Python: Dictionary as instance variable

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“Least Astonishment” in Python: The Mutable Default Argument

I'm very confused about the behavior of dictionaries as class instance variables in Python 3. The way I understand it, instance variables in Python have per-instance storage, unlike class variables which are per-class (similar to what some other languages call "static").

And this seems to hold true, except when the instance variable is a dictionary created from a default parameter. For example:

class Foo:
    def __init__(self, values = dict()):
        self.values = values

f1 = Foo()
f1.values["hello"] = "world"

f2 = Foo()
print(f2.values)

This program outputs:

{'hello': 'world'}

Huh? Why does the instance f2 have the same dictionary instance as f1?

I get the expected behavior if I don't pass in an empty dictionary as a default parameter, and just assign self.values to an empty dictionary explicitly:

class Foo:
    def __init__(self):
        self.values = dict()

But I can't see why this should make any difference.

It might be that the default parameters are only evaluated once, when the class is loaded. That way you're merely assigning the same reference as a default parameter. — Platinum Azure, May 16 '11 at 17:28
Stack overflow has a nice "FAQ" function per tag. Here is the FAQ for the Python tag: http://stackoverflow.com/questions/tagged/python?sort=faq&pagesize=30 Your question is answered in question number 4. — Lennart Regebro, May 16 '11 at 18:30

score 18 · Accepted Answer · answered May 16 '11 at 17:28

18

This is a well known surprise in Python. The default parameters are evaluated when the function is defined, not when it is called. So your default parameter is a reference to a common dict. It has nothing to do with assigning it to class/instance variables.

If you want to use a default parameter, use None, and check it:

if values is None:
    self.values = {}
else:
    self.values = values

answered May 16 '11 at 17:28

Thomas K

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You'll get the same behavior with a list as well – jaydel May 16 '11 at 17:30
3

You get the same behaviour with *every single* mutable object. – May 16 '11 at 17:50

score 3 · Answer 2 · answered May 16 '11 at 17:30

3

Default values are only evaluated once. You want something like this:

 class Foo:
     def __init__(self, values = None):
         self.values = values or dict()

If you supply a values, that'll get used. If not, values is evaluated as FALSE by the or operator and instantiates a new dict.

answered May 16 '11 at 17:30

eduffy

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This failes if anything falsy is passed as values - e.g. an empty instance of a custom `dict` subclass. – May 16 '11 at 18:02

Python: Dictionary as instance variable

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