Why the list creates in two different ways? Does it cause by the mutability of its elements? Where the Python document explains it?

[Chris Angelico]

On Tue, Jun 15, 2021 at 7:11 AM Rob Cliffe via Python-list
<python-list at python.org> wrote:
>
> This puzzled me, so I played around with it a bit (Python 3.8.3):
>
> n = []
> for i in range(3):
>      n.append((1,7,-3,None,"x"))
> for i in range(3):
>      n.append((1,7,-3,None,"x"))
> print([id(x) for x in n])
>
> a = 4
> n = []
> for i in range(3):
>      n.append((1,7,-3,a,None,"x"))
> for i in range(3):
>      n.append((1,7,-3,a,None,"x"))
> print([id(x) for x in n])
>
> Output:
>
> [27164832, 27164832, 27164832, 27164832, 27164832, 27164832]
> [30065208, 30065496, 30237192, 30239976, 30240024, 30343928]
>
> Evidently the compiler is clever enough to pick out a constant tuple and
> create (or cause to get created) a single instance of it which is used
> when required.  Indeed disassembling the code shows that LOAD_CONST is
> used to get the tuple.  But it obviously can't do that when the tuple
> contains a variable.

Correct. In theory, Python could intern the tuples (as can be done
with strings), noticing that it's constructing one that is identical
to one it already has, but the effort of doing that is hard to
justify. Simpler to just build a brand new tuple every time.

ChrisA