Splitting a list of sequences into two lists efficiently
python, python-3.x
Solution
>>> 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
Problem
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', '123'), ('foo', '456'), ('bar', '789'), ('test', '1337')] ``` Right now I'm just doing this: ``` names = [] vals = [] for product in catalog: names.append(product[0]) vals.append(product[1]) print (names) print (vals) ``` Which outputs two lists, and works just fine: ``` ['abc', 'foo', 'bar', 'test'] ['123', '456', '789', '1337'] ``` Is there a neater, more 'pythonic' way of doing this? Or should I stick with what I have? Any corrections or feedback on programming style is welcome, I'm new and trying to learn best practices.