This is because of the way Python resolves names with the .. When you write self.list the Python runtime tries to resolve the list name first by looking for it in the instance object, and if it is not found there, then in the class instance.
Let's look into it step by step
self.list.append(1)
- Is there a
listname into the objectself?- Yes: Use it! Finish.
- No: Go to 2.
- Is there a
listname into the class instance of objectself?- Yes: Use it! Finish
- No: Error!
But when you bind a name things are different:
self.list = []
- Is there a
listname into the objectself?- Yes: Overwrite it!
- No: Bind it!
So, that is always an instance variable.
Your first example creates a list into the class instance, as this is the active scope at the time (no self anywhere). But your second example creates a list explicitly in the scope of self.
More interesting would be the example:
class testClass():
list = ['foo']
def __init__(self):
self.list = []
self.list.append('thing')
x = testClass()
print x.list
print testClass.list
del x.list
print x.list
That will print:
['thing']
['foo']
['foo']
The moment you delete the instance name the class name is visible through the self reference.
This is because of the way Python resolves names with the .. When you write self.list the Python runtime tries to resolve the list name first by looking for it in the instance object, and if it is not found there, then in the class instance.
Let's look into it step by step
self.list.append(1)
- Is there a
listname into the objectself?- Yes: Use it! Finish.
- No: Go to 2.
- Is there a
listname into the class instance of objectself?- Yes: Use it! Finish
- No: Error!
But when you bind a name things are different:
self.list = []
- Is there a
listname into the objectself?- Yes: Overwrite it!
- No: Bind it!
So, that is always an instance variable.
Your first example creates a list into the class instance, as this is the active scope at the time (no self anywhere). But your second example creates a list explicitly in the scope of self.
More interesting would be the example:
class testClass():
list = ['foo']
def __init__(self):
self.list = []
self.list.append('thing')
x = testClass()
print x.list
print testClass.list
del x.list
print x.list
That will print:
['thing']
['foo']
['foo']
The moment you delete the instance name the class name is visible through the self reference.
Python has interesting rules about looking up names. If you really want to bend your mind, try this code:
class testClass():
l = []
def __init__(self):
self.l = ['fred']
This will give each instance a variable called l that masks the class variable l. You will still be able to get at the class variable if you do self.__class__.l.
The way I think of it is this... Whenever you do instance.variable (even for method names, they're just variables who's values happen to be functions) it looks it up in the instance's dictionary. And if it can't find it there, it tries to look it up in the instance's class' dictionary. This is only if the variable is being 'read'. If it's being assigned to, it always creates a new entry in the instance dictionary.
Videos
What is a Python namespace?
A Python namespace is a mapping from names to objects, with the property that there is zero relation between names in different namespaces. Namespaces are usually implemented as Python dictionaries, although this is abstracted away.
Python class method versus instance method: What’s the difference?
In Python, a class method is a method that is invoked with the class as the context. This is often called a static method in other programming languages. An instance method, on the other hand, is invoked with an instance as the context.
What happens if both instance attribute and class attribute are defined?
In that case, the instance namespace takes precedence over the class namespace. If there is an attribute with the same name in both, the instance namespace will be checked first and its value returned.
You can run the insertion code immediately after a class is created:
class Foo():
...
vars = ('tx', 'ty', 'tz') # plus plenty more
for v in vars:
setattr(Foo, v, 0)
Also, you can dynamically store the variable while the class is being created:
class Bar:
locals()['tx'] = 'texas'
Use the type class constructor!
Foo = type("Foo", (), {k: 0 for k in ("tx", "ty", "tz")})