[Tutor] Copying Variables

[Sander Sweers]

On 25 July 2011 17:17, naheed arafat <naheedcse at gmail.com> wrote:
> I got a question in this context.
> suppose
> a={'a': 3, 'b': [1, 2], 5: 100}
> ------------------b=a --------------    vs----------
> b=copy.copy(a)------------
> ----------------------------------------------------
> b[5]=6   ----------------------------------------      b[5]=6
> output: -----------------------------------------      output:
> b={'a': 3, 'b': [1, 2], 5: 6}-------------------     b={'a': 3, 'b': [1, 2],
> 5: 6}
> a={'a': 3, 'b': [1, 2], 5: 6} -------------------    a={'a': 3, 'b': [1, 2],
> 5: 100}
> that means b=a & b=copy.copy(a) aren't the same.
> but
> b['b'].append(3)
> output:
> b={'a': 3, 'b': [1, 2, 3], 5: 100}--------------b={'a': 3, 'b': [1, 2, 3],
> 5: 100}
> a={'a': 3, 'b': [1, 2, 3], 5: 100}--------------a={'a': 3, 'b': [1, 2, 3],
> 5: 100}
> now doesn't it mean that b=a & b=copy.copy(a) both are same?

No, b=a makes a new reference to the _same_ dict object.
b=copy.copy(a) creates a new dict object b. However it does not make a
copy of the *contents* of the dict object. To make a copy of the
contents of the dict use cop.deepcopy(). Play around with id() on the
a, b and their contents.

But do note that cpython caches small integers so the integer 3 will
have the same id (thus it is the same object).

greets
Sander