list comprehension question

[Terry Reedy]

J Kenneth King wrote:
>
> Keep in mind that nested comprehensions are still available because
> they do have a use case that justifies their existence.

Nested comprehensions are available because because the syntax makes 
them available by default and making a fiddly exception would be 
contrary to Python's style.  A list comp creates an iterable. A list 
comp requires use of an iterable.  Therefore, a list comp may use a list 
comp.

> However, I
> think the Python documentation warns against their use because people
> might rely on them for problems where they aren't necessary and since
> they are difficult to read... it can lead to difficult to read code.

Whenever the expression that results in the iterable used by a list comp 
is sufficiently complex, readability is improved by pulling it out as a 
separate statement. Nested list comps are often examplex of such 
sufficiently complex expressions, but not the only possible one.

tjr