From: Łukasz Langa Date: Wed, 28 Apr 2021 14:48:04 +0000 (+0200) Subject: Elaborate on what AST changes Black might perform X-Git-Url: https://git.madduck.net/etc/vim.git/commitdiff_plain/b39999da7f451c285befac217f1f9a685774b34d?ds=sidebyside Elaborate on what AST changes Black might perform --- diff --git a/CHANGES.md b/CHANGES.md index 5b9f574..6675c48 100644 --- a/CHANGES.md +++ b/CHANGES.md @@ -6,8 +6,9 @@ - Fix crash if the user configuration directory is inaccessible. (#2158) -- Clarify that _Black_ may change the AST, especially when cleaning up docstrings. - (#2159) +- Clarify + [circumstances](https://github.com/psf/black/blob/master/docs/the_black_code_style.md#pragmatism) + in which _Black_ may change the AST (#2159) #### _Packaging_ diff --git a/README.md b/README.md index c7577fb..41cea76 100644 --- a/README.md +++ b/README.md @@ -237,11 +237,11 @@ is that **until the formatter becomes stable, you should expect some formatting change in the future**. That being said, no drastic stylistic changes are planned, mostly responses to bug reports. -Also, as a temporary safety measure, _Black_ will check that the reformatted code still -produces a valid AST that is mostly equivalent to the original. This slows it down. If -you're feeling confident, use `--fast`. In a few contexts, Black does make changes to -the AST: it cleans up whitespace in docstrings, adds or removes parentheses in some -`del` statements, and may move around type comments. +Also, as a safety measure which slows down processing, _Black_ will check that the +reformatted code still produces a valid AST that is effectively equivalent to the +original (see the +[Pragmatism](https://github.com/psf/black/blob/master/docs/the_black_code_style.md#pragmatism) +section for details). If you're feeling confident, use `--fast`. ## The _Black_ code style diff --git a/docs/the_black_code_style.md b/docs/the_black_code_style.md index 67ccb8a..39a452f 100644 --- a/docs/the_black_code_style.md +++ b/docs/the_black_code_style.md @@ -464,3 +464,32 @@ exception to this rule is r-strings. It turns out that the very popular default by (among others) GitHub and Visual Studio Code, differentiates between r-strings and R-strings. The former are syntax highlighted as regular expressions while the latter are treated as true raw strings with no special semantics. + +### AST before and after formatting + +When run with `--safe`, _Black_ checks that the code before and after is semantically +equivalent. This check is done by comparing the AST of the source with the AST of the +target. There are three limited cases in which the AST does differ: + +1. _Black_ cleans up leading and trailing whitespace of docstrings, re-indenting them if + needed. It's been one of the most popular user-reported features for the formatter to + fix whitespace issues with docstrings. While the result is technically an AST + difference, due to the various possibilities of forming docstrings, all realtime use + of docstrings that we're aware of sanitizes indentation and leading/trailing + whitespace anyway. + +2. _Black_ manages optional parentheses for some statements. In the case of the `del` + statement, presence of wrapping parentheses or lack of thereof changes the resulting + AST but is semantically equivalent in the interpreter. + +3. _Black_ might move comments around, which includes type comments. Those are part of + the AST as of Python 3.8. While the tool implements a number of special cases for + those comments, there is no guarantee they will remain where they were in the source. + Note that this doesn't change runtime behavior of the source code. + +To put things in perspective, the code equivalence check is a feature of _Black_ which +other formatters don't implement at all. It is of crucial importance to us to ensure +code behaves the way it did before it got reformatted. We treat this as a feature and +there are no plans to relax this in the future. The exceptions enumerated above stem +from either user feedback or implementation details of the tool. In each case we made +due diligence to ensure that the AST divergence is of no practical consequence.