Why do the examples below behave differently?
Example 1: foo seems to behave like a class variable that is specific for various objects
class A:
foo = 1
a, b = A(), A()
a.foo = 5
print b.foo
----------------
Output: 1
Example 2: foo seems to behave like a static class variable that is the same for all object. Perhaps the behavior has something to do with lists working as pointers.
class A:
foo = []
a, b = A(), A()
a.foo.append(5)
print b.foo
----------------
Output: [5]
Example 3: Doesn't work
class A:
self.foo = []
a, b = A(), A()
a.foo.append(5)
print b.foo
----------------
Output: Error
The first two examples are both class attributes. The reason they seem different is because you're not doing the same thing in both cases: you're assigning a new value in the first case and modifying the existing value in the second case.
Notice that you are not doing the same thing in the first two examples. In the first example you do a.foo = 5, assigning a new value. In the second example, if you did the analogous thing, assigning, a.foo = [5], you would see the same kind of result as in the first example. But instead you altered the existing list with a.foo.append(5), so the behavior is different. a.foo = 5 changes only the variable (i.e., what value it points to); a.foo.append(5) changes the value itself.
(Notice that there is no way to do the equivalent of the second example in the first example. That is, there's nothing like a.foo.add(1) to add 1 to 5. That's because integers are not mutable but lists are. But what matters is not that lists "are" mutable, but that you mutated one. In other words, it doesn't matter what you can do with a list, it matters what you actually do in the specific code.)
Also, notice that although the foo you defined in the class definition is a class attribute, when you do a.foo = 5, you are creating a new attribute on the instance. It happens to have the same name as the class attribute, but it doesn't change the value of the class attribute, which is what b.foo still sees.
The last example doesn't work because, just like in the first two examples, code inside the class block is at the class scope. There is no self because there are no instances yet at the time the class is defined.
There are many, many other questions about this on StackOverflow and I urge you to search and read a bunch of them to gain a fuller understanding of how this works.
This doesn't work:
class A:
self.foo = []
Which raises an error.
NameError: name 'self' is not defined
Because self is not a keyword in Python, it's just a variable name commonly assigned to the instance of the class that is passed to a method of the class when the class is called.
Here's an example:
class A(object):
def __init__(self):
self.foo = []
a, b = A(), A()
a.foo.append(5)
print(b.foo)
Then returns:
[]
When each one is initialized, they each get their own list which can be accessed by the attribute foo, and when one is modified, the other, being a separate list stored at a different place in memory, is not affected.
The difference has not to do with mutability/immutability, but what operations are performed.
In example 1, the class has an attribute foo. After object creation, you give the object another attribute foo which shadows the former one. So the class attribute acts as a kind of "default" or "fallback".
In example 2, you have one object which you perform an operation on (which, admittedly, only works on mutable objects). So the object referred to by A.foo, which can be accessed as well via a.foo and b.foo due to the lack of an instance attribute with the same name, gets added a 5.
Example 3 doesn't work because self doesn't exist where you use it.
Note that example 1 would as well work with mutable objects, such as lists:
class A:
foo = []
a, b = A(), A()
a.foo = []
a.foo.append(5)
b.foo.append(10)
print a.foo # [5]
print b.foo # [10]
print A.foo # [10]
Here a.foo gets a new, empty list. b.foo, lacking an instance attribute, continues to refer to the class attribute. So we have two empty lists which are independent of each other, as we see when .append()ing.
Related
Is there any meaningful distinction between:
class A(object):
foo = 5 # some default value
vs.
class B(object):
def __init__(self, foo=5):
self.foo = foo
If you're creating a lot of instances, is there any difference in performance or space requirements for the two styles? When you read the code, do you consider the meaning of the two styles to be significantly different?
There is a significant semantic difference (beyond performance considerations):
when the attribute is defined on the instance (which is what we usually do), there can be multiple objects referred to. Each gets a totally separate version of that attribute.
when the attribute is defined on the class, there is only one underlying object referred to, so if operations on different instances of that class both attempt to set/(append/extend/insert/etc.) the attribute, then:
if the attribute is a builtin type (like int, float, boolean, string), operations on one object will overwrite (clobber) the value
if the attribute is a mutable type (like a list or a dict), we will get unwanted leakage.
For example:
>>> class A: foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[5]
>>> class A:
... def __init__(self): self.foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[]
The difference is that the attribute on the class is shared by all instances. The attribute on an instance is unique to that instance.
If coming from C++, attributes on the class are more like static member variables.
Here is a very good post, and summary it as below.
class Bar(object):
## No need for dot syntax
class_var = 1
def __init__(self, i_var):
self.i_var = i_var
## Need dot syntax as we've left scope of class namespace
Bar.class_var
## 1
foo = MyClass(2)
## Finds i_var in foo's instance namespace
foo.i_var
## 2
## Doesn't find class_var in instance namespace…
## So look's in class namespace (Bar.__dict__)
foo.class_var
## 1
And in visual form
Class attribute assignment
If a class attribute is set by accessing the class, it will override the value for all instances
foo = Bar(2)
foo.class_var
## 1
Bar.class_var = 2
foo.class_var
## 2
If a class variable is set by accessing an instance, it will override the value only for that instance. This essentially overrides the class variable and turns it into an instance variable available, intuitively, only for that instance.
foo = Bar(2)
foo.class_var
## 1
foo.class_var = 2
foo.class_var
## 2
Bar.class_var
## 1
When would you use class attribute?
Storing constants. As class attributes can be accessed as attributes of the class itself, it’s often nice to use them for storing Class-wide, Class-specific constants
class Circle(object):
pi = 3.14159
def __init__(self, radius):
self.radius = radius
def area(self):
return Circle.pi * self.radius * self.radius
Circle.pi
## 3.14159
c = Circle(10)
c.pi
## 3.14159
c.area()
## 314.159
Defining default values. As a trivial example, we might create a bounded list (i.e., a list that can only hold a certain number of elements or fewer) and choose to have a default cap of 10 items
class MyClass(object):
limit = 10
def __init__(self):
self.data = []
def item(self, i):
return self.data[i]
def add(self, e):
if len(self.data) >= self.limit:
raise Exception("Too many elements")
self.data.append(e)
MyClass.limit
## 10
Since people in the comments here and in two other questions marked as dups all appear to be confused about this in the same way, I think it's worth adding an additional answer on top of Alex Coventry's.
The fact that Alex is assigning a value of a mutable type, like a list, has nothing to do with whether things are shared or not. We can see this with the id function or the is operator:
>>> class A: foo = object()
>>> a, b = A(), A()
>>> a.foo is b.foo
True
>>> class A:
... def __init__(self): self.foo = object()
>>> a, b = A(), A()
>>> a.foo is b.foo
False
(If you're wondering why I used object() instead of, say, 5, that's to avoid running into two whole other issues which I don't want to get into here; for two different reasons, entirely separately-created 5s can end up being the same instance of the number 5. But entirely separately-created object()s cannot.)
So, why is it that a.foo.append(5) in Alex's example affects b.foo, but a.foo = 5 in my example doesn't? Well, try a.foo = 5 in Alex's example, and notice that it doesn't affect b.foo there either.
a.foo = 5 is just making a.foo into a name for 5. That doesn't affect b.foo, or any other name for the old value that a.foo used to refer to.* It's a little tricky that we're creating an instance attribute that hides a class attribute,** but once you get that, nothing complicated is happening here.
Hopefully it's now obvious why Alex used a list: the fact that you can mutate a list means it's easier to show that two variables name the same list, and also means it's more important in real-life code to know whether you have two lists or two names for the same list.
* The confusion for people coming from a language like C++ is that in Python, values aren't stored in variables. Values live off in value-land, on their own, variables are just names for values, and assignment just creates a new name for a value. If it helps, think of each Python variable as a shared_ptr<T> instead of a T.
** Some people take advantage of this by using a class attribute as a "default value" for an instance attribute that instances may or may not set. This can be useful in some cases, but it can also be confusing, so be careful with it.
There is one more situation.
Class and instance attributes is Descriptor.
# -*- encoding: utf-8 -*-
class RevealAccess(object):
def __init__(self, initval=None, name='var'):
self.val = initval
self.name = name
def __get__(self, obj, objtype):
return self.val
class Base(object):
attr_1 = RevealAccess(10, 'var "x"')
def __init__(self):
self.attr_2 = RevealAccess(10, 'var "x"')
def main():
b = Base()
print("Access to class attribute, return: ", Base.attr_1)
print("Access to instance attribute, return: ", b.attr_2)
if __name__ == '__main__':
main()
Above will output:
('Access to class attribute, return: ', 10)
('Access to instance attribute, return: ', <__main__.RevealAccess object at 0x10184eb50>)
The same type of instance access through class or instance return different result!
And i found in c.PyObject_GenericGetAttr definition，and a great post.
Explain
If the attribute is found in the dictionary of the classes which make up.
the objects MRO, then check to see if the attribute being looked up points to a Data Descriptor (which is nothing more that a class implementing both the __get__ and the __set__ methods).
If it does, resolve the attribute lookup by calling the __get__ method of the Data Descriptor (lines 28–33).
Is there any meaningful distinction between:
class A(object):
foo = 5 # some default value
vs.
class B(object):
def __init__(self, foo=5):
self.foo = foo
If you're creating a lot of instances, is there any difference in performance or space requirements for the two styles? When you read the code, do you consider the meaning of the two styles to be significantly different?
There is a significant semantic difference (beyond performance considerations):
when the attribute is defined on the instance (which is what we usually do), there can be multiple objects referred to. Each gets a totally separate version of that attribute.
when the attribute is defined on the class, there is only one underlying object referred to, so if operations on different instances of that class both attempt to set/(append/extend/insert/etc.) the attribute, then:
if the attribute is a builtin type (like int, float, boolean, string), operations on one object will overwrite (clobber) the value
if the attribute is a mutable type (like a list or a dict), we will get unwanted leakage.
For example:
>>> class A: foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[5]
>>> class A:
... def __init__(self): self.foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[]
The difference is that the attribute on the class is shared by all instances. The attribute on an instance is unique to that instance.
If coming from C++, attributes on the class are more like static member variables.
Here is a very good post, and summary it as below.
class Bar(object):
## No need for dot syntax
class_var = 1
def __init__(self, i_var):
self.i_var = i_var
## Need dot syntax as we've left scope of class namespace
Bar.class_var
## 1
foo = MyClass(2)
## Finds i_var in foo's instance namespace
foo.i_var
## 2
## Doesn't find class_var in instance namespace…
## So look's in class namespace (Bar.__dict__)
foo.class_var
## 1
And in visual form
Class attribute assignment
If a class attribute is set by accessing the class, it will override the value for all instances
foo = Bar(2)
foo.class_var
## 1
Bar.class_var = 2
foo.class_var
## 2
If a class variable is set by accessing an instance, it will override the value only for that instance. This essentially overrides the class variable and turns it into an instance variable available, intuitively, only for that instance.
foo = Bar(2)
foo.class_var
## 1
foo.class_var = 2
foo.class_var
## 2
Bar.class_var
## 1
When would you use class attribute?
Storing constants. As class attributes can be accessed as attributes of the class itself, it’s often nice to use them for storing Class-wide, Class-specific constants
class Circle(object):
pi = 3.14159
def __init__(self, radius):
self.radius = radius
def area(self):
return Circle.pi * self.radius * self.radius
Circle.pi
## 3.14159
c = Circle(10)
c.pi
## 3.14159
c.area()
## 314.159
Defining default values. As a trivial example, we might create a bounded list (i.e., a list that can only hold a certain number of elements or fewer) and choose to have a default cap of 10 items
class MyClass(object):
limit = 10
def __init__(self):
self.data = []
def item(self, i):
return self.data[i]
def add(self, e):
if len(self.data) >= self.limit:
raise Exception("Too many elements")
self.data.append(e)
MyClass.limit
## 10
Since people in the comments here and in two other questions marked as dups all appear to be confused about this in the same way, I think it's worth adding an additional answer on top of Alex Coventry's.
The fact that Alex is assigning a value of a mutable type, like a list, has nothing to do with whether things are shared or not. We can see this with the id function or the is operator:
>>> class A: foo = object()
>>> a, b = A(), A()
>>> a.foo is b.foo
True
>>> class A:
... def __init__(self): self.foo = object()
>>> a, b = A(), A()
>>> a.foo is b.foo
False
(If you're wondering why I used object() instead of, say, 5, that's to avoid running into two whole other issues which I don't want to get into here; for two different reasons, entirely separately-created 5s can end up being the same instance of the number 5. But entirely separately-created object()s cannot.)
So, why is it that a.foo.append(5) in Alex's example affects b.foo, but a.foo = 5 in my example doesn't? Well, try a.foo = 5 in Alex's example, and notice that it doesn't affect b.foo there either.
a.foo = 5 is just making a.foo into a name for 5. That doesn't affect b.foo, or any other name for the old value that a.foo used to refer to.* It's a little tricky that we're creating an instance attribute that hides a class attribute,** but once you get that, nothing complicated is happening here.
Hopefully it's now obvious why Alex used a list: the fact that you can mutate a list means it's easier to show that two variables name the same list, and also means it's more important in real-life code to know whether you have two lists or two names for the same list.
* The confusion for people coming from a language like C++ is that in Python, values aren't stored in variables. Values live off in value-land, on their own, variables are just names for values, and assignment just creates a new name for a value. If it helps, think of each Python variable as a shared_ptr<T> instead of a T.
** Some people take advantage of this by using a class attribute as a "default value" for an instance attribute that instances may or may not set. This can be useful in some cases, but it can also be confusing, so be careful with it.
There is one more situation.
Class and instance attributes is Descriptor.
# -*- encoding: utf-8 -*-
class RevealAccess(object):
def __init__(self, initval=None, name='var'):
self.val = initval
self.name = name
def __get__(self, obj, objtype):
return self.val
class Base(object):
attr_1 = RevealAccess(10, 'var "x"')
def __init__(self):
self.attr_2 = RevealAccess(10, 'var "x"')
def main():
b = Base()
print("Access to class attribute, return: ", Base.attr_1)
print("Access to instance attribute, return: ", b.attr_2)
if __name__ == '__main__':
main()
Above will output:
('Access to class attribute, return: ', 10)
('Access to instance attribute, return: ', <__main__.RevealAccess object at 0x10184eb50>)
The same type of instance access through class or instance return different result!
And i found in c.PyObject_GenericGetAttr definition，and a great post.
Explain
If the attribute is found in the dictionary of the classes which make up.
the objects MRO, then check to see if the attribute being looked up points to a Data Descriptor (which is nothing more that a class implementing both the __get__ and the __set__ methods).
If it does, resolve the attribute lookup by calling the __get__ method of the Data Descriptor (lines 28–33).
Is there any meaningful distinction between:
class A(object):
foo = 5 # some default value
vs.
class B(object):
def __init__(self, foo=5):
self.foo = foo
If you're creating a lot of instances, is there any difference in performance or space requirements for the two styles? When you read the code, do you consider the meaning of the two styles to be significantly different?
There is a significant semantic difference (beyond performance considerations):
when the attribute is defined on the instance (which is what we usually do), there can be multiple objects referred to. Each gets a totally separate version of that attribute.
when the attribute is defined on the class, there is only one underlying object referred to, so if operations on different instances of that class both attempt to set/(append/extend/insert/etc.) the attribute, then:
if the attribute is a builtin type (like int, float, boolean, string), operations on one object will overwrite (clobber) the value
if the attribute is a mutable type (like a list or a dict), we will get unwanted leakage.
For example:
>>> class A: foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[5]
>>> class A:
... def __init__(self): self.foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[]
The difference is that the attribute on the class is shared by all instances. The attribute on an instance is unique to that instance.
If coming from C++, attributes on the class are more like static member variables.
Here is a very good post, and summary it as below.
class Bar(object):
## No need for dot syntax
class_var = 1
def __init__(self, i_var):
self.i_var = i_var
## Need dot syntax as we've left scope of class namespace
Bar.class_var
## 1
foo = MyClass(2)
## Finds i_var in foo's instance namespace
foo.i_var
## 2
## Doesn't find class_var in instance namespace…
## So look's in class namespace (Bar.__dict__)
foo.class_var
## 1
And in visual form
Class attribute assignment
If a class attribute is set by accessing the class, it will override the value for all instances
foo = Bar(2)
foo.class_var
## 1
Bar.class_var = 2
foo.class_var
## 2
If a class variable is set by accessing an instance, it will override the value only for that instance. This essentially overrides the class variable and turns it into an instance variable available, intuitively, only for that instance.
foo = Bar(2)
foo.class_var
## 1
foo.class_var = 2
foo.class_var
## 2
Bar.class_var
## 1
When would you use class attribute?
Storing constants. As class attributes can be accessed as attributes of the class itself, it’s often nice to use them for storing Class-wide, Class-specific constants
class Circle(object):
pi = 3.14159
def __init__(self, radius):
self.radius = radius
def area(self):
return Circle.pi * self.radius * self.radius
Circle.pi
## 3.14159
c = Circle(10)
c.pi
## 3.14159
c.area()
## 314.159
Defining default values. As a trivial example, we might create a bounded list (i.e., a list that can only hold a certain number of elements or fewer) and choose to have a default cap of 10 items
class MyClass(object):
limit = 10
def __init__(self):
self.data = []
def item(self, i):
return self.data[i]
def add(self, e):
if len(self.data) >= self.limit:
raise Exception("Too many elements")
self.data.append(e)
MyClass.limit
## 10
Since people in the comments here and in two other questions marked as dups all appear to be confused about this in the same way, I think it's worth adding an additional answer on top of Alex Coventry's.
The fact that Alex is assigning a value of a mutable type, like a list, has nothing to do with whether things are shared or not. We can see this with the id function or the is operator:
>>> class A: foo = object()
>>> a, b = A(), A()
>>> a.foo is b.foo
True
>>> class A:
... def __init__(self): self.foo = object()
>>> a, b = A(), A()
>>> a.foo is b.foo
False
(If you're wondering why I used object() instead of, say, 5, that's to avoid running into two whole other issues which I don't want to get into here; for two different reasons, entirely separately-created 5s can end up being the same instance of the number 5. But entirely separately-created object()s cannot.)
So, why is it that a.foo.append(5) in Alex's example affects b.foo, but a.foo = 5 in my example doesn't? Well, try a.foo = 5 in Alex's example, and notice that it doesn't affect b.foo there either.
a.foo = 5 is just making a.foo into a name for 5. That doesn't affect b.foo, or any other name for the old value that a.foo used to refer to.* It's a little tricky that we're creating an instance attribute that hides a class attribute,** but once you get that, nothing complicated is happening here.
Hopefully it's now obvious why Alex used a list: the fact that you can mutate a list means it's easier to show that two variables name the same list, and also means it's more important in real-life code to know whether you have two lists or two names for the same list.
* The confusion for people coming from a language like C++ is that in Python, values aren't stored in variables. Values live off in value-land, on their own, variables are just names for values, and assignment just creates a new name for a value. If it helps, think of each Python variable as a shared_ptr<T> instead of a T.
** Some people take advantage of this by using a class attribute as a "default value" for an instance attribute that instances may or may not set. This can be useful in some cases, but it can also be confusing, so be careful with it.
There is one more situation.
Class and instance attributes is Descriptor.
# -*- encoding: utf-8 -*-
class RevealAccess(object):
def __init__(self, initval=None, name='var'):
self.val = initval
self.name = name
def __get__(self, obj, objtype):
return self.val
class Base(object):
attr_1 = RevealAccess(10, 'var "x"')
def __init__(self):
self.attr_2 = RevealAccess(10, 'var "x"')
def main():
b = Base()
print("Access to class attribute, return: ", Base.attr_1)
print("Access to instance attribute, return: ", b.attr_2)
if __name__ == '__main__':
main()
Above will output:
('Access to class attribute, return: ', 10)
('Access to instance attribute, return: ', <__main__.RevealAccess object at 0x10184eb50>)
The same type of instance access through class or instance return different result!
And i found in c.PyObject_GenericGetAttr definition，and a great post.
Explain
If the attribute is found in the dictionary of the classes which make up.
the objects MRO, then check to see if the attribute being looked up points to a Data Descriptor (which is nothing more that a class implementing both the __get__ and the __set__ methods).
If it does, resolve the attribute lookup by calling the __get__ method of the Data Descriptor (lines 28–33).
Is there any meaningful distinction between:
class A(object):
foo = 5 # some default value
vs.
class B(object):
def __init__(self, foo=5):
self.foo = foo
If you're creating a lot of instances, is there any difference in performance or space requirements for the two styles? When you read the code, do you consider the meaning of the two styles to be significantly different?
There is a significant semantic difference (beyond performance considerations):
when the attribute is defined on the instance (which is what we usually do), there can be multiple objects referred to. Each gets a totally separate version of that attribute.
when the attribute is defined on the class, there is only one underlying object referred to, so if operations on different instances of that class both attempt to set/(append/extend/insert/etc.) the attribute, then:
if the attribute is a builtin type (like int, float, boolean, string), operations on one object will overwrite (clobber) the value
if the attribute is a mutable type (like a list or a dict), we will get unwanted leakage.
For example:
>>> class A: foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[5]
>>> class A:
... def __init__(self): self.foo = []
>>> a, b = A(), A()
>>> a.foo.append(5)
>>> b.foo
[]
The difference is that the attribute on the class is shared by all instances. The attribute on an instance is unique to that instance.
If coming from C++, attributes on the class are more like static member variables.
Here is a very good post, and summary it as below.
class Bar(object):
## No need for dot syntax
class_var = 1
def __init__(self, i_var):
self.i_var = i_var
## Need dot syntax as we've left scope of class namespace
Bar.class_var
## 1
foo = MyClass(2)
## Finds i_var in foo's instance namespace
foo.i_var
## 2
## Doesn't find class_var in instance namespace…
## So look's in class namespace (Bar.__dict__)
foo.class_var
## 1
And in visual form
Class attribute assignment
If a class attribute is set by accessing the class, it will override the value for all instances
foo = Bar(2)
foo.class_var
## 1
Bar.class_var = 2
foo.class_var
## 2
If a class variable is set by accessing an instance, it will override the value only for that instance. This essentially overrides the class variable and turns it into an instance variable available, intuitively, only for that instance.
foo = Bar(2)
foo.class_var
## 1
foo.class_var = 2
foo.class_var
## 2
Bar.class_var
## 1
When would you use class attribute?
Storing constants. As class attributes can be accessed as attributes of the class itself, it’s often nice to use them for storing Class-wide, Class-specific constants
class Circle(object):
pi = 3.14159
def __init__(self, radius):
self.radius = radius
def area(self):
return Circle.pi * self.radius * self.radius
Circle.pi
## 3.14159
c = Circle(10)
c.pi
## 3.14159
c.area()
## 314.159
Defining default values. As a trivial example, we might create a bounded list (i.e., a list that can only hold a certain number of elements or fewer) and choose to have a default cap of 10 items
class MyClass(object):
limit = 10
def __init__(self):
self.data = []
def item(self, i):
return self.data[i]
def add(self, e):
if len(self.data) >= self.limit:
raise Exception("Too many elements")
self.data.append(e)
MyClass.limit
## 10
Since people in the comments here and in two other questions marked as dups all appear to be confused about this in the same way, I think it's worth adding an additional answer on top of Alex Coventry's.
The fact that Alex is assigning a value of a mutable type, like a list, has nothing to do with whether things are shared or not. We can see this with the id function or the is operator:
>>> class A: foo = object()
>>> a, b = A(), A()
>>> a.foo is b.foo
True
>>> class A:
... def __init__(self): self.foo = object()
>>> a, b = A(), A()
>>> a.foo is b.foo
False
(If you're wondering why I used object() instead of, say, 5, that's to avoid running into two whole other issues which I don't want to get into here; for two different reasons, entirely separately-created 5s can end up being the same instance of the number 5. But entirely separately-created object()s cannot.)
So, why is it that a.foo.append(5) in Alex's example affects b.foo, but a.foo = 5 in my example doesn't? Well, try a.foo = 5 in Alex's example, and notice that it doesn't affect b.foo there either.
a.foo = 5 is just making a.foo into a name for 5. That doesn't affect b.foo, or any other name for the old value that a.foo used to refer to.* It's a little tricky that we're creating an instance attribute that hides a class attribute,** but once you get that, nothing complicated is happening here.
Hopefully it's now obvious why Alex used a list: the fact that you can mutate a list means it's easier to show that two variables name the same list, and also means it's more important in real-life code to know whether you have two lists or two names for the same list.
* The confusion for people coming from a language like C++ is that in Python, values aren't stored in variables. Values live off in value-land, on their own, variables are just names for values, and assignment just creates a new name for a value. If it helps, think of each Python variable as a shared_ptr<T> instead of a T.
** Some people take advantage of this by using a class attribute as a "default value" for an instance attribute that instances may or may not set. This can be useful in some cases, but it can also be confusing, so be careful with it.
There is one more situation.
Class and instance attributes is Descriptor.
# -*- encoding: utf-8 -*-
class RevealAccess(object):
def __init__(self, initval=None, name='var'):
self.val = initval
self.name = name
def __get__(self, obj, objtype):
return self.val
class Base(object):
attr_1 = RevealAccess(10, 'var "x"')
def __init__(self):
self.attr_2 = RevealAccess(10, 'var "x"')
def main():
b = Base()
print("Access to class attribute, return: ", Base.attr_1)
print("Access to instance attribute, return: ", b.attr_2)
if __name__ == '__main__':
main()
Above will output:
('Access to class attribute, return: ', 10)
('Access to instance attribute, return: ', <__main__.RevealAccess object at 0x10184eb50>)
The same type of instance access through class or instance return different result!
And i found in c.PyObject_GenericGetAttr definition，and a great post.
Explain
If the attribute is found in the dictionary of the classes which make up.
the objects MRO, then check to see if the attribute being looked up points to a Data Descriptor (which is nothing more that a class implementing both the __get__ and the __set__ methods).
If it does, resolve the attribute lookup by calling the __get__ method of the Data Descriptor (lines 28–33).
To quote diveintopython,
"You already know about data attributes, which are variables owned by
a specific instance of a class. Python also supports class attributes,
which are variables owned by the class itself."
In what sense are class attributes owned by a class? If you change the value of a class attribute in a specific instance, that change is only reflected in that instance (and not in other instances of the class).
From my vantage point this makes class attributes fundamentally the same as data (i.e. instance) attributes (notwithstanding the syntactic differences).
In C++ change the value of a "class variable", and that change is reflected in all instances.
What is the difference between the two?
I think that this example will explain the meaning to you.
class A(object):
bar = 1
a = A()
b = A()
b.bar = 2
print a.bar # outputs 1
A.bar = 3
print a.bar # outputs 3
print b.bar # outputs 2
In this case b.bar will be owned by instance after b.bar = 2 but a.bar will still be owned by class. That is why it will be changed on instance after changing it on class and b.bar will not.
This question is a duplicate of this one:
>>> class B(object):
... foo = 1
...
>>> b = B()
>>> b.__dict__
{}
>>> b.foo = 2
>>> b.__dict__
{'foo': 2}
When you assign a value to b, you add an instance variable; you're not modifying the class attribute.