VPfB VPfB - 2 months ago 14
Python Question

Can I join lists with sum()?

Is it pythonic to use

sum()
for list concatenation?

>>> sum(([n]*n for n in range(1,5)),[])
[1, 2, 2, 3, 3, 3, 4, 4, 4, 4]

Answer

No it's not, Actually it's shlemiel the painter algorithm. Because each time it wants to concatenate a new list it has to traverse the whole list from beginning. (For more info read this article by Joel: http://www.joelonsoftware.com/articles/fog0000000319.html)

The most pythonic way is using a list comprehension:

In [28]: [t for n in range(1,5) for t in [n]*n ]
Out[28]: [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]

Or itertools.chain:

In [29]: from itertools import chain

In [32]: list(chain.from_iterable([n]*n for n in range(1,5)))
Out[32]: [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]

Or as a pute generator based approach you can use repeat instead of multiplying the list:

In [33]: from itertools import chain, repeat

# In python2.X use xrange instead of range
In [35]: list(chain.from_iterable(repeat(n, n) for n in range(1,5)))
Out[35]: [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]

Or if you are interest to numpy, or want a super fast approach here is one:

In [46]: import numpy as np
In [46]: np.repeat(np.arange(1, 5), np.arange(1, 5))
Out[46]: array([1, 2, 2, 3, 3, 3, 4, 4, 4, 4])
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