References

References
	Chapter 9. Data Structures

When you create an object and assign it to a variable, the variable only refers to the object and does not represent the object itself! That is, the variable name points to that part of your computer's memory where the object is stored. This is called as binding of the name to the object.

Generally, you don't need to be worried about this, but there is a subtle effect due to references which you need to be aware of. This is demonstrated by the following example.

Objects and References

Example 9.6. Objects and References

				
#!/usr/bin/python
# Filename: reference.py

print 'Simple Assignment'
shoplist = ['apple', 'mango', 'carrot', 'banana']
mylist = shoplist # mylist is just another name pointing to the same object!

del shoplist[0] # I purchased the first item, so I remove it from the list

print 'shoplist is', shoplist
print 'mylist is', mylist
# notice that both shoplist and mylist both print the same list without
# the 'apple' confirming that they point to the same object

print 'Copy by making a full slice'
mylist = shoplist[:] # make a copy by doing a full slice
del mylist[0] # remove first item

print 'shoplist is', shoplist
print 'mylist is', mylist
# notice that now the two lists are different

Output

				
$ python reference.py
Simple Assignment
shoplist is ['mango', 'carrot', 'banana']
mylist is ['mango', 'carrot', 'banana']
Copy by making a full slice
shoplist is ['mango', 'carrot', 'banana']
mylist is ['carrot', 'banana']

How It Works

Most of the explanation is available in the comments itself. What you need to remember is that if you want to make a copy of a list or such kinds of sequences or complex objects (not simple objects such as integers), then you have to use the slicing operation to make a copy. If you just assign the variable name to another name, both of them will refer to the same object and this could lead to all sorts of trouble if you are not careful.

Note for Perl programmers

Remember that an assignment statement for lists does not create a copy. You have to use slicing operation to make a copy of the sequence.


Sequences		More about Strings