Python, how come we can create class variables that were not defined in class creation?
Question:
Let’s say we have this simple Python code
class MyClass(object):
class_var = 1
def __init__(self, i_var):
self.i_var = i_var
Correct me if I get any of this wrong:
Class_Var is a class variable that is the same for all instances of MyClass object.
I_Var is an instance variable that only exists in instances of the MyClass object
foo = MyClass(2)
bar = MyClass(3)
foo.class_var, foo.i_var
## 1, 2
bar.class_var, bar.i_var
## 1, 3
Class variables are also properties of the class itself.
MyClass.class_var ##
## 1
MyClass.I_var should error out, correct?
Does that mean that class variables can be considered like instance variables of the class object itself (since all classes are objects) ?
MyClass.new_attribute = 'foo'
print(hasattr(ObjectCreator, 'new_attribute'))
That should return true. And
print (MyClass.new_attribute)
should return foo.
How come we can create a new class variable that was not defined in the original definition for that class?
Is
MyClass.new_attribute = 'foo'
the exact same thing as creating that class attribute in the original definition?
class MyClass(object):
class_var = 1
new_attribute = 'foo'
So we can create new class attributes at runtime? How does that not interfere with the init constructor that creates the class object and has those class variables as instance variables of the class object?
Answers:
A class
object is just an instance of yet another type, usually type
(though you can change this using the metaclass
parameter to the class
statement).
Like most other instances, you can add arbitrary instance attributes to a class
object at any time.
Class attributes and instance attributes are wholly separate; the former are stored on the class
object, the latter on instances of the class.
There’s nothing particularly special about __init__
; it’s just another method that, among other things, can attached new attributes to an object. What is special is that __init__
is called automatically when you create a new instance of the class by calling the class. foo = MyClass(2)
is equivalent to
foo = MyClass.__new__(MyClass, 2)
foo.__init__(2)
The class statement
class MyClass(object):
class_var = 1
def __init__(self, i_var):
self.i_var = i_var
is roughly equivalent to
def my_class_init(self, i_var):
self.i_var = i_var
MyClass = type('MyClass', (object,), {'class_var': 1, '__init__': my_class_init})
The 3-argument form of type
lets you pass a dict
that creates class attributes when you first create the class, but you can always assign attributes after the fact as well:
MyClass = type('MyClass', (object,), {})
MyClass.class_var = 1
MyClass.__init__ = my_class_init
Just to blow your mind a little bit more, the call to type
can be thought of as
MyClass = type.__new__(type, 'MyClass', (object,), {...})
MyClass.__init__('MyClass', (object,), {...})
though unless you define a custom metaclass (by subclassing type
), you never have to think about type
itself having __new__
and __init__
methods.
Does that mean that class variables can be considered like instance variables of the class object itself (since all classes are objects) ?
Yes.
How come we can create a new class variable that was not defined in the original definition for that class?
Because Python is a dynamic language. A class can be created at run time – in fact, it is created at run time when you run Python interactively.
So we can create new class attributes at runtime?
Yes, unless the metaclass (the class of the class) has forbidden it.
How does that not interfere with the init constructor that creates the class object and has those class variables as instance variables of the class object?
The only rule is that you cannot use something that has not yet be defined or something that has been deleted:
>>> class MyClass(object):
class_var = 1
def __init__(self, i_var):
self.i_var = i_var
self.j_var = self.class_var + 1
>>> a = MyClass(2)
>>> del MyClass.class_var
>>> b = MyClass(3)
Traceback (most recent call last):
File "<pyshell#39>", line 1, in <module>
b = MyClass(3)
File "<pyshell#36>", line 6, in __init__
self.j_var = self.class_var + 1
AttributeError: 'MyClass' object has no attribute 'class_var'
There is no magic here: anything can only exists between its definition point and its destruction point. Python allows you to add attributes to objects at any time, except that some classes (for example object
) forbid it.
With the previous a
object of class MyClass
, you could do:
a.z_var = 12
from that point, z_var
will be an attribute of a
but others objects of same class will not have it.
Simply object
forbids that:
>>> o = object()
>>> o.x=1
Traceback (most recent call last):
File "<pyshell#41>", line 1, in <module>
o.x=1
AttributeError: 'object' object has no attribute 'x'
Let’s say we have this simple Python code
class MyClass(object):
class_var = 1
def __init__(self, i_var):
self.i_var = i_var
Correct me if I get any of this wrong:
Class_Var is a class variable that is the same for all instances of MyClass object.
I_Var is an instance variable that only exists in instances of the MyClass object
foo = MyClass(2)
bar = MyClass(3)
foo.class_var, foo.i_var
## 1, 2
bar.class_var, bar.i_var
## 1, 3
Class variables are also properties of the class itself.
MyClass.class_var ##
## 1
MyClass.I_var should error out, correct?
Does that mean that class variables can be considered like instance variables of the class object itself (since all classes are objects) ?
MyClass.new_attribute = 'foo'
print(hasattr(ObjectCreator, 'new_attribute'))
That should return true. And
print (MyClass.new_attribute)
should return foo.
How come we can create a new class variable that was not defined in the original definition for that class?
Is
MyClass.new_attribute = 'foo'
the exact same thing as creating that class attribute in the original definition?
class MyClass(object):
class_var = 1
new_attribute = 'foo'
So we can create new class attributes at runtime? How does that not interfere with the init constructor that creates the class object and has those class variables as instance variables of the class object?
A class
object is just an instance of yet another type, usually type
(though you can change this using the metaclass
parameter to the class
statement).
Like most other instances, you can add arbitrary instance attributes to a class
object at any time.
Class attributes and instance attributes are wholly separate; the former are stored on the class
object, the latter on instances of the class.
There’s nothing particularly special about __init__
; it’s just another method that, among other things, can attached new attributes to an object. What is special is that __init__
is called automatically when you create a new instance of the class by calling the class. foo = MyClass(2)
is equivalent to
foo = MyClass.__new__(MyClass, 2)
foo.__init__(2)
The class statement
class MyClass(object):
class_var = 1
def __init__(self, i_var):
self.i_var = i_var
is roughly equivalent to
def my_class_init(self, i_var):
self.i_var = i_var
MyClass = type('MyClass', (object,), {'class_var': 1, '__init__': my_class_init})
The 3-argument form of type
lets you pass a dict
that creates class attributes when you first create the class, but you can always assign attributes after the fact as well:
MyClass = type('MyClass', (object,), {})
MyClass.class_var = 1
MyClass.__init__ = my_class_init
Just to blow your mind a little bit more, the call to type
can be thought of as
MyClass = type.__new__(type, 'MyClass', (object,), {...})
MyClass.__init__('MyClass', (object,), {...})
though unless you define a custom metaclass (by subclassing type
), you never have to think about type
itself having __new__
and __init__
methods.
Does that mean that class variables can be considered like instance variables of the class object itself (since all classes are objects) ?
Yes.
How come we can create a new class variable that was not defined in the original definition for that class?
Because Python is a dynamic language. A class can be created at run time – in fact, it is created at run time when you run Python interactively.
So we can create new class attributes at runtime?
Yes, unless the metaclass (the class of the class) has forbidden it.
How does that not interfere with the init constructor that creates the class object and has those class variables as instance variables of the class object?
The only rule is that you cannot use something that has not yet be defined or something that has been deleted:
>>> class MyClass(object):
class_var = 1
def __init__(self, i_var):
self.i_var = i_var
self.j_var = self.class_var + 1
>>> a = MyClass(2)
>>> del MyClass.class_var
>>> b = MyClass(3)
Traceback (most recent call last):
File "<pyshell#39>", line 1, in <module>
b = MyClass(3)
File "<pyshell#36>", line 6, in __init__
self.j_var = self.class_var + 1
AttributeError: 'MyClass' object has no attribute 'class_var'
There is no magic here: anything can only exists between its definition point and its destruction point. Python allows you to add attributes to objects at any time, except that some classes (for example object
) forbid it.
With the previous a
object of class MyClass
, you could do:
a.z_var = 12
from that point, z_var
will be an attribute of a
but others objects of same class will not have it.
Simply object
forbids that:
>>> o = object()
>>> o.x=1
Traceback (most recent call last):
File "<pyshell#41>", line 1, in <module>
o.x=1
AttributeError: 'object' object has no attribute 'x'