Deep copy of a dict in python

Deep copy of a dict in python

How about:

import copy
d = { ... }
d2 = copy.deepcopy(d)

Python 2 or 3:

Python 3.2 (r32:88445, Feb 20 2011, 21:30:00) [MSC v.1500 64 bit (AMD64)] on win32
Type help, copyright, credits or license for more information.
>>> import copy
>>> my_dict = {a: [1, 2, 3], b: [4, 5, 6]}
>>> my_copy = copy.deepcopy(my_dict)
>>> my_dict[a][2] = 7
>>> my_copy[a][2]

dict.copy() is a shallow copy function for dictionary

id is built-in function that gives you the address of variable

First you need to understand why is this particular problem is happening?

In [1]: my_dict = {a: [1, 2, 3], b: [4, 5, 6]}

In [2]: my_copy = my_dict.copy()

In [3]: id(my_dict)
Out[3]: 140190444167808

In [4]: id(my_copy)
Out[4]: 140190444170328

In [5]: id(my_copy[a])
Out[5]: 140190444024104

In [6]: id(my_dict[a])
Out[6]: 140190444024104

The address of the list present in both the dicts for key a is pointing to same location.
Therefore when you change value of the list in my_dict, the list in my_copy changes as well.

Solution for data structure mentioned in the question:

In [7]: my_copy = {key: value[:] for key, value in my_dict.items()}

In [8]: id(my_copy[a])
Out[8]: 140190444024176

Or you can use deepcopy as mentioned above.

Deep copy of a dict in python

Python 3.x

from copy import deepcopy

my_dict = {one: 1, two: 2}
new_dict_deepcopy = deepcopy(my_dict)

Without deepcopy, I am unable to remove the hostname dictionary from within my domain dictionary.

Without deepcopy I get the following error:

RuntimeError: dictionary changed size during iteration

…when I try to remove the desired element from my dictionary inside of another dictionary.

import socket
import xml.etree.ElementTree as ET
from copy import deepcopy

domain is a dictionary object

def remove_hostname(domain, hostname):
    domain_copy = deepcopy(domain)
    for domains, hosts in domain_copy.items():
        for host, port in hosts.items():
           if host == hostname:
                del domain[domains][host]
    return domain

Example output:
[orginal]domains = {localdomain: {localhost: {all: 4000}}}

[new]domains = {localdomain: {} }}

So whats going on here is I am iterating over a copy of a dictionary rather than iterating over the dictionary itself. With this method, you are able to remove elements as needed.

Leave a Reply

Your email address will not be published. Required fields are marked *