Deep copy of a dict in python


I would like to make a deep copy of a dict in python. Unfortunately the .deepcopy() method doesn’t exist for the dict. How do I do that?

>>> my_dict = {'a': [1, 2, 3], 'b': [4, 5, 6]}
>>> my_copy = my_dict.deepcopy()
Traceback (most recent calll last):
  File "<stdin>", line 1, in <module>
AttributeError: 'dict' object has no attribute 'deepcopy'
>>> my_copy = my_dict.copy()
>>> my_dict['a'][2] = 7
>>> my_copy['a'][2]

The last line should be 3.

I would like that modifications in my_dict don’t impact the snapshot my_copy.

How do I do that? The solution should be compatible with Python 3.x.

Asked By: Olivier Grégoire



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]
Answered By: Lasse V. Karlsen

Python 3.x

from copy import deepcopy

# define the original dictionary
original_dict = {'a': [1, 2, 3], 'b': {'c': 4, 'd': 5, 'e': 6}}

# make a deep copy of the original dictionary
new_dict = deepcopy(original_dict)

# modify the dictionary in a loop
for key in new_dict:
    if isinstance(new_dict[key], dict) and 'e' in new_dict[key]:
        del new_dict[key]['e']

# print the original and modified dictionaries
print('Original dictionary:', original_dict)
print('Modified dictionary:', new_dict)

Which would yield:

Original dictionary: {'a': [1, 2, 3], 'b': {'c': 4, 'd': 5, 'e': 6}}
Modified dictionary: {'a': [1, 2, 3], 'b': {'c': 4, 'd': 5}}

Without new_dict = deepcopy(original_dict), ‘e’ element is unable to be removed.

Why? Because if the loop was for key in original_dict, and an attempt is made to modify original_dict, a RuntimeError would be observed:

"RuntimeError: dictionary changed size during iteration"

So in order to modify a dictionary within an iteration, a copy of the dictionary must be used.

Here is an example function that removes an element from a dictionary:

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
Answered By: xpros

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.

Answered By: theBuzzyCoder
Categories: questions Tags: ,
Answers are sorted by their score. The answer accepted by the question owner as the best is marked with
at the top-right corner.