## Summary
The tokenizer was split into a forward and a backwards tokenizer. The
backwards tokenizer uses the same names as the forwards ones (e.g.
`next_token`). The backwards tokenizer gets the comment ranges that we
already built to skip comments.
---------
Co-authored-by: Micha Reiser <micha@reiser.io>
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## Summary
This PR implements Black's behavior where it first splits off parenthesized expressions before splitting before operands to avoid unnecessary parentheses:
```python
# We want
if a + [
b,
c
]:
pass
# Rather than
if (
a
+ [b, c]
):
pass
```
This is implemented by using the new IR elements introduced in #5596.
* We give the group wrapping the optional parentheses an ID (`parentheses_id`)
* We use `conditional_group` for the lower priority groups (all non-parenthesized expressions) with the condition that the `parentheses_id` group breaks (we want to split before operands only if the parentheses are necessary)
* We use `fits_expanded` to wrap all other parenthesized expressions (lists, dicts, sets), to prevent that expanding e.g. a list expands the `parentheses_id` group. We gate the `fits_expand` to only apply if the `parentheses_id` group fits (because we prefer `a\n+[b, c]` over expanding `[b, c]` if the whole expression gets parenthesized).
We limit using `fits_expanded` and `conditional_group` only to expressions that themselves are not in parentheses (checking the conditions isn't free)
## Test Plan
It increases the Jaccard index for Django from 0.915 to 0.917
## Incompatibilites
There are two incompatibilities left that I'm aware of (there may be more, I didn't go through all snapshot differences).
### Long string literals
I commented on the regression. The issue is that a very long string (or any content without a split point) may not fit when only breaking the right side. The formatter than inserts the optional parentheses. But this is kind of useless because the overlong string will still not fit, because there are no new split points.
I think we should ignore this incompatibility for now
### Expressions on statement level
I don't fully understand the logic behind this yet, but black doesn't break before the operators for the following example even though the expression exceeds the configured line width
```python
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa < bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb > ccccccccccccccccccccccccccccc == ddddddddddddddddddddd
```
But it would if the expression is used inside of a condition.
What I understand so far is that Black doesn't insert optional parentheses on the expression statement level (and a few other places) and, therefore, only breaks after opening parentheses. I propose to keep this deviation for now to avoid overlong-lines and use the compatibility report to make a decision if we should implement the same behavior.
