Can someone provides a detail explanation why this is happening? How does Python compiler create class variables in this case?
class A(object):
x = 1
y = x + 1
class B(A):
x = 10
>>> B.x
10
>>> B.y
2 # ---> I was expecting 11 here, why does this y still uses the A's value?
Because class variables are evaluated at the same time the class itself is evaluated. Here the sequence of events is: A
is defined and the values in it are set, so x
is 1 and y
is 2. Then B
is defined, and the x
entry in B
is set to 10. Then you access By
, and since there is no y
entry in B
, it checks its parent class. It does find a y
entry in A
, with a value of 2
. y
is defined only once.
If you really want such a variable, you may want to define a class method.
class A:
x = 1
@classmethod
def y(cls):
return cls.x + 1
class B(A):
x = 10
>>> B.y()
11
This is because y
is a class attribute that belongs to A, so changing the value of x in a class instance of B
does not change the value of y
. You can read more about that in the documentation: https://docs.python.org/2/tutorial/classes.html#class-objects
It does not do that.
>>> class A(object):
... x = 1
... y = x + 1
...
>>> class B(object):
... x = 10
...
>>> B.x
10
>>> B.y
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: type object 'B' has no attribute 'y'
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.