Class member persisting through other classes [duplicate]

Question

This question already has answers here:

Python Class Members Initialization (5 answers)

Closed 12 years ago.

I'm getting some strange behavior with Python 2.7.2.

If I have dictionary of classes, any lists inside those classes remain with the same value through all the class instances in the containing dictionary.

This will explain what I mean:

>>> class FooBar():
 somelist = []
>>> someFooBars = {}
>>> someFooBars["key1"] = FooBar()
>>> someFooBars["key2"] = FooBar()
>>> someFooBars["key3"] = FooBar()
>>> someFooBars["key1"].somelist.append("Hello")
>>> someFooBars["key1"].somelist
['Hello']
>>> someFooBars["key2"].somelist
['Hello']
>>> someFooBars["key1"].somelist.append("World!")
>>> someFooBars["key3"].somelist
['Hello', 'World!']

As you can see, I have filled the dictionary with three instances of FooBar, keyed with strings, but once I add objects to somelist, the objects are also in the other FooBars.

This isn't what I expect to happen (I expect them to be separate) but there is obviously a reason - please explain what this reason is, why this happens, and how I would fix it. Thanks!

• mgilson: Or this one: stackoverflow.com/questions/15489567/…
• toficofi: @PavelAnossov: You're correct, and I am sorry for making a duplicate - I did search beforehand but I obviously didn't use the correct terminology.
• eyquem: You will find precise explanation in this answer of me: (stackoverflow.com/a/15467310/551449)

Answer 1

What you have now is some kind of "static" member. To achieve "instance" member behavior, add __init__ method (serves as "constructor"):

class FooBar():
 def __init__(self):
 self.somelist = []

Answer 2

somelist is a class variable, so it's identical for every instance.

• toficofi: Thanks for further clarifying this. My previous experience with C++/Java etc tripped me up on this.