Hello,
I have the following problem:
Why is the instance B updating and how do i prevent that?
class myclass(): def __init__(self, x=[]): self.x = x def append_to(self): self.x.append('foo') A=myclass() B=myclass() A.append_to() B.xThis returns ['foo'].
Thank you for your help
You are using a list as a default. Defaults are evaluated once and stored, not evaluated each time you call the method. Since lists are mutable, self.x becomes a pointer to that list, not a copy of that list. That makes every instance share the same self.x, unless x is passed explicitly. You generally solve this with a default of None:
class myclass(): def __init__(self, x=None): if x is None: self.x = [] else: self.x = x
Thank you for your solution. I wonder however if there is a more elegant way of achieving the same results. My code so far looks like this:
class my_data(): def __init__(self, mn=None, r=None, x=None, y=None): if x is None: self.x=[] else: self.x = x if y is None: self.y=[] else: self.y = y if mn is None: self.mn=[] else: self.mn = mn if r is None: self.r=[] else: self.r= r
Thank you in advance
You can take advantage of Python 'returning operand' feature:
>>> scale = None >>> scale = scale or None >>> print(scale) None >>> scale = 10 >>> scale = scale or None >>> scale 10
This is much more compact! Thanks!
class my_data(): def __init__(self, mn=None, r=None, x=None, y=None): self.x = x or [] self.y = y or [] self.mn = mn or [] self.r = r or []