New submission from thautwarm <yaoxiansa...@gmail.com>:
Currently we can use `exprlist` as the iterator in a `for`
statement:
```
for i in 1, 2,:
print(i)
1
2
```
However, when it comes to comprehension expressions:
```
[i for i in 1, 2]
SyntaxError: invalid syntax
```
I know there might be some reason that leads to the absence of the
consistency between `for` expression and statement, but IMO until now it could
be better to allow `exprlist` to be the iterator of comprehensions. I'm not
sure whether our community has noticed this problem so I'm to propose it here.
A crucial benefit from this change is that we can avoid the ambiguity when a
comprehension is accepted as function parameter.
```
f(for i in [1, 2, 3], 1, 2)
```
We now get a SyntaxError when writing such codes, but people who know this
syntax might think the parameters can be distinguished from each other, but
it's still not allowed.
Allowing `exprlist` to be the iterator slot of a comprehension would be a
rational solution. If `f(for i in [1, 2, 3], 1, 2)` is equivalent to `f(for i
([1, 2, 3], 1, 2))`, it will be natural to restrict the number of parameters to
be just 1 when the parameter is a comprehension.
You can slightly accept this workaround and try following examples:
```
f(for i in 1,)
f(for i in 1, for j in 2, 3,)
f(for i in 1, 2 if cond(i) for j in 3, for k in 4,)
```
Obviously, in each of above cases, the number of parameters is 1,
just because a `exprlist` could the iterator of a comprehension.
The disadvantage of this change is, there is not too many related use cases,
for any `[expr for target in iter_elem1, iter_elem2, ...]` could be altered as
`[expr for target in (iter_elem1, iter_elem2, ...)]`. Meanwhile, that might not
be true when it comes to someone works frequently with interactive python.
Finally, I have finished this implementation at
https://github.com/thautwarm/cpython/tree/exprlist-in-comprehensions, and I do
want to make contributions to cpython projects. If most of you feel comfortable
with this change, may I make a PR next?
----------
messages: 326692
nosy: gvanrossum, serhiy.storchaka, thautwarm, yselivanov
priority: normal
severity: normal
status: open
title: allow exprlist as the iterators of comprehensions to be consistent with
for statements
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue34845>
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