Python list.append output values differ from list.extend -

saw question on site piece of python code driving nuts. small, straightforward-looking piece of code, looked @ it, figured out trying do, ran on local system, , discovered why driving original questioner nuts. hoping here can me understand what's going on.

the code seems straightforward "ask user 3 values (x,y,z) , sum (n); iterate values find tuples sum n, , add tuples list." solution. outputs is, instead of tuples sum n, list of tuples count of equal count of tuples sum n, contents of "[x,y,z]". trying wrap head around this, changed append call extend call (knowing un-list added tuples), see if behavior changed @ all. expected same output, "x,y,z,x,y,z..." repeatedly, instead of "[x,y,z],[x,y,z]" repeatedly, because read , understand python documentation, that's difference between append , extend on lists. got instead when used extend correct values of tuples summed n, broken out of tuple form extend.

here's problem code:

my = []  x = 3 y = 5 z = 7 n = 11  part = [0,0,0] in range(x+1):     part[0] =     j in range(y+1):         part[1] = j         k in range(z+1):             part[2] = k             if sum(part) == n:                 my.append(part) print(my) 

and output:

[[3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7], [3, 5, 7]] 

and here's extend output:

[0, 4, 7, 0, 5, 6, 1, 3, 7, 1, 4, 6, 1, 5, 5, 2, 2, 7, 2, 3, 6, 2, 4, 5, 2, 5, 4, 3, 1, 7, 3, 2, 6, 3, 3, 5, 3, 4, 4, 3, 5, 3] 

and extend code:

my = []  x = 3 y = 5 z = 7 n = 11  part = [0,0,0] in range(x+1):     part[0] =     j in range(y+1):         part[1] = j         k in range(z+1):             part[2] = k             if sum(part) == n:                 my.extend(part) print(my) 

any light shed on appreciated. i've dug around while on google , several q&a sites, , things found regarding python append/extend deltas things don't seem have relevance issue.

{edit: environment detail}

also, ran in both python 2.7.10 , python 3.4.3 (cygwin, under windows 10 home) same results.

extend adds items parameter list list object making call. more like, dump objects 1 list without emptying former.

append on other hand, appends; nothing more. therefore, appending list object list existing reference appended list damage - in case. after list has been appended, part still holds reference list (since you're modifying in place), you're modifying , (re-)appending same list object every time.

you can prevent either building new list @ start of each parent iteration of append case.

or appending copy of part list:

my.append(part[:])  my.append(list(part)) my.append(part.copy())  # python 3 

this append list has no other existing reference outside new parent list.