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copying an object - copy the reference
I call my father "Dad" but everyone else calls him "Norman". Same person, different name.
If Dad puts on the hat that he's got from the Christmas cracker before he eats his Christmas pudding, then everyone else sees that Norman has put on his hat before eating his Christmas pudding. Two names for one and the same person.
PYTHON - COPY BY REFERENCE
The same principle applies to the copying of objects in Python (and Java and PHP 5 and other object oriented languages); an assignment statement is just the addition of an extra name to the same object.
Let's see an example - in Python:
pie = ["apple","pear","strawberry"]
pudding = pie
pudding[1] = "apricot"
print pudding
print pie
pudding = pie
pudding[1] = "apricot"
print pudding
print pie
When I run that:
earth-wind-and-fire:~/feb05 grahamellis$ python obcop.py
['apple', 'apricot', 'strawberry']
['apple', 'apricot', 'strawberry']
earth-wind-and-fire:~/feb05 grahamellis$
['apple', 'apricot', 'strawberry']
['apple', 'apricot', 'strawberry']
earth-wind-and-fire:~/feb05 grahamellis$
By changing "pudding" I've also changed "pie".
PERL - COPY BY CONTENTS
In contrast, if I copy a list in Perl (and in PHP 4, Tcl and other languages which are not 100% object oriented), I duplicate the data. Example in Perl:
@pie = ("apple","pear","strawberry");
@pudding = @pie;
$pudding[1] = "apricot";
print "@pudding\n";
print "@pie\n";
@pudding = @pie;
$pudding[1] = "apricot";
print "@pudding\n";
print "@pie\n";
Which runs:
earth-wind-and-fire:~/feb05 grahamellis$ perl obcop.pl
apple apricot strawberry
apple pear strawberry
earth-wind-and-fire:~/feb05 grahamellis$
apple apricot strawberry
apple pear strawberry
earth-wind-and-fire:~/feb05 grahamellis$
See also Python programming course
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Related Material
Python - More on Collections and Sequences
Object Oriented Python
resource index - Python
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