The implementation of the new backtracking logic depends heavily on deepcopying the current state of the parser before seeing one of the new keywords, which by default is an very expensive operations. On my system, formatting these 3 files takes 1.3 seconds.
```
$ touch tests/data/pattern_matching_*; time python -m black -tpy310 tests/data/pattern_matching_* 19ms
All done! ✨🍰✨
3 files left unchanged.
python -m black -tpy310 tests/data/pattern_matching_* 2,09s user 0,04s system 157% cpu 1,357 total
```
which can be optimized 3X if we integrate the existing copying logic (`clone`) to the deepcopy system;
```
$ touch tests/data/pattern_matching_*; time python -m black -tpy310 tests/data/pattern_matching_* 1ms
All done! ✨🍰✨
3 files left unchanged.
python -m black -tpy310 tests/data/pattern_matching_* 0,66s user 0,02s system 147% cpu 0,464 total
```
This still might have some potential, but that would be way trickier than this initial patch.
* Improve Python 2 only syntax detection
First of all this fixes a mistake I made in Python 2 deprecation PR
using token.* to check for print/exec statements. Turns out that
for nodes with a type value higher than 256 its numeric type isn't
guaranteed to be constant. Using syms.* instead fixes this.
Also add support for the following cases:
print "hello, world!"
exec "print('hello, world!')"
def set_position((x, y), value):
pass
try:
pass
except Exception, err:
pass
raise RuntimeError, "I feel like crashing today :p"
`wow_these_really_did_exist`
10L
* Add octal support, more test cases, and fixup long ints
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
* Prepare for Python 2 depreciation
- Use BlackRunner and .stdout in command line test
So the next commit won't break this test. This is in its own commit so
we can just revert the depreciation commit when dropping Python 2
support completely.
* Deprecate Python 2 formatting support
Existing test was actually running a full black-primer
run which could be slow. This goes from 8 seconds to
0.4 seconds on my machine.
Needed to move to top level scope to leverage the caplog
feature of pytest in order to test that the command line
was parsing the bogus arguments and dumping to stderr.
* fix: allow tests to be run from the tests/ directory
* fix: try fixing windows build with MarcoGorelli's suggestion
* Windows hotfix + better respect test's spirit
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>
If the individual failures are verbose, it's useful to have
the summary at the end. Otherwise, it can be really difficult
to figure out which projects have an issue.
It currently prints both ASTs - this also
adds the line diff, making it much easier to visualize
the changes as well. Not too verbose since it's only a diff.
The main goals of this commit include:
* improving consistency on how strict the test suite is -- Jelle has
seen cases where a test did not fail to an incomplete test setup
even though it should've
* simplifying tests for both ease of creation and reading via
parametrization and helpers
* reorganizing the test suite by grouping more tests
* dropping test suite dependencies that aren't strictly necessary
The test suite could definitely do with more refactoring, but this is a
good first pass. Anyway it would've gotten too big to review effectively
if I did continue on this PR.
Commit history before squash merge:
* Drop parameterized dep and refactor format tests
Since the test suite is already using pytest-only features we can drop
the parameterized test dependency in favour of pytest's own offering.
I also added an utility function called assert_format that makes it
even easier to verify Black formats some code correctly. We already
have great tooling if the case is very simple in test_format.py but
any sort of complication makes it hard to use. Also if you're writing
a non-standard test case, you have to be careful to include all of
the steps so issues don't go undetected. assert_format aims to
1) improve consistency, 2) avoid wasted CPU cycles, and 3) avoid
logical errors that hide issues.
Finally, quite a few tests were either moved and/or simplified with
the new setup.
* Move file collection tests
* Add assert_collected_sources helper function
Testing source collection involves a lot of repetitive boilerplate,
something that black.files.get_sources's signature does not help with.
So to cut down on boilerplate like `report=black.Report()` I added
a convenience function to tests/test_black.py which wraps
black.get_sources. Its signature is designed to be much more lax to
make it much easier to use. Somehow this leads to cutting 100 lines!
Also IMO the test cases are much easier to read since it's more
declarative than really procedural now.
* Run isort on some test files
* Move cache tests
* Use pytest-style asserts & add parametrization
* Drop now unnecessary test dependencies
*pytest-cases might be interesting for further refactoring but I
haven't been able to wrap my head around it for the time being. We
can always revisit anyway.
* re-implement simple CORS middleware for blackd
* remove aiohttp-cors from setup.py
* Remove aiohttp-cors from Pipfile.lock
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>
Implementation stolen from PR davidhalter/parso#162. Thanks parso!
I could add support for these newer syntactical constructs in the
target version detection logic, but until I get diff-shades up
and running I don't feel very comfortable adding the code.
This fixes a bug where a trailing comma would be added to a
parenthesized return annotation changing its type to a tuple.
Here's one case where this bug shows up:
```
def spam() -> (
this_is_a_long_type_annotation_which_should_NOT_get_a_trailing_comma
):
pass
```
The root problem was that the type annotation was treated as if it was
a parameter & import list (is_body=True to linegen::bracket_split_build_line)
where a trailing comma is usually fine. Now there's another check in the
aforementioned function to make sure the body it's operating on isn't
a return annotation before truly adding a trailing comma.
* Add CPython repository into primer runs
- CPython tests is probably the best repo for black to test on as the stdlib's unittests should use all syntax
- Limit to running in recent versions of the python runtime - e.g. today >= 3.9
- This allows us to parse more syntax
- Exclude all failing files for now
- Definitely have bugs to explore there - Refer to #2407 for more details there
- Some test files on purpose have syntax errors, so we will never be able to parse them
- Add new black command arguments logging in debug mode; very handy for seeing how CLI arguments are formatted
CPython now succeeds ignoring 16 files:
```
Oh no! 💥💔💥
1859 files would be reformatted, 148 files would be left unchanged.
```
Testing
- Ran locally with and without string processing - Very little runtime difference BUT 3 more failed files
```
time /tmp/tb/bin/black --experimental-string-processing --check . 2>&1 | tee /tmp/black_cpython_esp
...
Oh no! 💥💔💥
1859 files would be reformatted, 148 files would be left unchanged, 16 files would fail to reformat.
real 4m8.563s
user 16m21.735s
sys 0m6.000s
```
- Add unittest for new covienence config file flattening that allows long arguments to be broke up into an array/list of strings
Addresses #2407
---
Commit history before merge:
* Add new `timeout_seconds` support into primer.json
- If present, will set forked process limit to that value in seconds
- Otherwise, stay with default 10 minutes (600 seconds)
* Add new "base_path" concept to black-primer
- Rather than start at the repo root start at a configured path within the repository
- e.g. for cpython only run black on `Lib`
* Disable by default - It's too much for GitHub Actions. But let's leave config for others to use
* Minor tweak to _flatten_cli_args
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>
we don't accidentally add backslashes to them when normalizing quotes
because that's invalid syntax!
The problem this commit fixes is that matches would eat too much
blocking important matches to occur. For example, here's one f-string
body:
{a}{b}{c}
I know there's no risk of introducing backslashes here, but the regex
already goes sideways with this. Throwing this example at regex101
I get:
{a}{b}{c} # The As and Bs are the two matches, and the upper
---- ---- # case letters are the groups with those matches.
aAaa bbBb
... we've missed the middle expression (so if any backslashes in a
more complex example were introduced there we wouldn't bail out
even though we should -- hence the bug). As it stands the regex
needs somesort of extra character (or the start/end of the body)
around the expressions but that isn't always the case as shown
above.
The fix implemented here is to turn the "eat a surrounding non-curly
bracket character" groups ie. `(?:[^{]|^)` and `(?:[^}]|$)` into
negative lookaheads and lookbehinds. This still guarantees the
already specified rules but without problematically eating extra
characters ^^
Fixes#2359.
This commit now makes Black exit with an user-friendly error message if a
.gitignore file couldn't be parsed -- a massive improvement over an opaque
traceback!
To summarise, based on what was discussed in that issue:
due to not being able to parse automagics (e.g. pip install black)
without a running IPython kernel, cells with syntax which is parseable
by neither ast.parse nor IPython will be skipped cells with multiline
magics will be skipped trailing semicolons will be preserved, as they
are often put there intentionally in Jupyter Notebooks to suppress
unnecessary output
Commit history before merge (excluding merge commits):
* wip
* fixup tests
* skip tests if no IPython
* install test requirements in ipynb tests
* if --ipynb format all as ipynb
* wip
* add some whole-notebook tests
* docstrings
* skip multiline magics
* add test for nested cell magic
* remove ipynb_test.yml, put ipynb tests in tox.ini
* add changelog entry
* typo
* make token same length as magic it replaces
* only include .ipynb by default if jupyter dependencies are found
* remove logic from const
* fixup
* fixup
* re.compile
* noop
* clear up
* new_src -> dst
* early exit for non-python notebooks
* add non-python test notebook
* add repo with many notebooks to black-primer
* install extra dependencies for black-primer
* fix planetary computer examples url
* dont run on ipynb files by default
* add scikit-lego (Expected to change) to black-primer
* add ipynb-specific diff
* fixup
* run on all (including ipynb) by default
* remove --include .ipynb from scikit-lego black-primer
* use tokenize so as to mirror the exact logic in IPython.core.displayhooks quiet
* fixup
* 🎨
* clarify docstring
* add test for when comment is after trailing semicolon
* enumerate(reversed) instead of [::-1]
* clarify docstrings
* wip
* use jupyter and no_jupyter marks
* use THIS_DIR
* windows fixup
* perform safe check cell-by-cell for ipynb
* only perform safe check in ipynb if not fast
* remove redundant Optional
* 🎨
* use typeguard
* dont process cell containing transformed magic
* require typing extensions before 3.10 so as to have TypeGuard
* use dataclasses
* mention black[jupyter] in docs as well as in README
* add faq
* add message to assertion error
* add test for indented quieted cell
* use tokenize_rt else we cant roundtrip
* fmake fronzet set for tokens to ignore when looking for trailing semicolon
* remove planetary code examples as recent commits result in changes
* use dataclasses which inherit from ast.NodeVisitor
* bump typing-extensions so that TypeGuard is available
* bump typing-extensions in Pipfile
* add test with notebook with empty metadata
* pipenv lock
* deprivative validate_cell
* Update README.md
* Update docs/getting_started.md
* dont cache notebooks if jupyter dependencies arent found
* dont write to cache if jupyter deps are not installed
* add notebook which cant be parsed
* use clirunner
* remove other subprocess calls
* add docstring
* make verbose and quiet keyword only
* 🎨
* run second many test on directory, not on file
* test for warning message when running on directory
* early return from non-python cell magics
* move NothingChanged to report to avoid circular import
* remove circular import
* reinstate --ipynb flag
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
toml unfortunately has a lack of maintainership issue right now. It's
evident by the fact toml only supports TOML v0.5.0. TOML v1.0.0 has
been recently released and right now Black crashes hard on its usage.
tomli is a brand new parse only TOML library. It supports TOML
v1.0.0. Although TBH we're switching to this one mostly because
pip is doing the same.
*The upper bound was included at the library maintainer's request.
Co-authored-by: Łukasz Langa <lukasz@langa.pl>
Co-authored-by: Taneli Hukkinen <3275109+hukkin@users.noreply.github.com>
Commit history before merge:
* Accept empty stdin (close#2337)
* Update tests/test_black.py
* Add changelog
* Assert Black reformats an empty string to an empty string (#2337) (#2346)
* fix
Click types have been moved to click repo itself. See pallets/click#1856
I've had some issues with typeshed types being outdated in another project
so might be good to avoid that here.
Commit history before merge:
* Get `click` types from main repo
* Fix mypy errors
* Require click v8 for type annotations
* Update Pipfile
* Add STDIN test to primer
- Check that out STDIN black support stays working
- Add asyncio.subprocess STDIN pip via communicate
- We just check we format python code from primer's `lib.py`
Fixes#2310
`black.strings.get_string_prefix` used to lowercase the extracted
prefix before returning it. This is wrong because 1) it ignores the
fact we should leave R prefixes alone because of MagicPython, and 2)
there is dedicated prefix casing handling code that fixes issue 1.
`.lower` is too naive.
This was originally fixed in 20.8b0, but was reintroduced since 21.4b0.
I also added proper prefix normalization for docstrings by using the
`black.strings.normalize_string_prefix` helper.
Some more test strings were added to make sure strings with capitalized
prefixes aren't treated differently (actually happened with my original
patch, Jelle had to point it out to me).
This commit adds a short section discussing the non-processing of docstrings
besides spacing improvements, mentions comment moving and links to the
AST equivalence discussion. I also added a simple spacing test for good
measure.
Commit history before merge:
* Mention comment non-processing in documentation, add spacing test
* Mention special cases for comment spacing
* Add all special cases, improve wording
Not sure the fix is right. Here is what I found: issue is connected
with line
first.prefix = prefix[comment.consumed :]
in `comments.py`. `first.prefix` is a prefix of the line, that ends
with `# fmt: skip`, but `comment.consumed` is the length of the
`" # fmt: skip"` string. If prefix length is greater than 14,
`first.prefix` will grow every time we apply formatting.
Fixes#2254
Closes#1246: This PR adds a new option (and automatically a toml entry, hooray for existing configuration management 🎉) to require a specific version of Black to be running.
For example: `black --required-version 20.8b -c "format = 'this'"`
Execution fails straight away if it doesn't match `__version__`.
PR #2286 did not fix the edge-cases (e.g. when the string is just long
enough to cause a line to be 89 characters long). This PR corrects that
mistake.
Resolves#2168 by disabling the insertion of a " " when the docstring is entirely empty.
Note that this PR is focussed only on the case of empty docstrings. In particular this does not make any changes to the behaviour that a " " is inserted if a non-empty docstring begins with the quoting character. That is, black still prefers:
""" "something" """
to:
""""something" """
and that:
""""Something""""
is not a legal docstring.
Commit history before merge:
Black now respects .gitignore files in all levels, not only root/.gitignore file
(apply .gitignore rules like git does).
* Fix: typo
* Fix: respect .gitignore files in all levels.
* Add: CHANGELOG note.
* Fix: TypeError: unsupported operand type(s) for +: 'NoneType' and 'PathSpec'
* Update docs.
* Fix: no parent .gitignore
* Add a comment since the if expression is a bit hard to understand
* Update tests - conver no parent .gitignore case.
* Use main's Pipfile.lock instead
The original changes in Pipfile.lock are whitespace only. The changes
turned the JSON's file indentation from 4 to 2. Effectively this
happened: `json.dumps(json.loads(old_pipfile_lock), indent=2) + "\n"`.
Just using main's Pipfile.lock instead of undoing the changes because
1) I don't know how to do that easily and quickly, and 2) there's a
merge conflict.
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>
* Merge remote-tracking branch 'upstream/main' into i1730 …
conflicts for days ay?
We've depended on Click 7.x ever since we broke CI systems across the
world (oops lol) and flake8-mypy was purged a fair bit back: #1867
Also remove the primer tests import in tests/test_black.py because it's
annoying when just trying to actually target tests/test_black.py tests.
`pytest -k test_black.py` doesn't do what you expect due to that import.
There's three optimizations in this commit:
1. Don't check if Black's output is stable or equivalant if no changes
were made in the first place. It's not like passing the same code
(for both source and actual) through black.assert_equivalent or
black.assert_stable is useful. It's not a big deal for the smaller
tests, but it eats a lot of time in tests/test_format.py since
its test cases are big. This is also closer to how Black works IRL.
2. Use a smaller file for `test_root_logger_not_used_directly` since
the logging it's checking happens during blib2to3's startup so the
file doesn't really matter.
3. If we're checking a file is formatting (i.e. test_source_is_formatted)
don't run Black over it again with `black.format_file_in_place`.
`tests/test_format.py::TestSimpleFormat.check_file` is good enough.
* Move string-related utility to functions to strings.py, const.py
* Move Leaf/Node-related functionality to nodes.py
* Move comment-related functions to comments.py
* Move caching to cache.py and Mode/TargetVersion/Feature to mode.py
* Move some leftover functions to nodes.py, comments.py, strings.py
* Add missing files to source list for test runs
* Move line-related functionality into lines.py, brackets into brackets.py
* Move transformers to trans.py
* Move file handling, output, parsing, concurrency, debug, and report
* Move two more functions to nodes.py
* Add CHANGES
* Add numeric.py
* Add linegen.py
* More docstrings
* Include new files in tests
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
This is a follow-up of #2203 that uses a pytest marker instead of a bunch of
`skipUnless`. Similarly to the Python 2 tests, they are running by default and
will crash on an unsuspecting contributor with missing dependencies. This is
by design, we WANT contributors to test everything. Unless we actually don't
and then we can run:
pytest --run-optional=no_blackd
Relatedly, bump required aiohttp to 3.6.0 at least to get rid of expected
failures on Python 3.8 (see 6b5eb7d465).
Closes#2164.
Changes behavior of how .gitignore is handled. With this change, the rules in .gitignore are only used as a fallback if no exclusion rule is explicitly passed on the command line or in pyproject.toml. Previously they were used regardless if explicit exclusion rules were specified, preventing any overriding of .gitignore rules.
Those that depend only on .gitignore for their exclusion rules will not be affected. Those that use both .gitignore and exclude will find that exclude will act more like actually specifying exclude and not just another extra-excludes. If the previous behavior was desired, they should move their rules from exclude to extra-excludes.
Previously the RELAXED_DECORATOR detection would be falsely True on that
example. The problem was that an argument-less parentheses pair didn't
pass the `is_simple_decorator_trailer` check even it should. OTOH a
parentheses pair containing an argument or more passed as expected.
- Allow runs with no code diff output
- This is handy for reducing output to see which file is erroring
Test:
- Edit config for 'channels' to expect no changes and run with `--no-diff` and see no diff output
We're only fixing them so fuzzers don't yell at us when we break "valid"
code. I mean "valid" because some of the examples aren't even accepted by
Python.
Black would previously strip the parenthesis away from statements like this these ones:
assert (spam := 12 + 1)
return (cheese := 1 - 12)
Which happens to be invalid code. Now before making the parenthesis invisible, Black
checks if the assignment expression's parent is an assert stamtment, aborting if True.
Raise, yield, and await are already handled fine.
I added a bunch of test cases from the PEP defining asssignment expressions (PEP 572).
Optional trailing commas put by Black become magic trailing commas on another
pass of the tool. Since they are influencing formatting around optional
parentheses, on rare occasions the tool changes its mind in terms of putting
parentheses or not.
Ideally this would never be the case but sadly the decision to put optional
parentheses or not (which looks at pre-existing "magic" trailing commas) is
happening around the same time as the decision to put an optional trailing
comma. Untangling the two proved to be impractically difficult.
This shameful workaround uses the fact that the formatting instability
introduced by magic trailing commas is deterministic: if the optional trailing
comma becoming a pre-existing "magic" trailing comma changes formatting, the
second pass becomes stable since there is no variable factor anymore on pass 3,
4, and so on.
For most files, this will introduce no performance penalty since `--safe` is
already re-formatting everything twice to ensure formatting stability. We're
using this result and if all's good, the behavior is equivalent. If there is
a difference, we treat the second result as the binding one, and check its
sanity again.
Fixes#1738. Fixes#1812.
Previously, Black removed leading and trailing spaces in multiline docstrings but failed to remove them from one-line docstrings.
* Added support for top-level user configuration
At the user level, a TOML config can be specified in the following locations:
* Windows: ~\.black
* Unix-like: $XDG_CONFIG_HOME/black (~/.config/black fallback)
Instead of changing env vars for the entire black-primer process, they
are now changed only for the black subprocess, using a tmpdir.
Fixes: #1662
Work-around for https://bugs.python.org/issue2142
The test has to slightly mess with its input data, because the utility
functions default to ensuring the test data has a final newline, which
defeats the point of the test.
Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>
* Add test case to illustrate the issue
* Accept carriage returns as valid separators while enumerating comments
Without this acceptance, escaped multi-line statments that use carriage returns will not be counted into the 'ignored_lines' variable since the emitted line values will end with a CR and not an escape character. That leads to comments associated with the line being incorrectly labeled with the STANDALONE_COMMENT type, affecting comment placement and line space management.
* Remove comment linking to ephemeral build log
* Provide a stdin-filename to allow stdin to respect exclude/force-exclude rules
This will allow automatic tools to enforce the project's
exclude/force-exclude rules even if they pass the file through stdin to
update its buffer.
This is a similar solution to --stdin-display-name in flake8.
* Update src/black/__init__.py
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>
* --stdin-filename should only respect --exclude-filename
* Update README with the new --stdin-filename option
* Write some tests for the new stdin-filename functionality
* Apply suggestions from code review
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
* Force stdin output when we asked for stdin even if the file exists
* Add an entry in the changelog regarding --stdin-filename
* Reduce disk reads if possible
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>
* Check for is_stdin and p.is_file before checking for p.is_dir()
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
The `fix_docstring` function expanded all tabs, which caused a
difference in the AST representation when those tabs were inline and not
leading. This changes the function to only expand leading tabs so inline
tabs are preserved.
Fixes#1601.
Black used to erroneously remove all empty lines between non-function
code and decorators when formatting typing stubs. Now a single empty
line is enforced.
I chose for putting empty lines around decorated classes that have empty
bodies since removing empty lines around such classes would cause a
formatting issue that seems to be impossible to fix.
For example:
```
class A: ...
@some_decorator
class B: ...
class C: ...
class D: ...
@some_other_decorator
def foo(): -> None: ...
```
It is easy to enforce no empty lines between class A, B, and C.
Just return 0, 0 for a line that is a decorator and precedes an stub
class. Fortunately before this commit, empty lines after that class
would be removed already.
Now let's look at the empty line between class D and function foo. In
this case, there should be an empty line there since it's class code next
to function code. The problem is that when deciding to add X empty lines
before a decorator, you can't tell whether it's before a class or a
function. If the decorator is before a function, then an empty line
is needed, while no empty lines are needed when the decorator is
before a class.
So even though I personally prefer no empty lines around decorated
classes, I had to go the other way surrounding decorated classes with
empty lines.
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
* Fix incorrect space before colon in if/while stmts
Previously Black would format this code
```
if (foo := True):
print(foo)
```
as
```
if (foo := True) :
print(foo)
```
adding an incorrect space after the RPAR. Buggy code in the
normalize_invisible_parens function caused the colon to be wrapped in
invisible parentheses. The LPAR of that pair was then prefixed with a
single space at the request of the whitespace function.
This commit fixes the accidental skipping of a pre-condition check
which must return True before parenthesis normalization of a specific
child Leaf or Node can happen. The pre-condition check being skipped
was why the colon was wrapped in invisible parentheses.
* Add an entry in CHANGES.md
The quotes of multiline docstrings are now only normalized when string
normalization is off, instead of the string normalization setting being
ignored and the quotes being *always* normalized.
I had to make a new test case and data file since the current pair for
docstrings only worked when there is no formatting difference between the
formatting results with string normalization on and off. I needed to add
tests for when there *are* differences between the two. So I split
test_docstring's test code when string normalization is disabled into a
new test case along with a new data file.
This addresses a few crashers, namely:
* producing non-equivalent code due to mangling escaped newlines,
* invalid hugging quote characters in the docstring body to the docstring outer
triple quotes (causing a quadruple quote which is a syntax error),
* lack of handling for docstrings that start on the same line as the `def`, and
* invalid stripping of outer triple quotes when the docstring contained
a string prefix.
As a bonus, tests now also run when string normalization is disabled.
This required some hackery. Long story short, we need to reuse the ability to
omit rightmost bracket pairs (which glues them together and splits on something
else instead), for use with pre-existing trailing commas.
This form of user-controlled formatting is brittle so we have to be careful not
to cause a scenario where Black first formats code without trailing commas in
one way, and then looks at the same file with pre-existing trailing commas
(that it itself put on the previous run) and decides to format the code again.
One particular ugly edge case here is handling of optional parentheses. In
particular, the long-standing `line_length=1` hack got in the way of
pre-existing trailing commas and had to be removed. Instead, a more
intelligent but costly solution was put in place: a "second opinion" if the
formatting that omits optional parentheses ended up causing lines to be too
long. Again, for efficiency purposes, Black reuses Leaf objects from blib2to3
and modifies them in place, which was invalid for having two separate
formattings. Line cloning was used to mitigate this.
Fixes#1619
