'str' object does not support item assignment

[jimbo1qaz]

spots[y][x]=mark fails with a "'str' object does not support item assignment" 
error,even though:
>>> a=[["a"]]
>>> a[0][0]="b"
and:
>>> a=[["a"]]
>>> a[0][0]=1000000
both work.
Spots is a nested list created as a copy of another list.
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