python Python双迭代
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Python double iteration
提问by Yuval Adam
What is the pythonic way of iterating simultaneously over two lists?
在两个列表上同时迭代的pythonic方法是什么？
Suppose I want to compare two files line by line (compare each ith line in one file with the ith line of the other file), I would want to do something like this:
假设我想逐行比较两个文件（将i一个文件中的每一行与另一个文件的i第 行进行比较），我想做这样的事情：
file1 = csv.reader(open(filename1),...)
file2 = csv.reader(open(filename2),...)
for line1 in file1 and line2 in file2: #pseudo-code!
if line1 != line2:
print "files are not identical"
break
What is the pythonic way of achieving this?
实现这一目标的pythonic方法是什么？
Edit:I am not using a file handler but rather a CSV reader (csv.reader(open(file),...)), and zip()doesn't seem to work with it...
编辑：我没有使用文件处理程序，而是使用 CSV 阅读器 ( csv.reader(open(file),...))，并且zip()似乎无法使用它...
Final edit:like @Alex M. suggested, zip()loads the files to memory on first iteration, so on big files this is an issue. On Python 2, using itertoolssolves the issue.
最终编辑：就像@Alex M. 建议的那样，zip()在第一次迭代时将文件加载到内存中，所以在大文件上这是一个问题。在 Python 2 上，使用itertools解决了这个问题。
采纳答案by Alex Martelli
In Python 2, you should import itertoolsand use its izip:
在 Python 2 中，你应该导入itertools并使用它的izip：
with open(file1) as f1:
with open(file2) as f2:
for line1, line2 in itertools.izip(f1, f2):
if line1 != line2:
print 'files are different'
break
with the built-in zip, both files will be entirely read into memory at once at the start of the loop, which may not be what you want. In Python 3, the built-in zipworks like itertools.izipdoes in Python 2 -- incrementally.
使用内置的zip，这两个文件将在循环开始时立即完全读入内存，这可能不是您想要的。在 Python 3 中，内置的zip工作方式与itertools.izipPython 2 中的一样——增量式。
回答by JAL
I vote for using zip. The manualsuggests "To loop over two or more sequences at the same time, the entries can be paired with the zip() function"
我投票赞成使用zip. 该手册建议“要同时遍历两个或多个序列，这些条目可以与拉链（）函数配对”
For example,
例如，
list_one = ['nachos', 'sandwich', 'name']
list_two = ['nachos', 'sandwich', 'the game']
for one, two in zip(list_one, list_two):
if one != two:
print "Difference found"
回答by kennytm
In lockstep (for Python ≥3):
步调一致（对于 Python ≥3）：
for line1, line2 in zip(file1, file2):
# etc.
As a "2D array":
作为“二维数组”：
for line1 in file1:
for line2 in file2:
# etc.
# you may need to rewind file2 to the beginning.
