Is it possible to only allow a user invoke any/all object methods when the object itself is called?
Looking for different examples on how to do this
Some example code
class Test:
def __init__(self, x=1):
self.x = x
def __call__(self, x=1):
self.x = x
return self
def get(self, y):
return (self.x * y)
t_obj = Test()
t_obj(2).get(1) # Acceptable case
t_obj.get(1) # Raises exception
t_obj().get(2) # Acceptable case
Right way to do it
It seems to me that what you want is for t_obj to actually be the class and not an instance of Test. It then gives the exact behavior you showed in your example.
class Test:
def __init__(self, x=1):
self.x = x
def get(self, y):
return (self.x * y)
t_obj = Test # not an object, actually a class
t_obj(2).get(1) # 2
t_obj.get(1) # Raises exception
t_obj().get(2) # 2
In particular, the exception states that method get() must be called with Test instance, i.e. you had to call t_obj to instantiate an object before being able to call get(), which is exactly what you want.
Fun way to do it
Although, suppose you really need your objects to be callable, here is a hacky way to get this working. It replaces the get method of your object when called, replacing a placeholder which only purpose is to raise.
class Test:
def __init__(self, x=1):
self.x = x
def __call__(self, x):
self.x = x
self.get = lambda y: self.x * y
return self
def get(self, y):
raise AttributeError('call object first')
t_obj = Test()
t_obj.get(1) # Raises exception
t_obj(2)
t_obj.get(1) # 2
Related
I have such an example of method:
class Class(OtherClass):
def __init__(self, x):
self.x = x
or the second example:
class Class():
def __init__(self, x):
self.x = x
I have no idea how to write a unit test to verify it.
I have experience only in classic functions like:
def add(x,y):
return x + y
Thanks a lot for help.
You have just written a class definition with a single method, which is called when the class is instantiated. So a unit test could be if you can instantiate the object and check if its attribute is correct.
testx = 'something'
assert Class(testx).x == testx
I have a situation where I'm using #classmethod to create a constructor for a class. Within this constructor, a function gets called, which then in turn calls another function. But either this doesn't work or (more probably) I'm doing something to make it not work. Here's an example in miniature:
class testclass:
def __init__(self, x):
self.x = x
#classmethod
def constructor(cls, x):
adj_x = cls.outer_adjust(cls, x)
return testclass(adj_x)
def outer_adjust(self, x):
return self.inner_adjust(x)
def inner_adjust(self, x):
return x + 1
test_instance = testclass.constructor(4)
This produces an error message:
inner_adjust() missing 1 required positional argument: 'x'
I can make it work by explicitly passing self to inner_adjust, eg
def outer_adjust(self, x):
return self.inner_adjust(self, x)
But this then means that the outer_adjust method can't be used outside of the constructor, which is not what I want.
Any assistance gratefully received.
Here's a more detailed example, with two constructors shown. I'm trying to follow the approach to constructors described in
What is a clean, pythonic way to have multiple constructors in Python?
Which is essentially that the constructors do some processing to figure out what variables they should pass to init when instantiating the class.
Both constructors give the same error:
if_char_is_z_make_it_a() missing 1 required positional argument: 'char_input'
As before, I need to be able to use the if_char_is_make_it_a function outside of the constructor (ie, when using the class normally).
class testclass:
def __init__(self, char):
self.char = char
#classmethod
def constructor_from_int(cls, int_input):
as_char = chr(int_input)
char = cls.process_char(cls, as_char)
return testclass(char)
#classmethod
def constructor_from_char(cls, char_input):
char = cls.process_char(cls, char_input)
return testclass(char)
def process_char(self, char_input):
processed_char = '(' + char_input + ')'
output_char = self.if_char_is_z_make_it_a(processed_char)
return output_char
def if_char_is_z_make_it_a(self, char_input):
if char_input == '(z)':
return '(a)'
return char_input
test_instance = testclass.constructor_from_char('a')
When you call cls.outer_adjust from constructor you are calling the unbound outer_adjust method.
Thus, you pass the class itself as self and not an instance to a method that expects to receive an instance as argument.
Although, there is no real reason to have a constructor method. This is exactly what __init__ is for.
class testclass:
def __init__(self, x):
self.x = self.outer_adjust(x)
def outer_adjust(self, x):
return self.inner_adjust(x)
def inner_adjust(self, x):
return x + 1
test_instance = testclass(4)
If you absolutely need the transformation on x to be done before the instantiation, then use __new__ instead. Although, this is generally not necessary.
Multiple constructors
If for some reason you still need to have a constructor method, by example if you want multiple constructors. Then keep in mind that outer_adjust and inner_adjust are instance methods, this means they must be called after you have created an instance.
class testclass:
def __init__(self, x):
self.x = x
#classmethod
def constructor1(cls, x):
instance = cls(x)
instance.outer_adjust()
return instance
#classmethod
def constructor2(cls, x):
instance = cls(x)
instance.inner_adjust()
return instance
def outer_adjust(self):
print('Do something else')
return self.inner_adjust()
def inner_adjust(self):
self.x += 1
As a sidenote, notice how I did not need to call testclass, but simply called cls in the constructor methods. Since this is a class method, we do not need to explicitly name the class. This is better, especially if you are to use inheritance.
Basically what you are doing here shall be done via the __new__ which serve as constructor.
class testclass:
def __init__(self, x):
self.x = x
def __new__(cls, *args, **kwargs):
instance = super(testclass, cls).__new__(cls, *args, **kwargs)
instance.outer_adjust(args[0])
return instance
def outer_adjust(self, x):
return self.inner_adjust(x)
def inner_adjust(self, x):
self.x = x + 1
test_instance = testclass(4)
You are abusing self. The point of the class method is to use the cls argument as constructor, instead of explicitly naming the class by testclass(adj_x). Also, during the cls.outer_adjust(cls, x) call, you are passing the class instead of the instance, which happens to work because you are not using any instance attributes.
As to your questions, there's no way to avoid the x argument. inner_adjust increases some value by 1, so you must give it something to increase. The idea would be to have
def constructor(cls, x):
return cls(x)
def inner_adjust(self):
return self.x += 1
and then do something like
object= testclass.constructor(12)
object.inner_adjust()
I am learning about class structure in python. Would like to know if it's possible to pass one argument through more than one method.
class Example(object):
def __init__(self, x):
self.x = x
def square(self):
return self.x**2
def cube(self):
return self.x**3
def squarethencube(y):
sq = Example.square(y)
cu = Example.cube(sq)
return cu
two = Example(2)
print(two.squarethencube())
Error is on line 10; AttributeError: 'int' object has no attribute 'x'
The goal is to use the 'squarethencube' method to pass '2' to square(), which is 4. Then pass '4' to cube(). The desired output is '64'. Obviously, you can write a function to do the math in a very simple way; the question here is how to use multiple methods.
I understand the error in that .x is getting assigned as an attribute onto the output of cube(sq). I was getting the same error, but on line 7, before I changed the argument to y (from self.x).
I've found some similar answers here but I need a simpler explanation.
Currently, square and cube are methods bound to the class; however, you are accessing them in squarethencube by class name, but they are methods, and thus rely on a reference to the class from an instance. Therefore, you can either create two new instances of the class or use classmethod:
Option1:
class Example(object):
def __init__(self, x):
self.x = x
def square(self):
return self.x**2
def cube(self):
return self.x**3
def squarethencube(self, y):
sq = Example(y).square()
cu = Example(y).cube()
return cu
Option 2: use a classmethod:
class Example(object):
def __init__(self, x):
self.x = x
#classmethod
def square(cls, x):
return x**2
#classmethod
def cube(cls, x):
return x**3
def squarethencube(self, y):
sq = Example.square(y)
cu = Example.cube(sq)
return cu
class Example:
def __init__(self, x):
self.x = x
def square(self):
return self.x**2
def cube(self):
return self.x**3
def squarethencube(self):
return (self.x**2)**3
two = Example(2)
print(two.squarethencube())
I try to use the composition relationship but I can't access to the compound class A:
with this code I'm trying to add to the list of class A, an object from the class B.
class B:
def __init__(self,X,Y,Z):
self.X
self.Y
self.Z
def Xreturner(self):
return self.X
def Yreturner(self):
return self.Y
def Zreturner(self):
return self.Z
class A:
def __init(self):
self.lst=[[1,2,3],[3,4,5],]
self.b=B()
def add(self): # trying to add b object to the list
self.lst.append(self.b)
#### TEST####
objA=A()
objA.add(6,7,8)
When I test I get this error:
Traceback (most recent call last):
File "home/testXYZ.py", line 28, in <module>
objA.add(6,7,8)
TypeError: add() takes exactly 1 argument (4 given)
Please help me to solve this.
The statement self.X in the __init__ method of B does nothing. You need to put self.X = X.
You are passing parameters to add() but it doesn't take any parameters. Maybe you want to add parameters (in the same way you have some for __init__ of X.
Maybe you even want to pass parameters to the __init__ of A (instead of to A). Then you can pass those to the constructor of B.
First, your class B initializer is incorrect:
class B:
def __init__(self, x, y, z): # <== should use snake_case for vars
self.x = x
self.y = y
self.z = z
Next, your class A add should create a new B object and add to the list:
def add(self, x, y, z):
new_b = B(x, y z)
self.lst.append(new_b)
NOTE on the question below. I think the 'proper' pythonic idiom is to a) create module functions, such as foo_math below, and then call their specific action against an instance within the class itself. The bottom piece of code reflects that approach.
I want to define a classmethod which takes two arguments and returns a value. I want the same method to be able to be called on a class instance with the instance value pass as one of the arguments. Can I do this without defining two distinct methods as I have done here?
class Foo(object):
__init__(x):
self.x = x
#classmethod
def foo_math(cls, x, y):
return x + y
def math(self, y):
return Foo.foo_math(self.x, y)
What I would like is:
>>> Foo.math(3, 4)
7
>>> f = Foo()
>>> f.x = 3
>>> f.math(4)
7
Short of subtyping int, here is my conclusion to this question:
def foo_math(cls, x, y):
return x + y
class Foo(object):
__init__(x):
self.x = x
def foo_math(self, y):
return foo_math(self, y)
i don't recommend doing this, but if you really want, it's this (thank you other guy on stackoverflow for first part):
class staticorinstancemethod(object):
def __init__(self, func):
self.func = func
def __get__(self, instance, owner):
return functools.partial(self.func, instance)
then, do something like
class F(object):
#staticorinstancemethod
def math(instOrNone, v1, v2=None):
return instOrNone.x + v1 if instOrNone else v1 + v2
but maybe you just want to define the __add__ and __radd__ methods...
I don't think that you can call a method from a class without defining an object of that class (class methods don't belong inside the methods of any one class), so things like Foo.math(3, 4) will return a NameError as Foo has not been defined.
With this in mind, you should modify your code to be like this (even though with the problem solved there are still some issues with the code):
# A class method would probably go here somewhere.
class Foo(object):
def __init__(self, x):
self.x = x
def foo_math(self, x, y):
return x + y
def math(self, y):
return self.foo_math(self.x, y)
Then you can do:
>>> f = Foo(3)
>>> f.math(4)
7