In my last blog post, I tried to understand how Python counts items in a sequence.

This week P2PU’s Mookie gave us to read on functions and amongst them the id() function, which shows the unique ID where that object is stored in memory.

Immediately I went and tested how last week’s theory works with this. And again I found it very interesting and also a bit odd.

First I tried what happens if I create two seemingly unrelated variables and assign them the same string.

>>> s = "MONTY"
>>> v = "MONTY"
>>> id(v)
140601249401952
>>> id(s)
140601249401952

It seems Python is smart enough that it assigns the same string to just one address in memory. I assume this way it conserves memory. Which is pretty cool.

And cooler still, that the same goes for items in the sequence.

>>> id(s[0])
140601249649048
>>> id(v[0])
140601249649048
>>> id(s[1])
140601250016472
>>> id(v[1])
140601250016472
>>> id(s[2])
140601250016432
>>> id(v[2])
140601250016432
>>> id(s[3])
140601250016832
>>> id(v[3])
140601250016832
>>> id(s[4])
140601249650768
>>> id(v[4])
140601249650768
>>> id(v[5])
Traceback (most recent call last):
  File "", line 1, in 
IndexError: string index out of range
>>> id(s[5])
Traceback (most recent call last):
  File "", line 1, in 
IndexError: string index out of range

And it also works backwards:

>>> id(s[-1])
140601249650768
>>> id(v[-1])
140601249650768
>>> id(s[-2])
140601250016832
>>> id(v[-2])
140601250016832

But does it also work with non-identical sequences that just happen to have some items identical?

>>> animals = ["dog", "sheep", "bird", "python"]
>>> mammals = ["dog", "sheep", "cow"]
>>> id(animals)
140601249224392
>>> id(mammals)
140601249258816
>>> id(mammals[0])
140601249407984
>>> id(animals[0])
140601249407984
>>> id(animals[2])
140601249402096
>>> id(mammals[2])
140601249407464

Yes it does! As you can see the ID of the two lists are different, but since in both the first item is "dog", that’s stored in the same place in memory. Of course the third item, which is not identical, has a different ID again.

So far so good, but again I have a question: Why the ID (and hence address in memory, I assume) of the sequence and the first item in it is so different?

hook out → playing a bit more with Python, and then back to boring legal books

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