Related
I have a situation like so...
class Outer(object):
def some_method(self):
# do something
class Inner(object):
def __init__(self):
self.Outer.some_method() # <-- this is the line in question
How can I access the Outer class's method from the Inner class?
You're trying to access Outer's class instance, from inner class instance. So just use factory-method to build Inner instance and pass Outer instance to it.
class Outer(object):
def createInner(self):
return Outer.Inner(self)
class Inner(object):
def __init__(self, outer_instance):
self.outer_instance = outer_instance
self.outer_instance.somemethod()
def inner_method(self):
self.outer_instance.anothermethod()
The methods of a nested class cannot directly access the instance attributes of the outer class.
Note that it is not necessarily the case that an instance of the outer class exists even when you have created an instance of the inner class.
In fact, it is often recommended against using nested classes, since the nesting does not imply any particular relationship between the inner and outer classes.
maybe I'm mad but this seems very easy indeed - the thing is to make your inner class inside a method of the outer class...
def do_sthg(self):
...
def mess_around(self):
outer_class_self = self
class Mooble():
def do_sthg_different(self):
...
outer_class_self.do_sthg()
Plus... "self" is only used by convention, so you could do this:
def do_sthg(self):
...
def mess_around(outer_class_self):
class Mooble():
def do_sthg_different(self):
...
outer_class_self.do_sthg()
It might be objected that you can't then create this inner class from outside the outer class... but this ain't true:
class Bumblebee():
def do_sthg(self):
print "sthg"
def give_me_an_inner_class(outer_class_self):
class Mooble():
def do_sthg_different(self):
print "something diff\n"
outer_class_self.do_sthg()
return Mooble
then, somewhere miles away:
blob = Bumblebee().give_me_an_inner_class()()
blob.do_sthg_different()
even push the boat out a bit and extend this inner class (NB to get super() to work you have to change the class signature of Mooble to class Mooble(object)).
class InnerBumblebeeWithAddedBounce(Bumblebee().give_me_an_inner_class()):
def bounce(self):
print "bounce"
def do_sthg_different(self):
super(InnerBumblebeeWithAddedBounce, self).do_sthg_different()
print "and more different"
ibwab = InnerBumblebeeWithAddedBounce()
ibwab.bounce()
ibwab.do_sthg_different()
later
mrh1997 raised an interesting point about the non-common inheritance of inner classes delivered using this technique. But it seems that the solution is pretty straightforward:
class Fatty():
def do_sthg(self):
pass
class InnerFatty(object):
pass
def give_me_an_inner_fatty_class(self):
class ExtendedInnerFatty(Fatty.InnerFatty):
pass
return ExtendedInnerFatty
fatty1 = Fatty()
fatty2 = Fatty()
innerFattyClass1 = fatty1.give_me_an_inner_fatty_class()
innerFattyClass2 = fatty2.give_me_an_inner_fatty_class()
print (issubclass(innerFattyClass1, Fatty.InnerFatty))
print (issubclass(innerFattyClass2, Fatty.InnerFatty))
I found this.
Tweaked to suite your question:
class Outer(object):
def some_method(self):
# do something
class _Inner(object):
def __init__(self, outer):
outer.some_method()
def Inner(self):
return _Inner(self)
I’m sure you can somehow write a decorator for this or something
related: What is the purpose of python's inner classes?
A few years late to the party.... but to expand on #mike rodent's wonderful answer, I've provided my own example below that shows just how flexible his solution is, and why it should be (or should have been) the accepted answer.
Python 3.7
class Parent():
def __init__(self, name):
self.name = name
self.children = []
class Inner(object):
pass
def Child(self, name):
parent = self
class Child(Parent.Inner):
def __init__(self, name):
self.name = name
self.parent = parent
parent.children.append(self)
return Child(name)
parent = Parent('Bar')
child1 = parent.Child('Foo')
child2 = parent.Child('World')
print(
# Getting its first childs name
child1.name, # From itself
parent.children[0].name, # From its parent
# Also works with the second child
child2.name,
parent.children[1].name,
# Go nuts if you want
child2.parent.children[0].name,
child1.parent.children[1].name
)
print(
# Getting the parents name
parent.name, # From itself
child1.parent.name, # From its children
child2.parent.name,
# Go nuts again if you want
parent.children[0].parent.name,
parent.children[1].parent.name,
# Or insane
child2.parent.children[0].parent.children[1].parent.name,
child1.parent.children[1].parent.children[0].parent.name
)
# Second parent? No problem
parent2 = Parent('John')
child3 = parent2.Child('Doe')
child4 = parent2.Child('Appleseed')
print(
child3.name, parent2.children[0].name,
child4.name, parent2.children[1].name,
parent2.name # ....
)
Output:
Foo Foo World World Foo World
Bar Bar Bar Bar Bar Bar Bar
Doe Doe Appleseed Appleseed John
Again, a wonderful answer, props to you mike!
You can easily access to outer class using metaclass: after creation of outer class check it's attribute dict for any classes (or apply any logic you need - mine is just trivial example) and set corresponding values:
import six
import inspect
# helper method from `peewee` project to add metaclass
_METACLASS_ = '_metaclass_helper_'
def with_metaclass(meta, base=object):
return meta(_METACLASS_, (base,), {})
class OuterMeta(type):
def __new__(mcs, name, parents, dct):
cls = super(OuterMeta, mcs).__new__(mcs, name, parents, dct)
for klass in dct.values():
if inspect.isclass(klass):
print("Setting outer of '%s' to '%s'" % (klass, cls))
klass.outer = cls
return cls
# #six.add_metaclass(OuterMeta) -- this is alternative to `with_metaclass`
class Outer(with_metaclass(OuterMeta)):
def foo(self):
return "I'm outer class!"
class Inner(object):
outer = None # <-- by default it's None
def bar(self):
return "I'm inner class"
print(Outer.Inner.outer)
>>> <class '__main__.Outer'>
assert isinstance(Outer.Inner.outer(), Outer)
print(Outer().foo())
>>> I'm outer class!
print(Outer.Inner.outer().foo())
>>> I'm outer class!
print(Outer.Inner().outer().foo())
>>> I'm outer class!
print(Outer.Inner().bar())
>>> I'm inner class!
Using this approach, you can easily bind and refer two classes between each other.
I've created some Python code to use an outer class from its inner class, based on a good idea from another answer for this question. I think it's short, simple and easy to understand.
class higher_level__unknown_irrelevant_name__class:
def __init__(self, ...args...):
...other code...
# Important lines to access sub-classes.
subclasses = self._subclass_container()
self.some_subclass = subclasses["some_subclass"]
del subclasses # Free up variable for other use.
def sub_function(self, ...args...):
...other code...
def _subclass_container(self):
_parent_class = self # Create access to parent class.
class some_subclass:
def __init__(self):
self._parent_class = _parent_class # Easy access from self.
# Optional line, clears variable space, but SHOULD NOT BE USED
# IF THERE ARE MULTIPLE SUBCLASSES as would stop their parent access.
# del _parent_class
class subclass_2:
def __init__(self):
self._parent_class = _parent_class
# Return reference(s) to the subclass(es).
return {"some_subclass": some_subclass, "subclass_2": subclass_2}
The main code, "production ready" (without comments, etc.). Remember to replace all of each value in angle brackets (e.g. <x>) with the desired value.
class <higher_level_class>:
def __init__(self):
subclasses = self._subclass_container()
self.<sub_class> = subclasses[<sub_class, type string>]
del subclasses
def _subclass_container(self):
_parent_class = self
class <sub_class>:
def __init__(self):
self._parent_class = _parent_class
return {<sub_class, type string>: <sub_class>}
Explanation of how this method works (the basic steps):
Create a function named _subclass_container to act as a wrapper to access the variable self, a reference to the higher level class (from code running inside the function).
Create a variable named _parent_class which is a reference to the variable self of this function, that the sub-classes of _subclass_container can access (avoids name conflicts with other self variables in subclasses).
Return the sub-class/sub-classes as a dictionary/list so code calling the _subclass_container function can access the sub-classes inside.
In the __init__ function inside the higher level class (or wherever else needed), receive the returned sub-classes from the function _subclass_container into the variable subclasses.
Assign sub-classes stored in the subclasses variable to attributes of the higher level class.
A few tips to make scenarios easier:
Making the code to assign the sub classes to the higher level class easier to copy and be used in classes derived from the higher level class that have their __init__ function changed:
Insert before line 12 in the main code:
def _subclass_init(self):
Then insert into this function lines 5-6 (of the main code) and replace lines 4-7 with the following code:
self._subclass_init(self)
Making subclass assigning to the higher level class possible when there are many/unknown quantities of subclasses.
Replace line 6 with the following code:
for subclass_name in list(subclasses.keys()):
setattr(self, subclass_name, subclasses[subclass_name])
Example scenario of where this solution would be useful and where the higher level class name should be impossible to get:
A class, named "a" (class a:) is created. It has subclasses that need to access it (the parent). One subclass is called "x1". In this subclass, the code a.run_func() is run.
Then another class, named "b" is created, derived from class "a" (class b(a):). After that, some code runs b.x1() (calling the sub function "x1" of b, a derived sub-class). This function runs a.run_func(), calling the function "run_func" of class "a", not the function "run_func" of its parent, "b" (as it should), because the function which was defined in class "a" is set to refer to the function of class "a", as that was its parent.
This would cause problems (e.g. if function a.run_func has been deleted) and the only solution without rewriting the code in class a.x1 would be to redefine the sub-class x1 with updated code for all classes derived from class "a" which would obviously be difficult and not worth it.
Do you mean to use inheritance, rather than nesting classes like this? What you're doing doesn't make a heap of sense in Python.
You can access the Outer's some_method by just referencing Outer.some_method within the inner class's methods, but it's not going to work as you expect it will. For example, if you try this:
class Outer(object):
def some_method(self):
# do something
class Inner(object):
def __init__(self):
Outer.some_method()
...you'll get a TypeError when initialising an Inner object, because Outer.some_method expects to receive an Outer instance as its first argument. (In the example above, you're basically trying to call some_method as a class method of Outer.)
Another possibility:
class _Outer (object):
# Define your static methods here, e.g.
#staticmethod
def subclassRef ():
return Outer
class Outer (_Outer):
class Inner (object):
def outer (self):
return _Outer
def doSomething (self):
outer = self.outer ()
# Call your static mehthods.
cls = outer.subclassRef ()
return cls ()
What we can do is pass the self variable of Outer Class inside the Inner Class as Class Argument and Under Outer init initialise the Inner Class with Outer self passed into Inner
class Outer:
def __init__(self):
self.somevalue=91
self.Inner=self.Inner(self)
def SomeMethod(self):
print('This is Something from Outer Class')
class Inner:
def __init__(self,Outer)
self.SomeMethod=Outer.SomeMethod
self.somevalue=Outer.somevalue
def SomeAnotherMethod(self):
print(self.somevalue)
self.SomeMethod()
>>>f=Outer()
>>>f.Inner.SomeAnotherMethod()
91
This is Something from Outer Class
Now After running this function it Works
Expanding on #tsnorri's cogent thinking, that the outer method may be a static method:
class Outer(object):
#staticmethod
def some_static_method(self):
# do something
class Inner(object):
def __init__(self):
self.some_static_method() # <-- this will work later
Inner.some_static_method = some_static_method
Now the line in question should work by the time it is actually called.
The last line in the above code gives the Inner class a static method that's a clone of the Outer static method.
This takes advantage of two Python features, that functions are objects, and scope is textual.
Usually, the local scope references the local names of the (textually) current function.
...or current class in our case. So objects "local" to the definition of the Outer class (Inner and some_static_method) may be referred to directly within that definition.
You may create a class, to decorate inner classes. In this case #inner.
Since this a decorator: Outer.A = inner(Outer.A). Once your code requires Outer.A it will be executed inner.__get__ method, which returns the original class (A) with a new attribute set on it: A.owner = Outer.
A classmethod in class A, in this case def add(cls, y=3), may use new attribute owner at return cls.owner.x + y + 1.
The line setattr(owner, name, self.inner), breaks the descriptor because owner.name => Outer.A => A is no longer an instance of the class inner.
Hope this helps.
class inner:
def __init__(self, inner):
self.inner = inner
def __get__(self, instance, owner):
print('__get__ method executed, only once... ')
name = self.inner.__name__
setattr(self.inner, 'owner', owner)
setattr(owner, name, self.inner) # breaks descriptor
return self.inner #returns Inner
class Outer:
x = 1
#inner
class A:
#classmethod
def add(cls, y=3):
return cls.owner.x + y + 1
print(Outer.A.add(0)) # First time executes inner.__get__ method
print(Outer.A.add(0)) # Second time not necessary.
>> __get__ method executed, only once...
>> 2
>> 2
It can be done by parsing the outer class object into inner class.
class Outer():
def __init__(self,userinput):
self.userinput = userinput
def outer_function(self):
self.a = self.userinput + 2
class Inner():
def inner_function(self):
self.b = self.a + 10
after defining this, it need to run the function
m = Outer(3)
m.outer_function()
print (m.a)
#this will output 5
Now it has the variable of outer class.
and then, it need to run inner class functions.
m.Inner.inner_function(m)
The object m of outer class is parsed into the function of inner class (inside the brackets)
Now, the inner class function is accessing self.a from the outer class.
print (m.b)
#this will output 15
It is too simple:
Input:
class A:
def __init__(self):
pass
def func1(self):
print('class A func1')
class B:
def __init__(self):
a1 = A()
a1.func1()
def func1(self):
print('class B func1')
b = A.B()
b.func1()
Output
class A func1
class B func1
Whenever I define a class whose instances create objects of other classes, I like defining the types of those other objects as class members:
class Foo(object):
DICT_TYPE = dict # just a trivial example
def __init__(self):
self.mydict = self.DICT_TYPE()
class Bar(Foo):
DICT_TYPE = OrderedDict # no need to override __init__ now
The idea is to allow potential subclasses to easily override it.
I've just found a problem with this habbit, when the "type" I use is not really a type, but a factory function. For example, RLock is confusingly not a class:
def RLock(*args, **kwargs):
return _RLock(*args, **kwargs)
Thus using it the same way is no good:
class Foo(object):
LOCK_TYPE = threading.RLock # alas, RLock() is a function...
def __init__(self):
self.lock = self.LOCK_TYPE()
The problem here is that since RLock is a function, self.LOCK_TYPE gets bound to self, resulting with a bound-method, consequently leading to an error.
Here's a quick demonstration of how things go wrong when a function is used instead of a class (for a case simpler than RLock above):
def dict_factory():
return {}
class Foo(object):
DICT_TYPE1 = dict
DICT_TYPE2 = dict_factory
f = Foo()
f.DICT_TYPE1()
=> {}
f.DICT_TYPE2()
=> TypeError: dict_factory() takes no arguments (1 given)
Does anybody have a good solution for this problem? Is my habbit of defining those class members fundamentally wrong?
I guess I could replace it with a factory method. Would that be a better approach?
class Foo(object);
def __init__(self):
self.lock = self._make_lock()
def _make_lock(self):
return threading.RLock()
you could use the staticmethod decorator to ensure your class does not get passed in
>>> class Foo(object):
... DICT_TYPE = staticmethod(my_dict)
...
>>> f = Foo()
>>> f.DICT_TYPE()
{}
The problem can be bypassed by using a classproperty (e.g. as defined in this answer):
class Foo(object):
#classproperty
def DICT_TYPE(cls):
return dict_factory
I would like to create a class in Python that manages above all static members. These members should be initiliazed during definition of the class already. Due to the fact that there will be the requirement to reinitialize the static members later on I would put this code into a classmethod.
My question: How can I call this classmethod from inside the class?
class Test():
# static member
x = None
# HERE I WOULD LOVE TO CALL SOMEHOW static_init!
# initialize static member in classmethod, so that it can be
#reinitialized later on again
#classmethod
def static_init(cls):
cls.x = 10
Any help is appreciated!
Thanks in advance,
Volker
At the time that x=10 is executed in your example, not only does the class not exist, but the classmethod doesn't exist either.
Execution in Python goes top to bottom. If x=10 is above the classmethod, there is no way you can access the classmethod at that point, because it hasn't been defined yet.
Even if you could run the classmethod, it wouldn't matter, because the class doesn't exist yet, so the classmethod couldn't refer to it. The class is not created until after the entire class block runs, so while you're inside the class block, there's no class.
If you want to factor out some class initialization so you can re-run it later in the way you describe, use a class decorator. The class decorator runs after the class is created, so it can call the classmethod just fine.
>>> def deco(cls):
... cls.initStuff()
... return cls
>>> #deco
... class Foo(object):
... x = 10
...
... #classmethod
... def initStuff(cls):
... cls.x = 88
>>> Foo.x
88
>>> Foo.x = 10
>>> Foo.x
10
>>> Foo.initStuff() # reinitialize
>>> Foo.x
88
You call a class method by appending the class name likewise:
class.method
In your code something like this should suffice:
Test.static_init()
You could also do this:
static_init(Test)
To call it inside your class, have your code do this:
Test.static_init()
My working code:
class Test(object):
#classmethod
def static_method(cls):
print("Hello")
def another_method(self):
Test.static_method()
and Test().another_method() returns Hello
You can't call a classmethod in the class definition because the class hasn't been fully defined yet, so there's nothing to pass the method as its first cls argument...a classic chicken-and-egg problem. However you can work around this limitation by overloading the __new__() method in a metaclass, and calling the classmethod from there after the class has been created as illustrated below:
class Test(object):
# nested metaclass definition
class __metaclass__(type):
def __new__(mcl, classname, bases, classdict):
cls = type.__new__(mcl, classname, bases, classdict) # creates class
cls.static_init() # call the classmethod
return cls
x = None
#classmethod
def static_init(cls): # called by metaclass when class is defined
print("Hello")
cls.x = 10
print Test.x
Output:
Hello
10
After re-reading your question carefully this time I can think of two solutions. The first one is to apply the Borg design pattern. The second one is to discard the class method and use a module level function instead. This appears to solve your problem:
def _test_static_init(value):
return value, value * 2
class Test:
x, y = _test_static_init(20)
if __name__ == "__main__":
print Test.x, Test.y
Old, incorrect answer:
Here's an example, I hope it helps:
class Test:
x = None
#classmethod
def set_x_class(cls, value):
Test.x = value
def set_x_self(self):
self.__class__.set_x_class(10)
if __name__ == "__main__":
obj = Test()
print Test.x
obj.set_x_self()
print Test.x
obj.__class__.set_x_class(15)
print Test.x
Anyway, NlightNFotis's answer is a better one: use the class name when accessing the class methods. It makes your code less obscure.
This seems like a reasonable solution:
from __future__ import annotations
from typing import ClassVar, Dict
import abc
import string
class Cipher(abc.ABC):
#abc.abstractmethod
def encrypt(self, plaintext: str) -> str:
pass
#abc.abstractmethod
def decrypt(self, ciphertext: str) -> str:
pass
class RotateCipher(Cipher, abc.ABC):
#staticmethod
def rotate(n: int) -> str:
return string.ascii_uppercase[n:] + string.ascii_uppercase[:n]
class VigenereCipher(RotateCipher):
_TABLE: ClassVar[Dict[str, str]] = dict({(chr(i + ord("A")), RotateCipher.rotate(i)) for i in range(26)})
def encrypt(self, plaintext: str) -> str:
pass
def decrypt(self, plaintext: str) -> str:
pass
vc = VigenereCipher()
The method is now a static method of the cipher, nothing outside the classes is referenced. You could opt to name RotateCipher _RotateCipher instead, if you don't want people using it by itself.
Note: I removed the Final, as I ran this on 3.7, but after reading the documentation on Final, I don't think it would affect the solution? Also added an import for string which the question was missing. And finally added an implementation for the abstract methods, alternatively, could have let VigenereCipher inherit from abc.ABC as well.
If your classmethod is not used very often do a lazy evaluation
class A() {
# this does not work: x=A.initMe()
#classmethod
def initMe(cls) {
if not hasattr(cls,"x"):
# your code her
cls.x=# your result
pass
#classmethod
def f1(cls) {
# needs initMe
cls.initMe()
# more code using cls.x
}
}
I've read What are Class methods in Python for? but the examples in that post are complex. I am looking for a clear, simple, bare-bones example of a particular use case for classmethods in Python.
Can you name a small, specific example use case where a Python classmethod would be the right tool for the job?
Helper methods for initialization:
class MyStream(object):
#classmethod
def from_file(cls, filepath, ignore_comments=False):
with open(filepath, 'r') as fileobj:
for obj in cls(fileobj, ignore_comments):
yield obj
#classmethod
def from_socket(cls, socket, ignore_comments=False):
raise NotImplemented # Placeholder until implemented
def __init__(self, iterable, ignore_comments=False):
...
Well __new__ is a pretty important classmethod. It's where instances usually come from
so dict() calls dict.__new__ of course, but there is another handy way to make dicts sometimes which is the classmethod dict.fromkeys()
eg.
>>> dict.fromkeys("12345")
{'1': None, '3': None, '2': None, '5': None, '4': None}
I don't know, something like named constructor methods?
class UniqueIdentifier(object):
value = 0
def __init__(self, name):
self.name = name
#classmethod
def produce(cls):
instance = cls(cls.value)
cls.value += 1
return instance
class FunkyUniqueIdentifier(UniqueIdentifier):
#classmethod
def produce(cls):
instance = super(FunkyUniqueIdentifier, cls).produce()
instance.name = "Funky %s" % instance.name
return instance
Usage:
>>> x = UniqueIdentifier.produce()
>>> y = FunkyUniqueIdentifier.produce()
>>> x.name
0
>>> y.name
Funky 1
The biggest reason for using a #classmethod is in an alternate constructor that is intended to be inherited. This can be very useful in polymorphism. An example:
class Shape(object):
# this is an abstract class that is primarily used for inheritance defaults
# here is where you would define classmethods that can be overridden by inherited classes
#classmethod
def from_square(cls, square):
# return a default instance of cls
return cls()
Notice that Shape is an abstract class that defines a classmethod from_square, since Shape is not really defined, it does not really know how to derive itself from a Square so it simply returns a default instance of the class.
Inherited classes are then allowed to define their own versions of this method:
class Square(Shape):
def __init__(self, side=10):
self.side = side
#classmethod
def from_square(cls, square):
return cls(side=square.side)
class Rectangle(Shape):
def __init__(self, length=10, width=10):
self.length = length
self.width = width
#classmethod
def from_square(cls, square):
return cls(length=square.side, width=square.side)
class RightTriangle(Shape):
def __init__(self, a=10, b=10):
self.a = a
self.b = b
self.c = ((a*a) + (b*b))**(.5)
#classmethod
def from_square(cls, square):
return cls(a=square.length, b=square.width)
class Circle(Shape):
def __init__(self, radius=10):
self.radius = radius
#classmethod
def from_square(cls, square):
return cls(radius=square.length/2)
The usage allows you to treat all of these uninstantiated classes polymorphically
square = Square(3)
for polymorphic_class in (Square, Rectangle, RightTriangle, Circle):
this_shape = polymorphic_class.from_square(square)
This is all fine and dandy you might say, but why couldn't I just use as #staticmethod to accomplish this same polymorphic behavior:
class Circle(Shape):
def __init__(self, radius=10):
self.radius = radius
#staticmethod
def from_square(square):
return Circle(radius=square.length/2)
The answer is that you could, but you do not get the benefits of inheritance because Circle has to be called out explicitly in the method. Meaning if I call it from an inherited class without overriding, I would still get Circle every time.
Notice what is gained when I define another shape class that does not really have any custom from_square logic:
class Hexagon(Shape):
def __init__(self, side=10):
self.side = side
# note the absence of classmethod here, this will use from_square it inherits from shape
Here you can leave the #classmethod undefined and it will use the logic from Shape.from_square while retaining who cls is and return the appropriate shape.
square = Square(3)
for polymorphic_class in (Square, Rectangle, RightTriangle, Circle, Hexagon):
this_shape = polymorphic_class.from_square(square)
I find that I most often use #classmethod to associate a piece of code with a class, to avoid creating a global function, for cases where I don't require an instance of the class to use the code.
For example, I might have a data structure which only considers a key valid if it conforms to some pattern. I may want to use this from inside and outside of the class. However, I don't want to create yet another global function:
def foo_key_is_valid(key):
# code for determining validity here
return valid
I'd much rather group this code with the class it's associated with:
class Foo(object):
#classmethod
def is_valid(cls, key):
# code for determining validity here
return valid
def add_key(self, key, val):
if not Foo.is_valid(key):
raise ValueError()
..
# lets me reuse that method without an instance, and signals that
# the code is closely-associated with the Foo class
Foo.is_valid('my key')
Another useful example of classmethod is in extending enumerated types. A classic Enum provides symbolic names which can be used later in the code for readability, grouping, type-safety, etc. This can be extended to add useful features using a classmethod. In the example below, Weekday is an enuerated type for the days of the week. It has been extended using classmethod so that instead of keeping track of the weekday ourselves, the enumerated type can extract the date and return the related enum member.
from enum import Enum
from datetime import date
class Weekday(Enum):
MONDAY = 1
TUESDAY = 2
WEDNESDAY = 3
THURSDAY = 4
FRIDAY = 5
SATURDAY = 6
SUNDAY = 7
#
#classmethod
def from_date(cls, date):
return cls(date.isoweekday())
Weekday.from_date(date.today())
<Weekday.TUESDAY: 2>
Source: https://docs.python.org/3/howto/enum.html
in class MyClass(object):
'''
classdocs
'''
obj=0
x=classmethod
def __init__(self):
'''
Constructor
'''
self.nom='lamaizi'
self.prenom='anas'
self.age=21
self.ville='Casablanca'
if __name__:
ob=MyClass()
print(ob.nom)
print(ob.prenom)
print(ob.age)
print(ob.ville)
I would like to replace an object instance by another instance inside a method like this:
class A:
def method1(self):
self = func(self)
The object is retrieved from a database.
It is unlikely that replacing the 'self' variable will accomplish whatever you're trying to do, that couldn't just be accomplished by storing the result of func(self) in a different variable. 'self' is effectively a local variable only defined for the duration of the method call, used to pass in the instance of the class which is being operated upon. Replacing self will not actually replace references to the original instance of the class held by other objects, nor will it create a lasting reference to the new instance which was assigned to it.
As far as I understand, If you are trying to replace the current object with another object of same type (assuming func won't change the object type) from an member function. I think this will achieve that:
class A:
def method1(self):
newObj = func(self)
self.__dict__.update(newObj.__dict__)
It is not a direct answer to the question, but in the posts below there's a solution for what amirouche tried to do:
Python object conversion
Can I dynamically convert an instance of one class to another?
And here's working code sample (Python 3.2.5).
class Men:
def __init__(self, name):
self.name = name
def who_are_you(self):
print("I'm a men! My name is " + self.name)
def cast_to(self, sex, name):
self.__class__ = sex
self.name = name
def method_unique_to_men(self):
print('I made The Matrix')
class Women:
def __init__(self, name):
self.name = name
def who_are_you(self):
print("I'm a women! My name is " + self.name)
def cast_to(self, sex, name):
self.__class__ = sex
self.name = name
def method_unique_to_women(self):
print('I made Cloud Atlas')
men = Men('Larry')
men.who_are_you()
#>>> I'm a men! My name is Larry
men.method_unique_to_men()
#>>> I made The Matrix
men.cast_to(Women, 'Lana')
men.who_are_you()
#>>> I'm a women! My name is Lana
men.method_unique_to_women()
#>>> I made Cloud Atlas
Note the self.__class__ and not self.__class__.__name__. I.e. this technique not only replaces class name, but actually converts an instance of a class (at least both of them have same id()). Also, 1) I don't know whether it is "safe to replace a self object by another object of the same type in [an object own] method"; 2) it works with different types of objects, not only with ones that are of the same type; 3) it works not exactly like amirouche wanted: you can't init class like Class(args), only Class() (I'm not a pro and can't answer why it's like this).
Yes, all that will happen is that you won't be able to reference the current instance of your class A (unless you set another variable to self before you change it.) I wouldn't recommend it though, it makes for less readable code.
Note that you're only changing a variable, just like any other. Doing self = 123 is the same as doing abc = 123. self is only a reference to the current instance within the method. You can't change your instance by setting self.
What func(self) should do is to change the variables of your instance:
def func(obj):
obj.var_a = 123
obj.var_b = 'abc'
Then do this:
class A:
def method1(self):
func(self) # No need to assign self here
In many cases, a good way to achieve what you want is to call __init__ again. For example:
class MyList(list):
def trim(self,n):
self.__init__(self[:-n])
x = MyList([1,2,3,4])
x.trim(2)
assert type(x) == MyList
assert x == [1,2]
Note that this comes with a few assumptions such as the all that you want to change about the object being set in __init__. Also beware that this could cause problems with inheriting classes that redefine __init__ in an incompatible manner.
Yes, there is nothing wrong with this. Haters gonna hate. (Looking at you Pycharm with your in most cases imaginable, there's no point in such reassignment and it indicates an error).
A situation where you could do this is:
some_method(self, ...):
...
if(some_condition):
self = self.some_other_method()
...
return ...
Sure, you could start the method body by reassigning self to some other variable, but if you wouldn't normally do that with other parametres, why do it with self?
One can use the self assignment in a method, to change the class of instance to a derived class.
Of course one could assign it to a new object, but then the use of the new object ripples through the rest of code in the method. Reassiging it to self, leaves the rest of the method untouched.
class aclass:
def methodA(self):
...
if condition:
self = replace_by_derived(self)
# self is now referencing to an instance of a derived class
# with probably the same values for its data attributes
# all code here remains untouched
...
self.methodB() # calls the methodB of derivedclass is condition is True
...
def methodB(self):
# methodB of class aclass
...
class derivedclass(aclass):
def methodB(self):
#methodB of class derivedclass
...
But apart from such a special use case, I don't see any advantages to replace self.
You can make the instance a singleton element of the class
and mark the methods with #classmethod.
from enum import IntEnum
from collections import namedtuple
class kind(IntEnum):
circle = 1
square = 2
def attr(y): return [getattr(y, x) for x in 'k l b u r'.split()]
class Shape(namedtuple('Shape', 'k,l,b,u,r')):
self = None
#classmethod
def __repr__(cls):
return "<Shape({},{},{},{},{}) object at {}>".format(
*(attr(cls.self)+[id(cls.self)]))
#classmethod
def transform(cls, func):
cls.self = cls.self._replace(**func(cls.self))
Shape.self = Shape(k=1, l=2, b=3, u=4, r=5)
s = Shape.self
def nextkind(self):
return {'k': self.k+1}
print(repr(s)) # <Shape(1,2,3,4,5) object at 139766656561792>
s.transform(nextkind)
print(repr(s)) # <Shape(2,2,3,4,5) object at 139766656561888>