results of tracking memory usage for alternatives to flatten a list of list in python (https://stackoverflow.com/a/59913424/3889948)

script.py

# --------------------------------------------------------------------------------
# option 1: using numpy.array(< list >, dtype=object).flat)
# --------------------------------------------------------------------------------

import tracemalloc
import functools
import operator
import itertools
import numpy

l1 = [[1,2,3],[4,5,6], [7], [8,9]] * 10_000

tracemalloc.start()
l2 = list(numpy.array(l1, dtype=object).flat)
size, peak = tracemalloc.get_traced_memory()
tracemalloc.reset_peak()
tracemalloc.stop()

print(f"{size=}, {peak=}")
# size=325_246, peak=647_522
# size=325_246, peak=647_522

# --------------------------------------------------------------------------------
# option 2: functools.reduce + operator.iconcat
# --------------------------------------------------------------------------------
import tracemalloc
import functools
import operator
import itertools

l1 = [[1,2,3],[4,5,6], [7], [8,9]] * 10_000

tracemalloc.start()
l2 = functools.reduce(operator.iconcat, l1, [])
size, peak = tracemalloc.get_traced_memory()
tracemalloc.reset_peak()
tracemalloc.stop()

print(f"{size=}, {peak=}")
# size=796_432, peak=806_712
# size=796_824, peak=807_128

# --------------------------------------------------------------------------------
# option 3: itertools.chain.from_iterable
# --------------------------------------------------------------------------------
import tracemalloc
import functools
import operator
import itertools

l1 = [[1,2,3],[4,5,6], [7], [8,9]] * 10_000

tracemalloc.start()
l2 = list(itertools.chain.from_iterable(l1))
size, peak = tracemalloc.get_traced_memory()
tracemalloc.reset_peak()
tracemalloc.stop()

print(f"{size=}, {peak=}")
# size=802_272, peak=812_448
# size=802_112, peak=812_408

# --------------------------------------------------------------------------------
# option 4: map + extend
# --------------------------------------------------------------------------------

import tracemalloc
import functools
import operator
import itertools

l1 = [[1,2,3],[4,5,6], [7], [8,9]] * 10_000

tracemalloc.start()
l2 = []
list(map(l2.extend, l1))
size, peak = tracemalloc.get_traced_memory()
tracemalloc.reset_peak()
tracemalloc.stop()

print(f"{size=}, {peak=}")
# size=796_488, peak=1_147_668
# size=796_880, peak=1_148_076

# --------------------------------------------------------------------------------

check tracemalloc - Trace memory allocations (from official docs 📄) to understand more about it