To get a new list from a list with an additional value, I used to think that list_b = [*list_a, value] is more performant than list_b = list_a + [value], as the latter generates an intermediate [value].

However, according to the benchmark (tested in Python 3.12.3 / Windows 10), it seems that the result is opposite:

from timeit import timeit
import random
import matplotlib.pyplot as plt

num_data_points = 1000
step = 10
methods = [
    # ordered from slowest to fastest to make the key easier to read
    # "list_b = list_a.copy(); list_b += (value,)",
    # "list_b = list_a.copy(); list_b.append(value)",
    # "list_b = list(list_a); list_b.append(value)",
    "list_b = [*list_a, value]",
    "list_b = list_a + [value]",
]

x = list(range(0, num_data_points * step, step))
y = [[] for _ in methods]
for i in x:
    list_a = list(range(i))
    random.shuffle(list_a)
    value = random.randint(0, num_data_points * step)
    setup = f"list_a = {list_a}; value = {value}"
    for method_index, method in enumerate(methods):
        y[method_index].append(timeit(method, setup=setup, number=800))
    print(i, "out of", num_data_points * step)

ax = plt.axes()
for method_index, method in enumerate(methods):
    ax.plot(x, y[method_index], label=method)
ax.set(xlabel="size of the list", ylabel="time (s) (lower is better)")
ax.legend()
ax.figure.canvas.get_default_filetype = lambda: 'svg'
plt.show()

Can someone help explain why?