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+--- Thread: strange slicing with [:] (/thread-21658.html)
strange slicing with [:] - Skaperen - Oct-08-2019
as i understand it, slicing with [:] yields a new copy of the sequence, or if assigned to, replace the entire sequence (for a mutable sequence). but i have seen an odd effect when using [:] for both the source and target. the effect is that nothing happens. if i do
a=[1,2,3] b=a a=a[:] b.append(4) print(a)i would not see the 4 append to b. but if i do
a=[1,2,3] b=a a[:]=a[:] b.append(4) print(a)i do get the 4 as if a retained a reference to the same list.
Output:lt2a/forums /home/forums 55> cat t1
a=[1,2,3]
b=a
b=a[:]
b.append(4)
print(a)
lt2a/forums /home/forums 56> cat t2
a=[1,2,3]
b=a
a[:]=a[:]
b.append(4)
print(a)
lt2a/forums /home/forums 57> python3 t1
[1, 2, 3]
lt2a/forums /home/forums 58> python3 t2
[1, 2, 3, 4]
lt2a/forums /home/forums 59>a[:]=a[:]?
RE: strange slicing with [:] - Yoriz - Oct-08-2019
Assign all of a, to all of a
RE: strange slicing with [:] - Skaperen - Oct-09-2019
but it's not a new list?
RE: strange slicing with [:] - ichabod801 - Oct-09-2019
No. In assigning to a[:], you are not assigning to a, you are assigning to a slice of a. So the reference which is a is still pointing to the same place, which is the same place b is pointing to.
RE: strange slicing with [:] - Skaperen - Oct-09-2019
oh, i see now, says the blind one. it's putting new contents, which are identical, into the same old place.