Splitting A List Of Sequences Into Two Lists Efficiently
Possible Duplicate: A Transpose/Unzip Function in Python I have a list of sequences, each sequence has two items. I would like to turn this into two lists. catalog = [('abc', '1
Solution 1:
>>> catalog = [('abc', '123'), ('foo', '456'), ('bar', '789'), ('test', '1337')]
>>> names, vals = zip(*catalog)
>>> names
('abc', 'foo', 'bar', 'test')
>>> vals
('123', '456', '789', '1337')
The *catalog
syntax here is called Unpacking Argument Lists, and zip(*catalog)
translates into the call zip(catalog[0], catalog[1], catalog[2], ...)
.
The zip()
builtin function groups iterables by indices, so when you pass a bunch of two-element tuples as above, you get a two-element list of tuples where the first tuple contains the first element of each tuple from catalog
, and the second tuple contains the second element from each tuple from catalog
.
In a quick timeit test the zip()
version outperforms a looping approach when I tested with 1,000,000 pairs:
In [1]: catalog = [(i, i+1) for i in range(1000000)]
In [2]: def with_zip():
...: return zip(*catalog)
...:
In [3]: def without_zip():
...: names, vals = [], []
...: for name, val in catalog:
...: names.append(name)
...: vals.append(val)
...: return names, vals
...:
In [4]: %timeit with_zip()
1 loops, best of 3: 176 ms per loop
In [5]: %timeit without_zip()
1 loops, best of 3: 250 ms per loop
Solution 2:
Sure, use this:
lst = [('abc', '123'), ('foo', '456'), ('bar', '789'), ('test', '1337')]
tup1, tup2 = zip(*lst)
The above will return a pair of tuples with the elements. If you need lists, then use this:
lst = [('abc', '123'), ('foo', '456'), ('bar', '789'), ('test', '1337')]
lst1, lst2 = map(list, zip(*lst))
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