# Re: [Python-ideas] Fwd: Fwd: unpacking generalisations for list comprehension

```On Thu, Oct 13, 2016, at 14:50, David Mertz wrote:
> Neither of those follows conventional Python semantics for function
> calling
> or sequence unpacking.  So maybe that remains a type error or syntax
> error.  But then we exclude a very common pattern of using comprehensions
> to create collections of *transformed* data, not simply of filtered data.```
```
[*map(math.exp, t) for t in [(1, 2), (3, 4)]]

[*(math.exp(x) for x in t) for t in [(1, 2), (3, 4)]]

I think "excluding" is a bit of a strong word - just because something
doesn't address a mostly unrelated need doesn't mean it doesn't have any
merit in its own right. Not every proposal is going to do everything. I
think the key is that the person originally asking this thought of *x as
a generalized "yield from x"-ish thing, for example:

"a, *b, c" becomes "def f(): yield a; yield from b; yield c;"

[a, *b, c] == list(f())
(a, *b, c) == tuple(f())

so,  under a similar 'transformation', "*foo for foo in bar" likewise
becomes "def f(): for foo in bar: yield from foo"

bar = [(1, 2), (3, 4)]
(*(1, 2), *(3, 4)) == == tuple(f())
[*(1, 2), *(3, 4)] == == list(f())

> In contrast, either of these are unambiguous and obvious:
>
>     [math.exp(t) for t in flatten([(1,2),(3,4)])]
>
> Or:
>
>     [math.exp(n) for t in [(1,2),(3,4)] for n in t]
>
> Obviously, picking math.exp() is arbitrary and any unary function would
> be
> the same issue.
>
>
> --
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> from the bellies of the hungry; books from the hands of the
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```