增量itertools.imap而不读取对象以提高性能

Question

我试着从屏幕截图的中心看像素。我正在使用PngPy读取屏幕截图，并希望快速到达像素。

import png
import itertools

r=png.Reader("myfile.png")
direct = r.asRGBA8()

bytesIter = direct[2] # returns itertools.imap object - 
                      # see https://pythonhosted.org/pypng/png.html
height = direct[1]

count=0
for row in bytesIter:
    if count >= (height/2):
        print "Half way"
        break
    count+=1

print count

是否可以在不将迭代器读取到新对象的情况下增加迭代器？这个操作在快速工作站上为768x1280PNG(它确实有一个阿尔法通道)花费了2秒时间。

Answer 1

您可以使用迭代工具中的 recipe：

from itertools import islice
from collections import deque

def consume(iterator, n):
    "Advance the iterator n-steps ahead. If n is none, consume entirely."
    # Use functions that consume iterators at C speed.
    if n is None:
        # feed the entire iterator into a zero-length deque
        deque(iterator, maxlen=0)
    else:
        # advance to the empty slice starting at position n
        next(islice(iterator, n, n), None)

所以，在你的例子中：

consume(bytesIter, height/2)