.The Python virtual machine looks up the len(.) function in a dictionary to find the associated implementation.When you call the function len(.) on a list object, here’s what happens (roughly): Python list objects keep track of their own length. Looking up the value of this counter takes constant time. Why? Because the list object maintains an integer counter that increases and decreases as you add and remove list elements. It takes constant runtime no matter how many elements are in the list. ![]() The runtime complexity of the len() function on your Python list is O(1).
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