Verifying exactly the same variable in python list when passing by reference

Question

Take the nested lists: list = [[foo, foo], [foo, foo]], [foo, foo]].

I pass list to a function, but I also separately pass by reference to the same function the second nested list as list[1].

Then, within the function at random indices, I add 3 more [foo, foo]'s to list.

Would it be possible to identify which [foo, foo] in list is the one I passed to the function?

In other words, I want to get the new index of the original [foo, foo] I separately passed earlier in the now-modified list.

Answer 1

What you are looking for is is, which tests if two variables refer to the same object.

Demonstrating in iPython shell:

In [1]: list = [['foo', 'foo'], ['foo', 'foo'], ['foo', 'foo']]

In [2]: sublist = list[1]

In [3]: sublist is list[1]
Out[3]: True

In [5]: list.insert(1, ['foo', 'foo'])

In [6]: list
Out[6]: [['foo', 'foo'], ['foo', 'foo'], ['foo', 'foo'], ['foo', 'foo']]

In [7]: list.insert(1, ['foo', 'foo'])

In [8]: list
Out[8]: 
[['foo', 'foo'],
 ['foo', 'foo'],
 ['foo', 'foo'],
 ['foo', 'foo'],
 ['foo', 'foo']]

In [9]: sublist is list [1]
Out[9]: False

In [10]: for count, l in enumerate(list):
   ...:     if l is sublist:
   ...:         print(count)
   ...: 
3

In [11]: sublist is list[3]
Out[11]: True