[Tutor] Copying Variables

[Prasad, Ramit]

From: tutor-bounces+ramit.prasad=jpmchase.com at python.org [mailto:tutor-bounces+ramit.prasad=jpmchase.com at python.org] On Behalf Of naheed arafat
Sent: Monday, July 25, 2011 10:17 AM
To: Steven D'Aprano
Cc: Tutor at python.org
Subject: Re: [Tutor] Copying Variables

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?
=========================================================================

Copy.copy is copies only a single level (shallow); it creates a new dictionary object but the key/values of the new dictionary object are the same as the source. In this case, the anonymous list with [1,2] is shared between both instances and a modification to one will modify it for any object containing a reference to that list. On the other hand, if you use copy.deepcopy you will get a different result because it will create a copy of the anonymous, internal list as well as the dictionary containing it.

>>> a={'a': 3, 'b': [1, 2], 5: 100}
>>> import copy
>>> b = copy.copy(a)
>>> b['b'].append(3)
>>> b
{'a': 3, 'b': [1, 2, 3], 5: 100}
>>> a
{'a': 3, 'b': [1, 2, 3], 5: 100}
>>> b = copy.deepcopy(a)
>>> a
{'a': 3, 'b': [1, 2, 3], 5: 100}
>>> b
{'a': 3, 'b': [1, 2, 3], 5: 100}
>>> b['b'].append(3)
>>> a
{'a': 3, 'b': [1, 2, 3], 5: 100}
>>> b
{'a': 3, 'b': [1, 2, 3, 3], 5: 100}






Ramit


Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology
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