TypeError: can only concatenate list (not "tuple") to list ((Python))

Question

When running the following code in a little script, I get the following error:

Traceback (most recent call last):
  File "/Users/PeterVanvoorden/Documents/GroepT/Thesis/f_python_standalone/python_files/Working_Files/testcase.py", line 9, in <module>
    permutations_01.determin_all_permutations()
  File "/Users/PeterVanvoorden/Documents/GroepT/Thesis/f_python_standalone/python_files/Working_Files/permutations_01.py", line 8, in determin_all_permutations
    config.permutations = [[1] + x for x in config.permutations]
TypeError: can only concatenate list (not "tuple") to list

Code:

import itertools
import config

def determin_all_permutations():
    L = list(xrange(config.number))
    List = [x+2 for x in L]
    config.permutations = list(itertools.permutations(List,config.number))
    config.permutations = [[1] + x for x in config.permutations]


def determin_lowest_price_permutation():
    length = len(L)
    a = 1
    while a <= length:
        point_a = L[a]
        point_b = L[a+1]

# Array with data cost connecting properties will exist of:
# * first column = ID first property [0]
# * second column = ID second property [1]
# * third column = distance between two properties [2]
# * forth column = estimated cost [3]

        position  = 0
        TF = False

        if TF == False:
            if point_a == config.properties_array[position][0] and point_b == config.properties_array[position][1] and TF == False:
                config.current_price = config.current_price + config.properties_array[position][3]
                config.current_path = config.current_path + [config.properties_array[position][2]]
                TF = True

            else:
                position += 1

        else:
            position = 0
            TF = False

I don't get why this error is occurring. When I test line 8

config.permutations = [[1] + x for x in config.permutations]

in a normal situation by making a list in Shell for ex:

List = [1,1,1],[1,1,1]
([1,1,1],[1,1,1])
List = [[0] + x for x in List]
List
([0,1,1,1],[0,1,1,1])

it works, but when using the exact same code in a method, I get the error about adding a tuple... Isn't [1] a list?

Can somebody help me?

Thanks!

Please provide the driver code you are using to produce the error. You have currently provided only function definitions so your example is not reproducible as is. — merlin2011
– merlin2011, Commented Apr 3, 2014 at 0:07

the Tin Man · Accepted Answer · 2015-01-08 23:10:30Z

5

In what you executed on the shell, List is made up of lists [1,1,1] and [1,1,1], not tuples. Hence doing List = [[0] + x for x in List] works without errors.

Where as in your code, list(itertools.permutations(List,config.number)) returns a list of tuples, like:

[(2, 4, 8, 5, 11, 10, 9, 3, 7, 6), (2, 4, 8, 5, 11, 10, 9, 6, 3, 7),...]

which explains the error.

Doing this:

config.permutations = [[1] + list(x) for x in config.permutations]

fixes the issue.

edited Jan 8, 2015 at 23:10

the Tin Man

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answered Apr 3, 2014 at 0:12

shaktimaan

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