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List of objects with sublist - medatib531 - Mar-01-2020
Suppose I have the following code
newList = []
for i in range(10):
newList.append([])
for i in range(len(newList)):
newList[i].append(1)
>>> newList[0]
[1]If however instead of having a list of lists, I have a list of objects containing a list attribute, the output is not the one I would expect:class testobj:
def __init__(tst,lst = []):
tst.lst = lst
newList = []
for i in range(10):
newList.append(testobj())
for i in range(len(newList)):
newList[i].lst.append(1)
>>> newList[0].lst
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1]Can someone explain why? How would I make the list of objects behave correctly?Thanks
RE: List of objects with sublist - buran - Mar-01-2020
you are using mutable default argument in the __init__
Read https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments
RE: List of objects with sublist - buran - Mar-01-2020
as a side note - don't use
for i in range(len(newList)):, read https://python-forum.io/Thread-Basic-Never-use-for-i-in-range-len-sequence
RE: List of objects with sublist - medatib531 - Mar-01-2020
Ok thanks, but what's the syntax to do this in the constructor of the object?
EDT:
got it
class testobj:
def __init__(tst,lst = None):
tst.lst = []
RE: List of objects with sublist - buran - Mar-01-2020
if you do it like this, then it doesn't make sense to have lst argument at all - you can remove it
class testobj:
def __init__(tst):
tst.lst = []by the way, convention is to use self. It's OK to use something different like tst but it make it more difficult for other developers to read