Subtle difference between any(a list) and any(a generator) with Python 3.9

[Terry Reedy]

On 7/29/2021 5:39 AM, Unknown wrote:
> Hello
> 
> Reading PEP572 about Python 3.9 assignment expressions,
> I discovered a subtle difference between any(a list)
> and any(a generator)
> 
> see:
> 
>  >>> lines = ["azerty", "#qsdfgh", "wxcvbn"]
>  >>> any((comment := line).startswith('#') for line in lines)
> True
>  >>> comment
> "#qsdfgh"
> 
>  >>> any([(comment := line).startswith('#') for line in lines])

Same as

 >>> booleans = [(comment := line).startswith('#') for line in lines]
 >>> # comment == last item.
 >>> any(booleans) # Iteration though booleans stops at 1st True.

 > True
   >>> comment
> 'wxcvbn'
> 
> The two code snippets which seems very similar provide a
> different value for "comment".
> 
> When "any" deals with a generator, it stops as soon it finds
> a True value and returns True.
> 
> When "any" deals with a list, the whole list is calculated
> first, and then "any" looks for a True.
> 
> Before 3.9 and the walrus operator, the two ways always provide
> the same result, in a faster way with a generator.

Since the 'two ways' involve the new :=, I have no idea what 'two ways' 
and 'same result' you mean before :=.

> With 3.9 and the walrus operator, result can be different



-- 
Terry Jan Reedy