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I'm generating a custom combination of sub lists without using itertools.

Here's what I've come up with:

input =  [[1, 2], [2, 3], [4, 3]]
output = [[1, 2, 2, 3], [1, 2, 4, 3], [2, 3, 4, 3]]

def getList():
    a = [[1, 2], [2, 3], [4, 3]]
    return(a)

c=[]                        # output list
l = len(getList())
for i in range(l):
    for j in range(i+1,l):
        a=getList()
        a[i].extend(a[j])
        c.append(a[i])

As the extend() updates the input list, I defined a function to redefine the input. Is it a recommended practice? What could be improved here?

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    \$\begingroup\$ Why don't you want to use itertools? Because using itertools is recommended practice... \$\endgroup\$ Commented Nov 20, 2017 at 9:52
  • \$\begingroup\$ @Peilonrayz no reason as such, was just trying how to achieve without it. \$\endgroup\$ Commented Nov 20, 2017 at 9:56
  • \$\begingroup\$ @VanPeer You have a hardcoded input() in get_List(), what would happen if the input changes? \$\endgroup\$ Commented Nov 20, 2017 at 10:55
  • \$\begingroup\$ These are not all combinations of a list \$\endgroup\$ Commented Nov 20, 2017 at 11:00
  • \$\begingroup\$ @Ludisposed thanks! assuming input doesn't change. i needed only the combinations specified in the output. i didn't mention all combinations, did i? anyways, updated. \$\endgroup\$ Commented Nov 20, 2017 at 11:04

2 Answers 2

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  1. Use a function.

  2. Don't mutate data you don't want to be mutated.

    Rather than using list.extend, use list.__add__. However don't use list.__iadd__, as that has the same problem as list.extend.

  3. Learn how to copy Python objects. You can use list([1, 2, 3]) to make a copy of the list. You can also use the copy library too.

  4. Don't waste memory, just yield. If you need a list use list(product(...))
def product(input):
    for i in range(len(input)):
        for j in range(i+1, len(input)):
            yield input[i] + input[j]
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    \$\begingroup\$ Yield is better then mine, memorywise, but don't use input as a variable name. :) \$\endgroup\$ Commented Nov 20, 2017 at 11:32
  • \$\begingroup\$ @Ludisposed You could always use a generator, rather than list, comprehension. And shadowing built-ins is fine IMO. It's not like the OP's going to use the original input in the function. \$\endgroup\$ Commented Nov 20, 2017 at 11:34
  • \$\begingroup\$ I just try to avoid shadowing built ins, at all costs! Maybe just a bit paranoid :$ \$\endgroup\$ Commented Nov 20, 2017 at 11:43
  • \$\begingroup\$ @Ludisposed There are good arguments for and good against. I don't think you'll confuse input as builtins.input in a small 4 lines of code function. If however it were a 200 line function I'd whole heartedly agree. I do admit it's not that great a variable name, but we don't have context to improve on that. \$\endgroup\$ Commented Nov 20, 2017 at 11:47
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What could be improved here?

@thnx Peilonrayz, a generator expression will work better on large inputs

  1. Use a nested generator expression.
  2. DON'T hardcode variables, this will make your code very static.
  3. Make a function, to accept different variables.
def custom_combo(L):
    return (L[i] + L[j] for i in range(len(L)) for j in range(i+1, len(L)))

if __name__ == '__main__':
    a = [[1, 2], [2, 3], [4, 3]]
    print(list(custom_combo(a)))
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