nested lists - utter newbie

[rahoool]

Hi,
why is this possible -

b = [1,2,3]
b[2] = b
b [1,2,[...]]
b[2] [1,2,[...]]
b[2][2][2][2][2]

[1,2,[...]]

but this is not -

x = [1,2,x]

Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: name 'x' is not defined

thanks
r_a_h_o_o_l

 

[Benjamin Niemann]

Hi,
why is this possible -

b = [1,2,3]
b[2] = b
b [1,2,[...]]
b[2] [1,2,[...]]
b[2][2][2][2][2]

[1,2,[...]]

but this is not -

x = [1,2,x]

Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: name 'x' is not defined

Because the right hand side ('[1,2,x]') is evaluated *before* the value is
bound to the name 'x' - and at this point there is obviously no name 'x'
defined.

 

[Terry Hancock]

Hi,
why is this possible -

b = [1,2,3]
b[2] = b
b [1,2,[...]]

b[2]

[1,2,[...]]
b[2][2][2][2][2]

[1,2,[...]]

but this is not -

x = [1,2,x]

Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: name 'x' is not defined

Because the right hand side ('[1,2,x]') is evaluated *before* the value is
bound to the name 'x' - and at this point there is obviously no name 'x'
defined.

OTOH, the name "b" was bound to the list, so it can be put into
the list, just like any other name. At that point, the list contains
a series of references, one of which happens to be to itself.

If you are familiar with Unix/Linux filesystems, you've probably
already seen behavior like this with either symbolic or hard links --
it's quite possible to create loops so that a directory is contained
within itself. Such a structure is not strictly a "tree" any more,
but a "directed graph". Those are the data structures terms for
them, and you might try a little googling to learn more about them.

The *really* smart thing is that Python *writes* the list as:

[1,2,[...]]

instead of printing a traceback due to "excessive recursion" which
is what it used to do. This is because the representation method
was changed to catch such circular references and stick in the "[...]"
instead.