迭代序列的 Pythonic 方式，一次 4 个项目
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Pythonic way to iterate over sequence, 4 items at a time
提问by blasted
Possible Duplicate:
What is the most “pythonic” way to iterate over a list in chunks?
I am reading in some PNG data, which has 4 channels per pixel. I would like to iterate over the data 1 pixel at a time (meaning every 4 elements = 1 pixel, rgba).
我正在读取一些 PNG 数据，每个像素有 4 个通道。我想一次迭代 1 个像素的数据（意思是每 4 个元素 = 1 个像素，rgba）。
red_channel = 0
while red_channel < len(raw_png_data):
green_channel, blue_channel, alpha_channel = red_channel +1, red_channel +2, red_channel +3
# do something with my 4 channels of pixel data ... raw_png_data[red_channel] etc
red_channel += 4
This way doesnt really seem "right". Is there a more Pythonic way to iterate over a sequence, 4 items at a time, and have those 4 items unpacked?
这种方式似乎并不“正确”。是否有一种更 Pythonic 的方式来迭代一个序列，一次 4 个项目，然后将这 4 个项目解包？
回答by excid3
Try something like this:
尝试这样的事情：
for red, green, blue, alpha in raw_png_data:
#do something
You can pull out multiple items and never have to use an iterator. :)
您可以提取多个项目，而不必使用迭代器。:)
Edit: This would mean that raw_png_data needs to be a list of 4 value tuples. It would be most pythonic to put each rgba group into a tuple and then append it to raw_png_data and iterate through like my example.
编辑：这意味着 raw_png_data 需要是 4 个值元组的列表。将每个 rgba 组放入一个元组中，然后将其附加到 raw_png_data 并像我的示例一样迭代将是最pythonic的。
回答by Tomasz Wysocki
vars = [1, 2, 3, 4, 5, 6, 7, 8]
for a, b, c, d in zip(*[iter(vars)]*4):
print a, b, c, d
回答by Sjoerd
for r, g, b, t in (data[i:i+4] for i in xrange(0, len(data)/4*4, 4)):
print r, g, b, t
回答by kennytm
(Python's itertools should really make all recipesas standard functions...)
（Python 的 itertools 应该真正将所有配方作为标准功能......）
You could use the grouperfunction:
您可以使用该grouper功能：
from itertools import zip_longest
def grouper(n, iterable, fillvalue=None):
"grouper(3, 'ABCDEFG', 'x') --> ABC DEF Gxx"
args = [iter(iterable)] * n
return izip_longest(fillvalue=fillvalue, *args)
Then you can iterate the pixels by
然后你可以迭代像素
for r,g,b,a in grouper(4, raw_png_data):
....
Alternatively, you could use
或者，您可以使用
irpd = iter(raw_png_data)
for r,g,b,a in zip(irpd, irpd, irpd, irpd): # use itertools.izip in Python 2.x
....
Note that this will chop the last few bytes if the iterable's length is not a multiple of 4. OTOH, the grouperfunction uses izip_longest, so the extra bytes will be padded with None for that.
请注意，如果迭代的长度不是 4 的倍数，这将截断最后几个字节。 OTOH，该grouper函数使用izip_longest，因此额外的字节将用 None 填充。
回答by John La Rooy
from itertools import izip
for r,g,b,a in izip(*[iter(data)]*4):
...
