The Python zip built-in function makes it easy to iterate through multiple iterables all at once. It aggregates elements from each iterable to create an iterator of tuples. The i-th tuple contains the i-th element of each of the constituent iterables. For example:
>>> x = [1, 2, 3] >>> y = ['a', 'b', 'c'] >>> zipped = zip(x, y) >>> for x, y in zipped: ... print(x, y) ... 1 a 2 b 3 c Getting the zipped elements as a list
The zip function returns a -- wait for it -- zip object. You can confirm this by printing the zipped variable out in the console. You should see something like this:
>>> zipped = zip(x, y) <zip object at ox7f45ddba2c08> To convert it into a list you can manipulate, you use the very aptly named list function like so:
>>> list(zipped) [(1, 'a'), (2, 'b'), (3, 'c')] Unzipping a zip object
If for any reason you need to reverse a zip operation, you can do so with the zip function, in conjunction with the destructuring operator (*) to return the original itertables as tuples:
>>> x = [1, 2, 3] >>> y = ['a', 'b', 'c'] >>> x2, y2 = zip(*zip(x, y)) >>> x == list(x2) and y == list(y2) True Zipping unequal list lengths
Zipping unequal list lengths causes the elements of the longer lists to be dropped:
>>> x = [1, 2, 3] >>> y = ['a', 'b', 'c', 'd'] >>> list(zip(x, y)) [(1, 'a'), (2, 'b'), (3, 'c')] Fortunately, we can use the itertools.zip_longest function to solve this. zip_longest takes iterables as paramaters like zip, but with an additional fillvalue parameter which substitutes for missing values. It defaults to None. Here's how you use it:
>>> from itertools import zip_longest >>> x = [1, 2, 3] >>> y = ['a', 'b', 'c', 'd'] >>> list(zip_longest(x, y)) [(1, 'a'), (2, 'b'), (3, 'c'), (None, 'd') And with a fillvalue:
>>> list(zip_longest(x, y, fillvalue=-1)) [(1, 'a'), (2, 'b'), (3, 'c'), (-1, 'd')] As you can see, with Python, it's very simple to iterate over multiple lists at once.
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