overriding equals operation

[Pradipto Banerjee]

I am trying to define class, where if I use a statement a = b, then instead of "a" pointing to the same instance as "b", it should point to a copy of "b", but I can't get it right.

Currently, I have the following:

----

class myclass(object):
def __init__(self, name='')
self.name = name

def copy(self):
newvar = myclass(self.name)
return newvar

def __eq__(self, other):
if instance(other, myclass):
return self == other.copy()
return NotImplemented
----

Now if I try:
'test2'

I wanted b=a to make a new copy of "a", but then when I assigned b.name = 'test2', even a.name became 'test2'.

How can I rectify my code to make the __eq__() behave like copy()?

Thanks


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[Thomas Rachel]

Am 16.10.2012 15:51 schrieb Pradipto Banerjee:

I am trying to define class, where if I use a statement a = b, then instead of "a" pointing to the same instance as "b", it should point to a copy of "b", but I can't get it right.

This is not possible.

Currently, I have the following:

Myclass or MyClass, see http://www.python.org/dev/peps/pep-0008/.

def __eq__(self, other):
if instance(other, myclass):
return self == other.copy()
return NotImplemented

This redefines the == operator, not the = operator.

It is not possible to redefine =.

One way could be to override assignment of a class attribute. But this
won't be enough, I think.

Let me explain:

class MyContainer(object):
@property
def content(self):
return self._content
@content.setter
def content(self, new):
self._content = new.copy()

Then you can do:

a = MyClass()
b = MyContainer()
b.content = a
print b.content is a # should print False; untested...

But something like

a = MyClass()
b = a

will always lead to "b is a".

This communication is for informational purposes only. It is not
intended to be, nor should it be construed or used as, financial,
legal, tax or investment advice or an offer to sell, or a
solicitation of any offer to buy, an interest in any fund advised by
Ada Investment Management LP, the Investment advisor.

What?


Thomas

 

[Nobody]

I am trying to define class, where if I use a statement a = b, then
instead of "a" pointing to the same instance as "b", it should point to a
copy of "b", but I can't get it right.

It cannot be done.

Name binding ("variable = value") is a language primitive; it is not
delegated to the object on the LHS (if there even is one; the name doesn't
have to exist at the point that the statement is executed).

You'll have to change the syntax to something which is delegated to
objects, e.g. "a[:] = b" will call "a.__setitem__(slice(None), b)"

 

[88888 Dihedral]

Pradipto Banerjeeæ–¼ 2012å¹´10月16日星期二UTC+8下åˆ9時59分05秒寫é“：

I am trying to define class, where if I use a statement a = b, then instead of "a" pointing to the same instance as "b", it should point to a copyof "b", but I can't get it right.



Currently, I have the following:



----



class myclass(object):

def __init__(self, name='')

self.name = name



def copy(self):

newvar = myclass(self.name)

return newvar



def __eq__(self, other):

if instance(other, myclass):

return self == other.copy()

return NotImplemented

----



Now if I try:

What you really want is b=a.copy()
not b=a to disentangle two objects.

__eq__ is used in the comparison operation.

 

[88888 Dihedral]

Pradipto Banerjeeæ–¼ 2012å¹´10月16日星期二UTC+8下åˆ9時59分05秒寫é“：

I am trying to define class, where if I use a statement a = b, then instead of "a" pointing to the same instance as "b", it should point to a copyof "b", but I can't get it right.



Currently, I have the following:



----



class myclass(object):

def __init__(self, name='')

self.name = name



def copy(self):

newvar = myclass(self.name)

return newvar



def __eq__(self, other):

if instance(other, myclass):

return self == other.copy()

return NotImplemented

----



Now if I try:

What you really want is b=a.copy()
not b=a to disentangle two objects.

__eq__ is used in the comparison operation.

 

[Mark Lawrence]

What you really want is b=a.copy()
not b=a to disentangle two objects.

__eq__ is used in the comparison operation.

The winner Smartest Answer by a Bot Award 2012