[Tutor] True and 1 [was Re: use of the newer dict types]

[Walter Prins]

Hi,

On 27 July 2013 11:36, Steven D'Aprano <steve at pearwood.info> wrote:
> However, this will test your knowledge:
>
> L = []
> L.append(L)
> [L] == L
>
> True or false? Can you explain why?

On 27 July 2013 20:14, Don Jennings <dfjennings at gmail.com> wrote:

> In [20]: bool(L)
> Out[20]: True
>
> An empty list does evaluate to False, but a list which contains another
> list (even if the contained list happens to be empty) evaluates to True.
>
>
True but besides the point for an expression where you're comparing 2 lists
--  Lists are considered equal when corresponding values are equal, not if
their truthyness is the same as the truthyness of the list on the other
side of the expression.

In the case above, you have 2 lists.  On the right hand side of the boolean
expression you have L, which is a plain list, and which (crucially)
contains one element, a reference to a list L.

On the *left* hand side, you have another list, which also contains 1
element, a reference to list L.

So, since lists are considered equal when the *values* of their
corresponding elements are the same, and since each list contains 1
element, being a reference to the object (list) L, it follows that the
expression [L] and L would be considered equal by Python's comparison
rules, which means the expression evaluates to True.

Analogously, consider the following:

l1=[1]
l2=l1
l3=[l1]
l4=[l2]
l3==l4 # True or false?  Why?

Walter
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