How can I change a class object with some rules when it assigned?
for example:
class Foo:
x=0
if x < 0:
x = -1
else:
x = 1
p = Foo()
p.x = 1234
print(p.X)
So when I print p.x I expect 1 to be printed. but 1234 is printed. What should I do?
The first x = 0 you declare is a class attribute. It is the same for all objects of the class.
When you write p.x, you create an instance attribute, a value of x that belongs to that specific object. That hides the Foo.x in that particular object (that's how the python attribute lookup works).
However, that value remains the default for other objects of the class, eg, if you create a new object
foo2 = Foo()
print(f2.x) # prints 1
You have to make x a property, with a setter function:
class Foo:
def __init__(self):
self.x = 1
#property
def x(self):
return self._x
#x.setter
def x(self, value):
if value < 0:
value = -1
else:
value = 1
self._x = value
This makes it so the function decorated with #x.setter is executed whenever an assignment to the x attribute is performed:
>>> foo = Foo()
>>> foo.x = 1234
>>> foo.x
1
Related
what's best way to make the class Foo():
>>> p=Foo()
>>> print (p.x) => p.x = 0
>>> p.x = 125
>>> print (p.x) => p.x = 25 (tens of 125)
You can use getters and setters. Depending on whether you want to store the remainder or the unmodified value in the instance, place the logic to calculate the remainder in either the setter or getter, respectively.
class Foo:
def __init__(self):
self._x = 0
#property
def x(self):
return self._x
#x.setter
def x(self, x):
self._x = x % 100
(As a side note, defaulting to using getters and setters (as is common in some other languages) is considered unpythonic. Here they (or some variation of it) are needed to alter the value set or retrieved according to your rule/requirement.)
I want to use a class variable as a default argument value in a static method.
But when I reference the class, I get an error NameError: name 'MyClass' is not defined
class MyClass:
x = 100
y = 200
#staticmethod
def foo(x = MyClass.x, y = MyClass.y):
return x*y
MyClass is not defined yet when Python wants to bind the default arguments, but x and y are already defined in the classes' scope.
In other words, you can write:
class MyClass:
x = 100
y = 200
#staticmethod
def foo(x=x, y=y):
return x*y
Note that foo will not recognize reassignments to MyCLass.x and MyClass.y because the default arguments are bound once, when the function is created.
>>> MyClass.foo()
20000
>>> MyClass.x = 0
>>> MyClass.foo()
20000
Ive been taking online Python 3 course and there was an exercise.
You should write a class called Foo that has a property called x, which is set according to these rules:
The initial value of x is 0 when creating the Foo class.
When setting x with a number:
If the number is nonnegative, the two digits to the right of it are stored in x.
p=Foo()
print(p.x) -----> output:0
p.x=123
print(p.x) -----> output :23
I just wondering how is the x getting the assignment through the object.
>>> p=Foo()
>>> p.x = 1234
>>> p.x == 34
True
>>> type(p.x)
<class 'int'>
class Foo():
def __init__(self):
self.n=0
#property
def x(self):
return self.n
#x.setter
def x(self, num):
if num<100 and num>=0:
self.n=num
elif num>100 and num%100!=0 :
self.n=num%100
elif num<0:
self.n=-1
else:
self.n=0
Im trying to understand how the #property decorator works.
Here I have used method y as a property for field x,
After the attribute-self.x has a property, does it mean that we can't set the value explicitly..
I thought the last statement--> c.x = 2 will not work once you have the property method set on a variable?
class C(object):
def __init__(self):
self.x = 0
self.list = [1,2,3,4,10]
#property
def y(self):
print 'getting'
self.x = sum(self.list)
return self.x
#y.setter
def y(self, value):
print 'setting'
self.x = value
if __name__ == '__main__':
c = C()
print 'Value of c.y=',c.y
print '-'*80
c.y = 50
print '-'*80
print c.y
print '-'*80
if c.y >5:
print 'Hi'
You can always set x explicitly.
class Foo(object):
def __init__(self):
self.x = 1
self.lst = [1,2,3]
#property
def y(self):
self.x = sum(self.lst)
return self.x
#y.setter
def y(self,value):
self.x = value
f = Foo()
print f.y #6
print f.x #6
f.x = 3
print f.x #3
print f.y #6
print f.x #6
The problem is that in this example, calling the getter (y) also sets the value of the x attribute, so you'll never see the change of x if you're doing all of the changing via y because the act of looking at y changes the value of x.
One way that you might try to get around that limitation is:
class Foo(object):
def __init__(self):
self.x = None
self.lst = [1,2,3]
#property
def y(self):
return sum(self.lst) if self.x is None else self.x
#y.setter
def y(self,value):
self.x = value
Now if you explicitly set a value for x (or y), that value will stick until you set it back to None which you could even do in another function decorated with #y.deleter if you really wanted.
There is limited support for private instance variables in Python via name-mangling
to avoid exposing x, you need two leading underscores, i.e. __x
You cant prohibit to change attribute directly using property decorator but You can do such a trick I think
class A(object):
def __init__(self):
self.x = 0
#property
def x(self):
return self.__dict__['x']
#x.setter
def x(self, value):
self.__dict__['x']=value
this will allow You to implement behavior like You have described
Python does not provide any capability for preventing callers from accessing variables. In other words, there is no "private" in Python. By convention, a variable or method prefixed with an underscore is not intended for external use. E.g.,
class C(object):
def __init__(self):
self._x = 0
self.list = [1,2,3,4,10]
.
.
.
I can still access _x if I really want to, and nothing prevents me from setting it.
>>> c = C()
>>> c._x
10
>>> c._x = 20
>>> c._x
20
However, by convention, the underscore tells me I'm doing something dangerous and ill advised. It's up to me, the programmer, to determine if I broke anything by doing it.
This is a conscious design decision made when creating Python. The idea is that whoever uses your class is responsible for what they do with it; if they misuse it and it breaks, that's their fault. You warned them with the underscore. I think the notion that a clever programmer can get around your attempts to lock them out anyway may have played a role in the decision (such as reflection libraries or interacting with the compiled bytecode directly), but don't hold me to that.
On a mildly related note, the underscore does actually do something if the variable (including other imported modules, functions, etc.) is a member of a module. Members beginning with an underscore are not imported by import *. E.g.,
a.py
_a = 10
b = 50
Command prompt:
>>> from a import *
>>> b
50
>>> _a
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name '_a' is not defined
In your particular example, x and its setter are relatively useless since you're overriding its value any time the getter is called.
class Foo(object):
def init(self):
self.x = None
self.lst = [1,2,3]
#property
def y(self):
return sum(self.lst) if self.x is None else self.x
#y.setter
def y(self,value):
self.x = value
How do these 2 classes differ?
class A():
x=3
class B():
def __init__(self):
self.x=3
Is there any significant difference?
A.x is a class variable.
B's self.x is an instance variable.
i.e. A's x is shared between instances.
It would be easier to demonstrate the difference with something that can be modified like a list:
#!/usr/bin/env python
class A:
x = []
def add(self):
self.x.append(1)
class B:
def __init__(self):
self.x = []
def add(self):
self.x.append(1)
x = A()
y = A()
x.add()
y.add()
print("A's x:", x.x)
x = B()
y = B()
x.add()
y.add()
print("B's x:", x.x)
Output
A's x: [1, 1]
B's x: [1]
Just as a side note: self is actually just a randomly chosen word, that everyone uses, but you could also use this, foo, or myself or anything else you want, it's just the first parameter of every non static method for a class. This means that the word self is not a language construct but just a name:
>>> class A:
... def __init__(s):
... s.bla = 2
...
>>>
>>> a = A()
>>> a.bla
2
A.x is a class variable, and will be shared across all instances of A, unless specifically overridden within an instance.
B.x is an instance variable, and each instance of B has its own version of it.
I hope the following Python example can clarify:
>>> class Foo():
... i = 3
... def bar(self):
... print 'Foo.i is', Foo.i
... print 'self.i is', self.i
...
>>> f = Foo() # Create an instance of the Foo class
>>> f.bar()
Foo.i is 3
self.i is 3
>>> Foo.i = 5 # Change the global value of Foo.i over all instances
>>> f.bar()
Foo.i is 5
self.i is 5
>>> f.i = 3 # Override this instance's definition of i
>>> f.bar()
Foo.i is 5
self.i is 3
I used to explain it with this example
# By TMOTTM
class Machine:
# Class Variable counts how many machines have been created.
# The value is the same for all objects of this class.
counter = 0
def __init__(self):
# Notice: no 'self'.
Machine.counter += 1
# Instance variable.
# Different for every object of the class.
self.id = Machine.counter
if __name__ == '__main__':
machine1 = Machine()
machine2 = Machine()
machine3 = Machine()
#The value is different for all objects.
print 'machine1.id', machine1.id
print 'machine2.id', machine2.id
print 'machine3.id', machine3.id
#The value is the same for all objects.
print 'machine1.counter', machine1.counter
print 'machine2.counter', machine2.counter
print 'machine3.counter', machine3.counter
The output then will by
machine1.id 1
machine2.id 2
machine3.id 3
machine1.counter 3
machine2.counter 3
machine3.counter 3
I've just started learning Python and this confused me as well for some time. Trying to figure out how it all works in general I came up with this very simple piece of code:
# Create a class with a variable inside and an instance of that class
class One:
color = 'green'
obj2 = One()
# Here we create a global variable(outside a class suite).
color = 'blue'
# Create a second class and a local variable inside this class.
class Two:
color = "red"
# Define 3 methods. The only difference between them is the "color" part.
def out(self):
print(self.color + '!')
def out2(self):
print(color + '!')
def out3(self):
print(obj2.color + '!')
# Create an object of the class One
obj = Two()
When we call out() we get:
>>> obj.out()
red!
When we call out2():
>>> obj.out2()
blue!
When we call out3():
>>> obj.out3()
green!
So, in the first method self specifies that Python should use the variable(attribute), that "belongs" to the class object we created, not a global one(outside the class). So it uses color = "red". In the method Python implicitly substitutes self for the name of an object we created(obj). self.color means "I am getting color="red" from the obj"
In the second method there is no self to specify the object where the color should be taken from, so it gets the global one color = 'blue'.
In the third method instead of self we used obj2 - a name of another object to get color from. It gets color = 'green'.