哪个更快Counter()+=Counter或Counter.update(Counter)

Question

哪个更快? Counter()+=Counter或Counter.update(Counter) ?

为什么一个比另一个快?

我已经尝试了一些简单的分析，但我认为这不足以最终保存Counter+=Counter比Counter.update(Counter)快:

from collections import Counter
import time
x = Counter(['abc','def', 'abc'])
y = Counter(['xyz', 'xyz', 'uvw', 'abc'])
start = time.time()
x.update(y)
end = time.time() - start
print x
print 'update:', end
print 
x = Counter(['abc','def', 'abc'])
start = time.time()
x+=y
end = time.time() - start
print x
print 'plus:', end

[出]:

Counter({'abc': 3, 'xyz': 2, 'def': 1, 'uvw': 1})
update: 4.48226928711e-05
Counter({'abc': 3, 'xyz': 2, 'def': 1, 'uvw': 1})
plus: 2.28881835938e-05

Answer 1

Counter.update()方法的设计是为了更快。__add__()方法做了更多的工作，因为它必须消除非负值:

# heart of the update() loop in Python 2:
for elem, count in iterable.iteritems():
    self[elem] = self_get(elem, 0) + count
# heart of the __add__() loop in Python 2:
result = Counter()
for elem, count in self.items():
    newcount = count + other[elem]
    if newcount > 0:
        result[elem] = newcount
for elem, count in other.items():
    if elem not in self and count > 0:
        result[elem] = count
return result

如您所见，__add__方法做了相当多的工作。

在Python 3的后续版本中还有另一个不同之处，__iadd__()方法执行真正的就地更新，而__add__()方法创建一个新的计数器，然后赋值来替换旧的计数器:

def __iadd__(self, other):
    for elem, count in other.items():
        self[elem] += count
    return self._keep_positive()