Python double iteration
python
Solution
In Python 2, you should import itertools and use its 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 `zip` works like `itertools.izip` does in Python 2 -- incrementally.
Problem
What is the pythonic way of iterating simultaneously over two lists? Suppose I want to compare two files line by line (compare each `i`th line in one file with the `i`th line of the other file), I would want to do something like this: ``` 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? 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... 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 `itertools` solves the issue.