Question

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How do you split a list into evenly sized chunks in Python?

I have a list such as:

L = [1,2,3,4,5,6,7,8]

Let’s say I want to divide this L into 3 parts into something like:

result = [[1,2,3],[4,5,6],[7,8]]

How can I do this without using higher programming methods (ONLY USING SIMPLE PYTHON METHOD)?

Answer 1

You can try something like:

In [13]: l = [1,2,3,4,5,6,7,8]

In [14]: result = [l[i:i+3] for i in xrange(0, len(l), 3)]

In [15]: result
Out[15]: [[1, 2, 3], [4, 5, 6], [7, 8]]

This pulls out slices from the list that are of length n, where n is the number that you add to i and is also the step value in your range. As @Eric pointed out, this breaks the list into chunks of three, but not three chunks. In order to get it into three chunks, you can do something like:

In [21]: l = [1,2,3,4,5,6,7,8]

In [22]: chunk = int(round(len(l)/3.0))

In [23]: result = [l[i:i+chunk] for i in range(0,len(l),chunk)]

In [24]: result
Out[24]: [[1, 2, 3], [4, 5, 6], [7, 8]]

In [25]: l = [1,2,3,4,5]

In [26]: chunk = int(round(len(l)/3.0))

In [27]: result = [l[i:i+chunk] for i in range(0,len(l),chunk)]

In [28]: result
Out[28]: [[1, 2], [3, 4], [5]]

As it sounds like you have certain constraints, this could be written in a for loop as well (although this actually has more function calls than the one above 🙂 ):

In [17]: result = []

In [18]: for i in xrange(0, len(l), 3):
   ....:     result.append(l[i:i+3])
   ....:     
   ....:     

In [19]: result
Out[19]: [[1, 2, 3], [4, 5, 6], [7, 8]]