What is the difference between x[:]=y and x=y[:]?

Peter Otten __peter__ at web.de
Wed Apr 12 04:41:04 EDT 2017


jfong at ms4.hinet.net wrote:

Assuming both x and y are lists

x[:] = y 

replaces the items in x with the items in y while


x = y[:]

makes a copy of y and binds that to the name x. In both cases x and y remain 
different lists, but in only in the second case x is rebound. This becomes 
relevant when initially there are other names bound to x. Compare:


>>> z = x = [1, 2]
>>> y = [10, 20, 30]
>>> x[:] = y # replace the values, z affected
>>> z
[10, 20, 30]


>>> z = x = [1, 2]
>>> y = [10, 20, 30]
>>> x = y[:] # rebind. x and z are now different lists
>>> z
[1, 2]




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