List as a member of a python class, why is its contents being shared across all instances of the class?

by

You don’t want the members declared inside the class, but just set in the __init__ method:

class Listener:
    def __init__(self, id):
        self.id = id
        self.artists = []

    def addArtist(self, artist, plays):
        print self.id # debugging...
        print "pre: ", self.artists
        self.artists.append(artist)
        print "post: ", self.artists

If you have a class like

class A:
  x=5

Then x is a member of the class and not a member of instances of that class. This can be confusing, since python lets you access class members through the instance:

>>> a=A()
>>> print a.x
5

But you can also access it through the class itself:

>>> print A.x
5

It would even appear that this works properly:

>>> a1=A()
>>> a2=A()
>>> a1.x=6
>>> print a1.x
6
>>> print a2.x
5

but what has actually happened is that you’ve put a new x into the a1 instance, which will be printed instead of the class member, which still has its original value:

>>> print A.x
5

You only start to see a difference when you have something that can be changed, like a list:

class A:
  l=[]

>>> a1=A()
>>> print a1.l
[]
>>> a2=A()
>>> print a2.l
[]
>>> a1.l.append(5)
>>> print a1.l
[5]
>>> print a2.l
[5]
>>> print A.l
[5]

More Related Contents:

  1. Is there a way to circumvent Python list.append() becoming progressively slower in a loop as the list grows?
  2. Accessing the index in ‘for’ loops
  3. Finding the index of an item in a list
  4. Get a list of numbers as input from the user
  5. python list by value not by reference [duplicate]
  6. Access multiple elements of list knowing their index
  7. Comparing two lists using the greater than or less than operator
  8. Appending the same string to a list of strings in Python
  9. Removing item from list causes the list to become NoneType
  10. What is the difference between class and instance variables?
  11. How to unzip a list of tuples into individual lists? [duplicate]
  12. Getting only element from a single-element list in Python?
  13. Removing a list of characters in string
  14. Attaching a decorator to all functions within a class
  15. Appending a dictionary to a list in a loop
  16. Generating all possible combinations of a list, “itertools.combinations” misses some results
  17. How to read an array of integers from single line of input in python3
  18. Splitting List That Contains Strings and Integers
  19. Replace special characters in a string in Python
  20. List with duplicated values and suffix
  21. How to group elements in python by n elements [duplicate]
  22. Unexpected IndexError while removing list items [duplicate]
  23. How do I reverse a list or loop over it backwards?
  24. “__init__” vs “__call__” in python class
  25. Ellipsis lists […] and concatenating a list to itself [duplicate]
  26. Creating sublists [duplicate]
  27. Can’t sort my list because it is NoneType? Simple Python [duplicate]
  28. How to sort a list of dictionaries by a value of the dictionary in Python?
  29. Why Python `Memory Error` with list `append()` lots of RAM left
  30. Store functions in list and call them later [duplicate]

Leave a Comment