Adding values to a Python list is not completely free: Python reserves storage as the list grows and keeps some spare capacity for future appends. This reduces the need to resize on every addition. section 3.5
Inspecting list storage
Use sys.getsizeof to compare a list's length with the storage currently reserved for it. The reported size stays constant across several appends, then jumps when Python allocates another block of space. section 3.5
>>> import sys
>>> mylist = []
>>> for i in range(25):
... l = len(mylist)
... s = sys.getsizeof(mylist)
... print(f'len = {l}, size = {s}')
... mylist.append(i)
Run this listing to observe list length and allocated size after each append. section 3.5
len = 0, size = 64 len = 1, size = 96 len = 2, size = 96 len = 3, size = 96 len = 4, size = 96 len = 5, size = 128 len = 6, size = 128 len = 7, size = 128 len = 8, size = 128 len = 9, size = 192 len = 10, size = 192 len = 11, size = 192 len = 12, size = 192 len = 13, size = 192 len = 14, size = 192 len = 15, size = 192 len = 16, size = 192 len = 17, size = 264 len = 18, size = 264 len = 19, size = 264 len = 20, size = 264 len = 21, size = 264 len = 22, size = 264 len = 23, size = 264 len = 24, size = 264
Compare the output to see spare capacity and stepwise allocation rather than one-byte-per-item growth. section 3.5
Treat these details as useful background for optimization or implementation awareness, not as routine application concerns. Python's garbage collector generally handles memory well, while offering limited fine-grained control. section 3.5
