test.py

import collections

a = [(i, i - 1) for i in range(10000)]


def t1():
    d = collections.defaultdict(lambda: [].append)
    for i in a:
        d[i[0]](i)


def t2():
    d = collections.defaultdict(list)
    for i in a:
        d[i[0]].append(i)

testing.py

import timeit

timeit.timeit('''
import collections

a = [(i, i - 1) for i in range(10000)]


def t1():
    d = collections.defaultdict(lambda: [].append)
    for i in a:
        d[i[0]](i)

t1()''', number=1000)


timeit.timeit('''
import collections

a = [(i, i - 1) for i in range(10000)]


def t2():
    d = collections.defaultdict(list)
    for i in a:
        d[i[0]].append(i)

t2()''', number=1000)

>>> import timeit
>>>
>>> timeit.timeit('''
... import collections
... a = [(i, i - 1) for i in range(10000)]
... def t1():
...     d = collections.defaultdict(lambda: [].append)
...     for i in a:
...         d[i[0]](i)
... t1()''', number=1000)
15.552219152450562
>>>
>>>
>>> timeit.timeit('''
... import collections
... a = [(i, i - 1) for i in range(10000)]
... def t2():
...     d = collections.defaultdict(list)
...     for i in a:
...         d[i[0]].append(i)
... t2()''', number=1000)
10.282384157180786
>>>