Python list Internals: How Dynamic Arrays Work Under the Hood
The article explains how Python lists are implemented as dynamic arrays in CPython, focusing on the internal mechanisms that manage capacity and resizing. It details the conditions under which lists expand or shrink and the formula used to determine new capacity. The resizing behavior ensures efficient performance for append operations through amortized constant time complexity.
- ▪Python lists are implemented as dynamic arrays in CPython.
- ▪The list capacity is adjusted using a formula that includes a 1/8 size increase plus a fixed slack of 3 or 6 for small lists.
- ▪Resizing is triggered when the new size exceeds the current capacity or drops below half of it.
- ▪The function list_resize in Objects/listobject.c manages all capacity changes for Python lists.
- ▪PyMem_Realloc handles memory reallocation by allocating new space, copying data, and freeing the old memory block.
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