Often you need to loop through every item of multiple lists and compare them. This can be done without a using a counter.
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a = ['a1', 'a2', 'a3'] b = ['b1', 'b2'] # will iterate 3 times, # the last iteration, b will be None print "Map:" for x, y in map(None, a, b): print x, y # will iterate 2 times, # the third value of a will not be used print "Zip:" for x, y in zip(a, b): print x, y # will iterate 6 times, # it will iterate over each b, for each a # producing a slightly different outpu print "List:" for x, y in [(x,y) for x in a for y in b]: print x, y
Using map with "None" as the first argument is discussed in the docs:
"If function is None, the identity function is assumed; if there are multiple list arguments, map() returns a list consisting of tuples containing the corresponding items from all lists (i.e. a kind of transpose operation). The list arguments may be any kind of sequence; the result is always a list."
Note: this returns None for sequences where there is no element. Output:<pre>a1 b1 a2 b2 a3 None</pre>
Zip allows you to iterate over the lists in a similar way, but only up to the number of elements of the smallest list.
Output:<pre>a1 b1 a2 b2</pre>
Python 2.0 introduced list comprehension which explains the rather strange syntax: <pre>[(x,y) for x in a for y in b]</pre> this iterates over the b list for every element in a. These are put into a tuple x, y. We then iterate through that tuple in the outermost for loop.
The result is quite different:<pre>a1 b1 a1 b2 a2 b1 a2 b2 a3 b1 a3 b2</pre>