Python Copy Through Assignment?

Question

I would expect that the following code would just initialise the dict_a, dict_b and dict_c dictionaries. But it seams to have a copy through effect:

dict_a = dict_b = dict_c = {}
dict_c['hello'] = 'goodbye'

print dict_a
print dict_b
print dict_c

As you can see the result is as follows:

{'hello': 'goodbye'}
{'hello': 'goodbye'}
{'hello': 'goodbye'}

Why does that program give the previous result, When I would expect it to return:

{}
{}
{'hello': 'goodbye'}

Answer 1

This is because in Python, objects are just references. When you assign dict_a = dict_b, you are really copying a memory address (or pointer, if you will) from dict_b to dict_a. There is still one instance of a dictionary.

To get the desired behavior, use the dict.copy() method.

>>> a = {1:2}
>>> b = a.copy()
>>> b
{1: 2}
>>> b[3] = 4
>>> a
{1: 2}
>>> b
{1: 2, 3: 4}
>>> 

Answer 2

Your first assignment assigns the same dictionary object to the variables dict_a, dict_b, and dict_c. It is equivalent to dict_c = {}; dict_b = dict_c; dict_a = dict_c.

Answer 3

As danben previously said, you're just copying the same dict into 3 variables, so that each one reffers to the same object.

To get the behaviour you want, you should instanciate a different dict in each variable:

>>> dict_a, dict_b, dict_c = {}, {}, {}
>>> dict_c['hello'] = 'goodbye'
>>> print dict_a
{}
>>> print dict_b
{}
>>> print dict_c
{'hello': 'goodbye'}
>>>

Answer 4

Even though

>>> dict_a, dict_b, dict_c = {}, {}, {}

is the right way to go in most cases, when it get more than 3 it looks weird

Imagine

>>> a, b, c, d, e, f = {}, {}, {}, {}, {}, {}

In cases where I wanna initialize more than 3 things, I use

>>> a, b, c, d, e, f, = [dict() for x in range(6)]

Caution: Do not use [{} for x in range(6)]