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## Summary
I started working on this because I assumed that I would need access to options inside of `NeedsParantheses` but it then turned out that I won't.
Anyway, it kind of felt nice to pass fewer arguments. So I'm gonna put this out here to get your feedback if you prefer this over passing individual fiels.
Oh, I sneeked in another change. I renamed `context.contents` to `source`. `contents` is too generic and doesn't tell you anything.
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## Test Plan
It compiles
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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.
This formats slice expressions and subscript expressions.
Spaces around the colons follows the same rules as black
(https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#slices):
```python
e00 = "e"[:]
e01 = "e"[:1]
e02 = "e"[: a()]
e10 = "e"[1:]
e11 = "e"[1:1]
e12 = "e"[1 : a()]
e20 = "e"[a() :]
e21 = "e"[a() : 1]
e22 = "e"[a() : a()]
e200 = "e"[a() : :]
e201 = "e"[a() :: 1]
e202 = "e"[a() :: a()]
e210 = "e"[a() : 1 :]
```
Comment placement is different due to our very different infrastructure.
If we have explicit bounds (e.g. `x[1:2]`) all comments get assigned as
leading or trailing to the bound expression. If a bound is missing
`[:]`, comments get marked as dangling and placed in the same section as
they were originally in:
```python
x = "x"[ # a
# b
: # c
# d
]
```
to
```python
x = "x"[
# a
# b
:
# c
# d
]
```
Except for the potential trailing end-of-line comments, all comments get
formatted on their own line. This can be improved by keeping end-of-line
comments after the opening bracket or after a colon as such but the
changes were already complex enough.
I added tests for comment placement and spaces.
* A basic StmtAssign formatter and better dummies for expressions
The goal of this PR was formatting StmtAssign since many nodes in the black tests (and in python in general) are after an assignment. This caused unstable formatting: The spacing of power op spacing depends on the type of the two involved expressions, but each expression was formatted as dummy string and re-parsed as a ExprName, so in the second round the different rules of ExprName were applied, causing unstable formatting.
This PR does not necessarily bring us closer to black's style, but it unlocks a good porting of black's test suite and is a basis for implementing the Expr nodes.
* fmt
* Review
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## Summary
This PR replaces the `verbatim_text` builder with a `not_yet_implemented` builder that emits `NOT_YET_IMPLEMENTED_<NodeKind>` for not yet implemented nodes.
The motivation for this change is that partially formatting compound statements can result in incorrectly indented code, which is a syntax error:
```python
def func_no_args():
a; b; c
if True: raise RuntimeError
if False: ...
for i in range(10):
print(i)
continue
```
Get's reformatted to
```python
def func_no_args():
a; b; c
if True: raise RuntimeError
if False: ...
for i in range(10):
print(i)
continue
```
because our formatter does not yet support `for` statements and just inserts the text from the source.
## Downsides
Using an identifier will not work in all situations. For example, an identifier is invalid in an `Arguments ` position. That's why I kept `verbatim_text` around and e.g. use it in the `Arguments` formatting logic where incorrect indentations are impossible (to my knowledge). Meaning, `verbatim_text` we can opt in to `verbatim_text` when we want to iterate quickly on nodes that we don't want to provide a full implementation yet and using an identifier would be invalid.
## Upsides
Running this on main discovered stability issues with the newline handling that were previously "hidden" because of the verbatim formatting. I guess that's an upside :)
## Test Plan
None?
