Question (and answer) about deepcopy in Sage
 
  
 
 
     
 
 
 
 
 
 
 
    
        
        asked 6 years ago  
 
 al gravatar image  
 
 
 
 
    
        
        updated 6 years ago  
 
 
 
 
 
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/2...
 
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.
 
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For instance, typing
If we define `f` by

    def f(x, y, z):
        return x * y * z

then `f(2, 3, 5)` returns `30` but `f(2*3*5)` gives:

    TypeError: f() takes exactly 3 arguments (1 given)
produces:
If we define f by
def f(x, y, z):
    return x * y * z
then f(2, 3, 5) returns 30 but f(2*3*5) gives:
TypeError: f() takes exactly 3 arguments (1 given)
Please edit your question to do that.
slelievre gravatar imageslelievre (
    
        6 years ago
    )
Also, please separate the answer from the question. The question should be a question, and the answer should be posted as an answer, for clarity.
slelievre gravatar imageslelievre (
    
        6 years ago
    )
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