Python prevent copying object as reference

Is it possible to copy an object in Python without copying a reference?

For example, if I define a class

class SomeClass:

    def __init__(self):

        self.value = 0

and then create an instance

someObject = SomeClass()

someObject.value = 12

and I try to copy it to another instance:

anotherObject = someObject

and try to modify a property,

anotherObject.value = 10

the original property gets modified:

print someObject.value #prints 10

Is there any way to prevent this from happening? To clarify, I want the anotherObject.value to contain 10, but I want someObject.value to still contain the original 12. Is this possible in python?

Thanks in advance.

asked Oct 6 '13 at 16:12

Raghav Malik

158111

add a comment |

Is it possible to copy an object in Python without copying a reference?

For example, if I define a class

class SomeClass:

    def __init__(self):

        self.value = 0

and then create an instance

someObject = SomeClass()

someObject.value = 12

and I try to copy it to another instance:

anotherObject = someObject

and try to modify a property,

anotherObject.value = 10

the original property gets modified:

print someObject.value #prints 10

Thanks in advance.

asked Oct 6 '13 at 16:12

Raghav Malik

158111

add a comment |

Is it possible to copy an object in Python without copying a reference?

For example, if I define a class

class SomeClass:

    def __init__(self):

        self.value = 0

and then create an instance

someObject = SomeClass()

someObject.value = 12

and I try to copy it to another instance:

anotherObject = someObject

and try to modify a property,

anotherObject.value = 10

the original property gets modified:

print someObject.value #prints 10

Thanks in advance.

asked Oct 6 '13 at 16:12

Raghav Malik

158111

Is it possible to copy an object in Python without copying a reference?

For example, if I define a class

class SomeClass:

    def __init__(self):

        self.value = 0

and then create an instance

someObject = SomeClass()

someObject.value = 12

and I try to copy it to another instance:

anotherObject = someObject

and try to modify a property,

anotherObject.value = 10

the original property gets modified:

print someObject.value #prints 10

Thanks in advance.

python

asked Oct 6 '13 at 16:12

Raghav Malik

158111

asked Oct 6 '13 at 16:12

Raghav Malik

158111

asked Oct 6 '13 at 16:12

Raghav Malik

158111

asked Oct 6 '13 at 16:12

Raghav Malik

158111

asked Oct 6 '13 at 16:12

Raghav Malik

158111

add a comment |

5 Answers
5

active

oldest

votes

The problem is, with

anotherObject = someObject

you don't copy the object, but just add another reference to it. To copy an object, try this:

from copy import copy



anotherObject = copy(someObject)

answered Oct 6 '13 at 16:17

Constantinius

23.9k65279

add a comment |

import copy



obj2 = copy.deepcopy(obj2)

answered Oct 6 '13 at 16:17

Steve Barnes

20.5k43750

add a comment |

As you've noticed, anotherObject = someObject doesn't make a copy - if you want a copy, try

import copy

otherObject = copy.copy(someObject)

The copy.copy vs copy.deepcopy distinction is important here - you can get away with copy.copy for the simple object you've described, but a more nested object would require copy.deepcopy.

copy.copy(someObject) makes a copy only of the object someObject, but if someObject contains references to other objects that can be changed ("mutable" objects) as in

someObject.value.this_one_has_values_too = 4

someObject.value[0] = 1

someObject.value['key'] = 'value'

then references to those objects will be made in the copy. If you use copy.deepcopy, they'll also be copied over.

A great way to gain an understanding of this is playing with the Online Python Tutor (see linked example) but here's a straightforward demonstration of the behavior without the useful diagraming Online Python Tutor provides.

>>> import copy

>>> class Foo(object):

...     pass

...

>>> f = Foo()

>>> f.value = 1

>>> f.nested_value = [2,3,4]

>>> deep = copy.deepcopy(f)

>>> deep.value = 5

>>> f.value

1

>>> deep.nested_value.append(6)

>>> f.nested_value

[2, 3, 4]

>>> shallow = copy.copy(f)

>>> shallow.value = 7

>>> f.value

1

>>> shallow.nested_value.append(8)

>>> f.nested_value

[2, 3, 4, 8]

Edit: But what about the integer in the first example? It's actually being shared between the f object and the shallow object, but that's not a problem - it's not editable; we can't change an integer object like 1 to be any different, so we might as well save memory and use that one object anytime any Python object needs a reference to 1.

The thing to read about this is Ned's Facts and Myths about Python names and values.

edited Oct 7 '13 at 3:09

answered Oct 6 '13 at 16:18

Thomas

4,77412340

What is the difference between a copy and a deepcopy? Also, what do you mean by nested object? I'm new to python and I just want to learn as much as possible.

– Raghav Malik
Oct 6 '13 at 16:25

@coder108 does this make sense? If not, please let me know what doesn't so I can improve the answer.

– Thomas
Oct 6 '13 at 16:33

Oh, I see what you mean by nested objects. Just out of curiosity, isn't an integer also an object? Why isn't deepcopy necessary for that? Thank you for the link, btw. It is very useful.

– Raghav Malik
Oct 6 '13 at 16:47

@coder108 terrific question - both objects actually use the same integer. Why isn't this a problem? Because it's an immutable object; there's no way to take an integer object 1 and change it.

– Thomas
Oct 6 '13 at 16:53

That makes sense. Thank you so much!

– Raghav Malik
Oct 6 '13 at 20:19

add a comment |

You can use copy for that. Try the following:

someObject = SomeClass()

someObject.value = 12

anotherObject = copy.copy(someObject)

anotherObject.value = 10

Now someObject.value will still be 12. You will have to place import copy at the top of your script though.

answered Oct 6 '13 at 16:18

Patrick Kostjens

4,24462237

add a comment |

import copy

In some cases just:

copy_obj = copy.copy(obj)

is enough but in other cases you we have to use:

copy_obj = copy.deepcopy(obj)

edited Nov 20 '18 at 14:56

answered Nov 20 '18 at 14:21

Juba FOURALI

193

add a comment |

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5 Answers
5

active

oldest

votes

5 Answers
5

active

oldest

votes

The problem is, with

anotherObject = someObject

you don't copy the object, but just add another reference to it. To copy an object, try this:

from copy import copy



anotherObject = copy(someObject)

answered Oct 6 '13 at 16:17

Constantinius

23.9k65279

add a comment |

The problem is, with

anotherObject = someObject

you don't copy the object, but just add another reference to it. To copy an object, try this:

from copy import copy



anotherObject = copy(someObject)

answered Oct 6 '13 at 16:17

Constantinius

23.9k65279

add a comment |

The problem is, with

anotherObject = someObject

you don't copy the object, but just add another reference to it. To copy an object, try this:

from copy import copy



anotherObject = copy(someObject)

answered Oct 6 '13 at 16:17

Constantinius

23.9k65279

The problem is, with

anotherObject = someObject

you don't copy the object, but just add another reference to it. To copy an object, try this:

from copy import copy



anotherObject = copy(someObject)

answered Oct 6 '13 at 16:17

Constantinius

23.9k65279

answered Oct 6 '13 at 16:17

Constantinius

23.9k65279

answered Oct 6 '13 at 16:17

Constantinius

23.9k65279

answered Oct 6 '13 at 16:17

Constantinius

23.9k65279

add a comment |

import copy



obj2 = copy.deepcopy(obj2)

answered Oct 6 '13 at 16:17

Steve Barnes

20.5k43750

add a comment |

import copy



obj2 = copy.deepcopy(obj2)

answered Oct 6 '13 at 16:17

Steve Barnes

20.5k43750

add a comment |

import copy



obj2 = copy.deepcopy(obj2)

answered Oct 6 '13 at 16:17

Steve Barnes

20.5k43750

import copy



obj2 = copy.deepcopy(obj2)

answered Oct 6 '13 at 16:17

Steve Barnes

20.5k43750

answered Oct 6 '13 at 16:17

Steve Barnes

20.5k43750

answered Oct 6 '13 at 16:17

Steve Barnes

20.5k43750

answered Oct 6 '13 at 16:17

Steve Barnes

20.5k43750

add a comment |

As you've noticed, anotherObject = someObject doesn't make a copy - if you want a copy, try

import copy

otherObject = copy.copy(someObject)

The copy.copy vs copy.deepcopy distinction is important here - you can get away with copy.copy for the simple object you've described, but a more nested object would require copy.deepcopy.

copy.copy(someObject) makes a copy only of the object someObject, but if someObject contains references to other objects that can be changed ("mutable" objects) as in

someObject.value.this_one_has_values_too = 4

someObject.value[0] = 1

someObject.value['key'] = 'value'

then references to those objects will be made in the copy. If you use copy.deepcopy, they'll also be copied over.

>>> import copy

>>> class Foo(object):

...     pass

...

>>> f = Foo()

>>> f.value = 1

>>> f.nested_value = [2,3,4]

>>> deep = copy.deepcopy(f)

>>> deep.value = 5

>>> f.value

1

>>> deep.nested_value.append(6)

>>> f.nested_value

[2, 3, 4]

>>> shallow = copy.copy(f)

>>> shallow.value = 7

>>> f.value

1

>>> shallow.nested_value.append(8)

>>> f.nested_value

[2, 3, 4, 8]

The thing to read about this is Ned's Facts and Myths about Python names and values.

edited Oct 7 '13 at 3:09

answered Oct 6 '13 at 16:18

Thomas

4,77412340

What is the difference between a copy and a deepcopy? Also, what do you mean by nested object? I'm new to python and I just want to learn as much as possible.

– Raghav Malik
Oct 6 '13 at 16:25

@coder108 does this make sense? If not, please let me know what doesn't so I can improve the answer.

– Thomas
Oct 6 '13 at 16:33

Oh, I see what you mean by nested objects. Just out of curiosity, isn't an integer also an object? Why isn't deepcopy necessary for that? Thank you for the link, btw. It is very useful.

– Raghav Malik
Oct 6 '13 at 16:47

@coder108 terrific question - both objects actually use the same integer. Why isn't this a problem? Because it's an immutable object; there's no way to take an integer object 1 and change it.

– Thomas
Oct 6 '13 at 16:53

That makes sense. Thank you so much!

– Raghav Malik
Oct 6 '13 at 20:19

add a comment |

As you've noticed, anotherObject = someObject doesn't make a copy - if you want a copy, try

import copy

otherObject = copy.copy(someObject)

The copy.copy vs copy.deepcopy distinction is important here - you can get away with copy.copy for the simple object you've described, but a more nested object would require copy.deepcopy.

copy.copy(someObject) makes a copy only of the object someObject, but if someObject contains references to other objects that can be changed ("mutable" objects) as in

someObject.value.this_one_has_values_too = 4

someObject.value[0] = 1

someObject.value['key'] = 'value'

then references to those objects will be made in the copy. If you use copy.deepcopy, they'll also be copied over.

>>> import copy

>>> class Foo(object):

...     pass

...

>>> f = Foo()

>>> f.value = 1

>>> f.nested_value = [2,3,4]

>>> deep = copy.deepcopy(f)

>>> deep.value = 5

>>> f.value

1

>>> deep.nested_value.append(6)

>>> f.nested_value

[2, 3, 4]

>>> shallow = copy.copy(f)

>>> shallow.value = 7

>>> f.value

1

>>> shallow.nested_value.append(8)

>>> f.nested_value

[2, 3, 4, 8]

The thing to read about this is Ned's Facts and Myths about Python names and values.

edited Oct 7 '13 at 3:09

answered Oct 6 '13 at 16:18

Thomas

4,77412340

What is the difference between a copy and a deepcopy? Also, what do you mean by nested object? I'm new to python and I just want to learn as much as possible.

– Raghav Malik
Oct 6 '13 at 16:25

@coder108 does this make sense? If not, please let me know what doesn't so I can improve the answer.

– Thomas
Oct 6 '13 at 16:33

Oh, I see what you mean by nested objects. Just out of curiosity, isn't an integer also an object? Why isn't deepcopy necessary for that? Thank you for the link, btw. It is very useful.

– Raghav Malik
Oct 6 '13 at 16:47

@coder108 terrific question - both objects actually use the same integer. Why isn't this a problem? Because it's an immutable object; there's no way to take an integer object 1 and change it.

– Thomas
Oct 6 '13 at 16:53

That makes sense. Thank you so much!

– Raghav Malik
Oct 6 '13 at 20:19

add a comment |

As you've noticed, anotherObject = someObject doesn't make a copy - if you want a copy, try

import copy

otherObject = copy.copy(someObject)

The copy.copy vs copy.deepcopy distinction is important here - you can get away with copy.copy for the simple object you've described, but a more nested object would require copy.deepcopy.

copy.copy(someObject) makes a copy only of the object someObject, but if someObject contains references to other objects that can be changed ("mutable" objects) as in

someObject.value.this_one_has_values_too = 4

someObject.value[0] = 1

someObject.value['key'] = 'value'

then references to those objects will be made in the copy. If you use copy.deepcopy, they'll also be copied over.

>>> import copy

>>> class Foo(object):

...     pass

...

>>> f = Foo()

>>> f.value = 1

>>> f.nested_value = [2,3,4]

>>> deep = copy.deepcopy(f)

>>> deep.value = 5

>>> f.value

1

>>> deep.nested_value.append(6)

>>> f.nested_value

[2, 3, 4]

>>> shallow = copy.copy(f)

>>> shallow.value = 7

>>> f.value

1

>>> shallow.nested_value.append(8)

>>> f.nested_value

[2, 3, 4, 8]

The thing to read about this is Ned's Facts and Myths about Python names and values.

edited Oct 7 '13 at 3:09

answered Oct 6 '13 at 16:18

Thomas

4,77412340

As you've noticed, anotherObject = someObject doesn't make a copy - if you want a copy, try

import copy

otherObject = copy.copy(someObject)

The copy.copy vs copy.deepcopy distinction is important here - you can get away with copy.copy for the simple object you've described, but a more nested object would require copy.deepcopy.

copy.copy(someObject) makes a copy only of the object someObject, but if someObject contains references to other objects that can be changed ("mutable" objects) as in

someObject.value.this_one_has_values_too = 4

someObject.value[0] = 1

someObject.value['key'] = 'value'

then references to those objects will be made in the copy. If you use copy.deepcopy, they'll also be copied over.

>>> import copy

>>> class Foo(object):

...     pass

...

>>> f = Foo()

>>> f.value = 1

>>> f.nested_value = [2,3,4]

>>> deep = copy.deepcopy(f)

>>> deep.value = 5

>>> f.value

1

>>> deep.nested_value.append(6)

>>> f.nested_value

[2, 3, 4]

>>> shallow = copy.copy(f)

>>> shallow.value = 7

>>> f.value

1

>>> shallow.nested_value.append(8)

>>> f.nested_value

[2, 3, 4, 8]

The thing to read about this is Ned's Facts and Myths about Python names and values.

edited Oct 7 '13 at 3:09

answered Oct 6 '13 at 16:18

Thomas

4,77412340

edited Oct 7 '13 at 3:09

answered Oct 6 '13 at 16:18

Thomas

4,77412340

answered Oct 6 '13 at 16:18

Thomas

4,77412340

answered Oct 6 '13 at 16:18

Thomas

4,77412340

What is the difference between a copy and a deepcopy? Also, what do you mean by nested object? I'm new to python and I just want to learn as much as possible.

– Raghav Malik
Oct 6 '13 at 16:25

@coder108 does this make sense? If not, please let me know what doesn't so I can improve the answer.

– Thomas
Oct 6 '13 at 16:33

Oh, I see what you mean by nested objects. Just out of curiosity, isn't an integer also an object? Why isn't deepcopy necessary for that? Thank you for the link, btw. It is very useful.

– Raghav Malik
Oct 6 '13 at 16:47

@coder108 terrific question - both objects actually use the same integer. Why isn't this a problem? Because it's an immutable object; there's no way to take an integer object 1 and change it.

– Thomas
Oct 6 '13 at 16:53

That makes sense. Thank you so much!

– Raghav Malik
Oct 6 '13 at 20:19

add a comment |

What is the difference between a copy and a deepcopy? Also, what do you mean by nested object? I'm new to python and I just want to learn as much as possible.

– Raghav Malik
Oct 6 '13 at 16:25

@coder108 does this make sense? If not, please let me know what doesn't so I can improve the answer.

– Thomas
Oct 6 '13 at 16:33

Oh, I see what you mean by nested objects. Just out of curiosity, isn't an integer also an object? Why isn't deepcopy necessary for that? Thank you for the link, btw. It is very useful.

– Raghav Malik
Oct 6 '13 at 16:47

@coder108 terrific question - both objects actually use the same integer. Why isn't this a problem? Because it's an immutable object; there's no way to take an integer object 1 and change it.

– Thomas
Oct 6 '13 at 16:53

That makes sense. Thank you so much!

– Raghav Malik
Oct 6 '13 at 20:19

What is the difference between a copy and a deepcopy? Also, what do you mean by nested object? I'm new to python and I just want to learn as much as possible.

– Raghav Malik
Oct 6 '13 at 16:25

@coder108 does this make sense? If not, please let me know what doesn't so I can improve the answer.

– Thomas
Oct 6 '13 at 16:33

Oh, I see what you mean by nested objects. Just out of curiosity, isn't an integer also an object? Why isn't deepcopy necessary for that? Thank you for the link, btw. It is very useful.

– Raghav Malik
Oct 6 '13 at 16:47

@coder108 terrific question - both objects actually use the same integer. Why isn't this a problem? Because it's an immutable object; there's no way to take an integer object 1 and change it.

– Thomas
Oct 6 '13 at 16:53

That makes sense. Thank you so much!

– Raghav Malik
Oct 6 '13 at 20:19

add a comment |

You can use copy for that. Try the following:

someObject = SomeClass()

someObject.value = 12

anotherObject = copy.copy(someObject)

anotherObject.value = 10

Now someObject.value will still be 12. You will have to place import copy at the top of your script though.

answered Oct 6 '13 at 16:18

Patrick Kostjens

4,24462237

add a comment |

You can use copy for that. Try the following:

someObject = SomeClass()

someObject.value = 12

anotherObject = copy.copy(someObject)

anotherObject.value = 10

Now someObject.value will still be 12. You will have to place import copy at the top of your script though.

answered Oct 6 '13 at 16:18

Patrick Kostjens

4,24462237

add a comment |

You can use copy for that. Try the following:

someObject = SomeClass()

someObject.value = 12

anotherObject = copy.copy(someObject)

anotherObject.value = 10

Now someObject.value will still be 12. You will have to place import copy at the top of your script though.

answered Oct 6 '13 at 16:18

Patrick Kostjens

4,24462237

You can use copy for that. Try the following:

someObject = SomeClass()

someObject.value = 12

anotherObject = copy.copy(someObject)

anotherObject.value = 10

Now someObject.value will still be 12. You will have to place import copy at the top of your script though.

answered Oct 6 '13 at 16:18

Patrick Kostjens

4,24462237

answered Oct 6 '13 at 16:18

Patrick Kostjens

4,24462237

answered Oct 6 '13 at 16:18

Patrick Kostjens

4,24462237

answered Oct 6 '13 at 16:18

Patrick Kostjens

4,24462237

add a comment |

import copy

In some cases just:

copy_obj = copy.copy(obj)

is enough but in other cases you we have to use:

copy_obj = copy.deepcopy(obj)

edited Nov 20 '18 at 14:56

answered Nov 20 '18 at 14:21

Juba FOURALI

193

add a comment |

import copy

In some cases just:

copy_obj = copy.copy(obj)

is enough but in other cases you we have to use:

copy_obj = copy.deepcopy(obj)

edited Nov 20 '18 at 14:56

answered Nov 20 '18 at 14:21

Juba FOURALI

193

add a comment |

import copy

In some cases just:

copy_obj = copy.copy(obj)

is enough but in other cases you we have to use:

copy_obj = copy.deepcopy(obj)

edited Nov 20 '18 at 14:56

answered Nov 20 '18 at 14:21

Juba FOURALI

193

import copy

In some cases just:

copy_obj = copy.copy(obj)

is enough but in other cases you we have to use:

copy_obj = copy.deepcopy(obj)

edited Nov 20 '18 at 14:56

answered Nov 20 '18 at 14:21

Juba FOURALI

193

edited Nov 20 '18 at 14:56

answered Nov 20 '18 at 14:21

Juba FOURALI

193

answered Nov 20 '18 at 14:21

Juba FOURALI

193

answered Nov 20 '18 at 14:21

Juba FOURALI

193

add a comment |

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