How should I declare default values for instance variables in Python?

Question

Should I give my class members default values like this:

class Foo:
    num = 1

or like this?

class Foo:
    def __init__(self):
        self.num = 1

In this question I discovered that in both cases,

bar = Foo()
bar.num += 1

is a well-defined operation.

I understand that the first method will give me a class variable while the second one will not. However, if I do not require a class variable, but only need to set a default value for my instance variables, are both methods equally good? Or one of them more 'pythonic' than the other?

One thing I've noticed is that in the Django tutorial, they use the second method to declare Models. Personally I think the second method is more elegant, but I'd like to know what the 'standard' way is.

Alec · Accepted Answer · 2019-04-01 01:52:56Z

186

Extending bp's answer, I wanted to show you what he meant by immutable types.

First, this is okay:

>>> class TestB():
...     def __init__(self, attr=1):
...         self.attr = attr
...     
>>> a = TestB()
>>> b = TestB()
>>> a.attr = 2
>>> a.attr
2
>>> b.attr
1

However, this only works for immutable (unchangable) types. If the default value was mutable (meaning it can be replaced), this would happen instead:

>>> class Test():
...     def __init__(self, attr=[]):
...         self.attr = attr
...     
>>> a = Test()
>>> b = Test()
>>> a.attr.append(1)
>>> a.attr
[1]
>>> b.attr
[1]
>>>

Note that both a and b have a shared attribute. This is often unwanted.

This is the Pythonic way of defining default values for instance variables, when the type is mutable:

>>> class TestC():
...     def __init__(self, attr=None):
...         if attr is None:
...             attr = []
...         self.attr = attr
...     
>>> a = TestC()
>>> b = TestC()
>>> a.attr.append(1)
>>> a.attr
[1]
>>> b.attr
[]

The reason my first snippet of code works is because, with immutable types, Python creates a new instance of it whenever you want one. If you needed to add 1 to 1, Python makes a new 2 for you, because the old 1 cannot be changed. The reason is mostly for hashing, I believe.

edited Apr 1, 2019 at 1:52

Alec

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answered Apr 21, 2010 at 9:02

Xavier Ho

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4 Comments

Bill Over a year ago

I recently came across this much simpler way of implementing default values for mutable attributes. Just write self.attr = attr or [] within the __init__ method. It achieves the same result (I think) and is still clear and readable.

Bill Over a year ago

Sorry folks. I did some more research on the suggestion in my above comment and apparently it's not quite the same and is not recommended.

ChrisG Over a year ago

Why does the class TestC have the empty parentheses? Is that a Python 2 legacy?

elano7 Over a year ago

And a nice one-liner self.attr = [] if attr is None else attr

talekeDskobeDa · Accepted Answer · 2020-04-07 22:18:19Z

82

The two snippets do different things, so it's not a matter of taste but a matter of what's the right behaviour in your context. Python documentation explains the difference, but here are some examples:

Exhibit A

class Foo:
  def __init__(self):
    self.num = 1

This binds num to the Foo instances. Change to this field is not propagated to other instances.

Thus:

>>> foo1 = Foo()
>>> foo2 = Foo()
>>> foo1.num = 2
>>> foo2.num
1

Exhibit B

class Bar:
  num = 1

This binds num to the Bar class. Changes are propagated!

>>> bar1 = Bar()
>>> bar2 = Bar()
>>> bar1.num = 2 #this creates an INSTANCE variable that HIDES the propagation
>>> bar2.num
1
>>> Bar.num = 3
>>> bar2.num
3
>>> bar1.num
2
>>> bar1.__class__.num
3

Actual answer

If I do not require a class variable, but only need to set a default value for my instance variables, are both methods equally good? Or one of them more 'pythonic' than the other?

The code in exhibit B is plain wrong for this: why would you want to bind a class attribute (default value on instance creation) to the single instance?

The code in exhibit A is okay.

If you want to give defaults for instance variables in your constructor I would however do this:

class Foo:
  def __init__(self, num = None):
    self.num = num if num is not None else 1

...or even:

class Foo:
  DEFAULT_NUM = 1
  def __init__(self, num = None):
    self.num = num if num is not None else DEFAULT_NUM

...or even: (preferrable, but if and only if you are dealing with immutable types!)

class Foo:
  def __init__(self, num = 1):
    self.num = num

This way you can do:

foo1 = Foo(4)
foo2 = Foo() #use default

edited Apr 7, 2020 at 22:18

talekeDskobeDa

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answered Apr 21, 2010 at 8:20

badp

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4 Comments

Eliethesaiyan Over a year ago

is the init method used in this example different from underscore___init__underscore for initialization @badp

r4bb1t Over a year ago

Shouldn't the first example be def __init__(self, num = None)

Hairy Over a year ago

Can't you just add the following and it be done? def __init__(self, num = None): self.num = num if num else DEFAULT_NUM

Bilbottom Over a year ago

@Hairy you can go a step further: def __init__(self, num = None): self.num = num or DEFAULT_NUM

Duncan · Accepted Answer · 2010-04-21 08:35:26Z

Using class members to give default values works very well just so long as you are careful only to do it with immutable values. If you try to do it with a list or a dict that would be pretty deadly. It also works where the instance attribute is a reference to a class just so long as the default value is None.

I've seen this technique used very successfully in repoze which is a framework that runs on top of Zope. The advantage here is not just that when your class is persisted to the database only the non-default attributes need to be saved, but also when you need to add a new field into the schema all the existing objects see the new field with its default value without any need to actually change the stored data.

I find it also works well in more general coding, but it's a style thing. Use whatever you are happiest with.

Johannes Gehrs · Accepted Answer · 2020-07-06 14:24:06Z

5

With dataclasses, a feature added in Python 3.7, there is now yet another (quite convenient) way to achieve setting default values on class instances. The decorator dataclass will automatically generate a few methods on your class, such as the constructor. As the documentation linked above notes, "[t]he member variables to use in these generated methods are defined using PEP 526 type annotations".

Considering OP's example, we could implement it like this:

from dataclasses import dataclass

@dataclass
class Foo:
    num: int = 0

When constructing an object of this class's type we could optionally overwrite the value.

print('Default val: {}'.format(Foo()))
# Default val: Foo(num=0)
print('Custom val: {}'.format(Foo(num=5)))
# Custom val: Foo(num=5)

edited Jul 6, 2020 at 14:24

answered Jul 6, 2020 at 14:17

Johannes Gehrs

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3 Comments

kva1966 Over a year ago

Should mark the num attrib as a dataclasses.field, otherwise the propagation behaviour described in @badp's (sorry, can't get a mention right!) Exhibit B still occurs. python from dataclasses import dataclass, field @dataclass class Foo: num: int = field(default=0)

Johannes Gehrs Over a year ago

Hi @kva1966 docs say that "The dataclass() decorator examines the class to find fields. A field is defined as a class variable that has a type annotation". Therefore my understanding is that the int type annotation is sufficient.

kva1966 Over a year ago

My apologies, you are right. I may have missed something in my earlier test. The type annotation is what makes the difference.

Max Shawabkeh · Accepted Answer · 2010-04-21 08:24:08Z

2

Using class members for default values of instance variables is not a good idea, and it's the first time I've seen this idea mentioned at all. It works in your example, but it may fail in a lot of cases. E.g., if the value is mutable, mutating it on an unmodified instance will alter the default:

>>> class c:
...     l = []
... 
>>> x = c()
>>> y = c()
>>> x.l
[]
>>> y.l
[]
>>> x.l.append(10)
>>> y.l
[10]
>>> c.l
[10]

answered Apr 21, 2010 at 8:24

Max Shawabkeh

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2 Comments

Phlip Over a year ago

except only the very first [] got cloned around there; x.l and y.l are both evil names and different immediate values linking to the same referent. (Language lawyer terms made up)

NeilG Over a year ago

This will not fail in a lot of cases. It can only fail if the value is mutable and it is mutated by users in different ways. As far as I can tell the only case that fails is the classic mutable default scenario. No other cases explained here and no other cases I can think of.

Realistic · Accepted Answer · 2015-05-28 19:07:50Z

2

You can also declare class variables as None which will prevent propagation. This is useful when you need a well defined class and want to prevent AttributeErrors. For example:

>>> class TestClass(object):
...     t = None
... 
>>> test = TestClass()
>>> test.t
>>> test2 = TestClass()
>>> test.t = 'test'
>>> test.t
'test'
>>> test2.t
>>>

Also if you need defaults:

>>> class TestClassDefaults(object):
...    t = None
...    def __init__(self, t=None):
...       self.t = t
... 
>>> test = TestClassDefaults()
>>> test.t
>>> test2 = TestClassDefaults([])
>>> test2.t
[]
>>> test.t
>>>

Of course still follow the info in the other answers about using mutable vs immutable types as the default in __init__.

answered May 28, 2015 at 19:07

Realistic

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