[newbie] A question about lists and strings

[Mok-Kong Shen]

In an earlier question about lists, I was told about the issue of
creation of local names in a function. However, I still can't
understand why the program below outputs:

[999] sss
[999]

and not two identical lines of output. For both operators "+=" should
anyway work in similar manner in the function xx in my view.

Thanks for your help in advance.

M. K. Shen

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def xx(list,str):
   list+=[999]
   str+="sss"

lista=[]
stra=""
lista+=[999]
stra+="sss"
print(lista,stra)

listb=[]
strb=""
xx(listb,strb)
print(listb,strb)