python - What does [:] do? -

return self.var[:] 

what return?

python permits "slice" various container types; shorthand notation taking subcollection of ordered collection. instance, if have list

foo = [1,2,3,4,5] 

and want second, third, , fourth elements, can do:

foo[1:4] 

if omit 1 of numbers in slice, defaults start of list. instance

foo[1:] == [2,3,4,5] foo[:4] == [1,2,3,4] 

naturally, if omit both numbers in slice entire list back! however, copy of list instead of original; in fact, standard notation copying list. note difference:

>>> = [1,2,3,4] >>> b = >>> b.append(5) >>> [1, 2, 3, 4, 5] >>> >>> = [1,2,3,4] >>> b = a[:] >>> b.append(5) >>> [1, 2, 3, 4] 

this occurs because b = a tells b point same object a, appending b same appending a. copying list a avoids this. notice runs 1 level of indirection deep -- if a contained list, say, , appended list in b, still change a.

by way, there optional third argument slice, step parameter -- lets move through list in jumps of greater 1. write range(100)[0::2] numbers 100.