This was driving me crazy but I just found the answer, slightly different from what I found in a link below. I will explain. I hope it may be useful to other beginners!
If you have a list of numbers and want to change one, but save the old list, no problem; you can use = as follows:
T=[1,2,3]
S=T
S[0]=5
now you find that S= [5,2,3] while T= [1,2,3]
If you do this with a list of lists, changing T changes S also, as they are identified. The way to solve this is with the copy command, either
S=T{:}
or
S=copy(T).
However, if T is a list of lists of lists, which is very easy to come across, you are in trouble. There is a Python command called "deepcopy" which deals with this.
See https://stackoverflow.com/questions/28684154/python-copy-a-list-of-lists
The point I just discovered is that to use this instead of e.g. S=copy.deepcopy(T) as suggested in the link -- this gave me an error message!!! --maybe it's ok in Pythin?? what works in SAGEMATH is simply S=deepcopy(T).
example:
T=[] S=[]
T=[[1],[2],[3]]
S=T[:]
(or equivalently S=copy(T) )
then if you do
S[0][0]=5
now both S and T are [[5],[2],[3]].
if instead you do
S= deepcopy(T)
as suggested by the links (slight modification) then this works!!! The explanation for why is in the links.
