lists v. tuples

Ninereeds stephenhorne100 at
Mon Mar 17 14:06:59 CET 2008

On Mar 17, 12:28 pm, castiro... at wrote:

> > Why is the immutable form the default?
> Using a house definition from some weeks ago, a tuple is a data
> structure such which cannot contain a refrence to itself.  Can a
> single expression refer to itself ever?

Can't imagine why that feature was highlighted in particular, but a
list can reference itself even though an expression can't.

The following example looks a bit self-referential, but isn't...

  a = 2
  a = [1, a, 3]   #  result [1, 2, 3]

The following additional line, however does create a self-referencing

  a [1] = a

The result being [1, [...], 2]

It's nice to see that Python can handle the output for this without
going into an infinite recursion - which is exactly what it used to do
in the distant past.

A tuple cannot be made to reference itself because it cannot be
modified after creation. The key point is that lists are mutable,
whereas tuples are not.

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