## Code style
-_Black_ reformats entire files in place. Style configuration options are deliberately
-limited and rarely added. It doesn't take previous formatting into account, except for
-the magic trailing comma and preserving newlines. It doesn't reformat blocks that start
-with `# fmt: off` and end with `# fmt: on`, or lines that ends with `# fmt: skip`.
-`# fmt: on/off` have to be on the same level of indentation. It also recognizes
+_Black_ aims for consistency, generality, readability and reducing git diffs. Similar
+language constructs are formatted with similar rules. Style configuration options are
+deliberately limited and rarely added. Previous formatting is taken into account as
+little as possible, with rare exceptions like the magic trailing comma. The coding style
+used by _Black_ can be viewed as a strict subset of PEP 8.
+
+_Black_ reformats entire files in place. It doesn't reformat lines that end with
+`# fmt: skip` or blocks that start with `# fmt: off` and end with `# fmt: on`.
+`# fmt: on/off` must be on the same level of indentation and in the same block, meaning
+no unindents beyond the initial indentation level between them. It also recognizes
[YAPF](https://github.com/google/yapf)'s block comments to the same effect, as a
courtesy for straddling code.
+The rest of this document describes the current formatting style. If you're interested
+in trying out where the style is heading, see [future style](./future_style.md) and try
+running `black --preview`.
+
### How _Black_ wraps lines
_Black_ ignores previous formatting and applies uniform horizontal and vertical
whitespace to your code. The rules for horizontal whitespace can be summarized as: do
-whatever makes `pycodestyle` happy. The coding style used by _Black_ can be viewed as a
-strict subset of PEP 8.
+whatever makes `pycodestyle` happy.
As for vertical whitespace, _Black_ tries to render one full expression or simple
statement per line. If this fits the allotted line length, great.
...
```
+If a data structure literal (tuple, list, set, dict) or a line of "from" imports cannot
+fit in the allotted length, it's always split into one element per line. This minimizes
+diffs as well as enables readers of code to find which commit introduced a particular
+entry. This also makes _Black_ compatible with
+[isort](../guides/using_black_with_other_tools.md#isort) with the ready-made `black`
+profile or manual configuration.
+
+You might have noticed that closing brackets are always dedented and that a trailing
+comma is always added. Such formatting produces smaller diffs; when you add or remove an
+element, it's always just one line. Also, having the closing bracket dedented provides a
+clear delimiter between two distinct sections of the code that otherwise share the same
+indentation level (like the arguments list and the docstring in the example above).
+
(labels/why-no-backslashes)=
_Black_ prefers parentheses over backslashes, and will remove backslashes if found.
slightly refactor your code. I hope some of the examples above show you that there are
many ways in which you can do it.
-You might have noticed that closing brackets are always dedented and that a trailing
-comma is always added. Such formatting produces smaller diffs; when you add or remove an
-element, it's always just one line. Also, having the closing bracket dedented provides a
-clear delimiter between two distinct sections of the code that otherwise share the same
-indentation level (like the arguments list and the docstring in the example above).
-
-If a data structure literal (tuple, list, set, dict) or a line of "from" imports cannot
-fit in the allotted length, it's always split into one element per line. This minimizes
-diffs as well as enables readers of code to find which commit introduced a particular
-entry. This also makes _Black_ compatible with
-[isort](../guides/using_black_with_other_tools.md#isort) with the ready-made `black`
-profile or manual configuration.
+(labels/line-length)=
### Line length
side-by-side diff review on typical screen resolutions. Long lines also make it harder
to present code neatly in documentation or talk slides.
-If you're using Flake8, you can bump `max-line-length` to 88 and mostly forget about it.
-However, it's better if you use [Bugbear](https://github.com/PyCQA/flake8-bugbear)'s
-B950 warning instead of E501, and bump the max line length to 88 (or the `--line-length`
-you used for black), which will align more with black's _"try to respect
-`--line-length`, but don't become crazy if you can't"_. You'd do it like this:
-
-```ini
-[flake8]
-max-line-length = 88
-...
-select = C,E,F,W,B,B950
-extend-ignore = E203, E501
-```
+#### Flake8
-Explanation of why E203 is disabled can be found further in this documentation. And if
-you're curious about the reasoning behind B950,
-[Bugbear's documentation](https://github.com/PyCQA/flake8-bugbear#opinionated-warnings)
-explains it. The tl;dr is "it's like highway speed limits, we won't bother you if you
-overdo it by a few km/h".
+If you use Flake8, you have a few options:
-**If you're looking for a minimal, black-compatible flake8 configuration:**
+1. Recommended is using [Bugbear](https://github.com/PyCQA/flake8-bugbear) and enabling
+ its B950 check instead of using Flake8's E501, because it aligns with Black's 10%
+ rule. Install Bugbear and use the following config:
-```ini
-[flake8]
-max-line-length = 88
-extend-ignore = E203
-```
+ ```ini
+ [flake8]
+ max-line-length = 80
+ ...
+ select = C,E,F,W,B,B950
+ extend-ignore = E203, E501, E704
+ ```
+
+ The rationale for E950 is explained in
+ [Bugbear's documentation](https://github.com/PyCQA/flake8-bugbear#opinionated-warnings).
+
+2. For a minimally compatible config:
+
+ ```ini
+ [flake8]
+ max-line-length = 88
+ extend-ignore = E203, E704
+ ```
+
+An explanation of why E203 is disabled can be found in the [Slices section](#slices) of
+this page.
### Empty lines
_Black_ will allow single empty lines inside functions, and single and double empty
lines on module level left by the original editors, except when they're within
parenthesized expressions. Since such expressions are always reformatted to fit minimal
-space, this whitespace is lost.
+space, this whitespace is lost. The other exception is that it will remove any empty
+lines immediately following a statement that introduces a new indentation level.
+
+```python
+# in:
+
+def foo():
+
+ print("All the newlines above me should be deleted!")
+
+
+if condition:
+
+ print("No newline above me!")
+
+ print("There is a newline above me, and that's OK!")
+
+
+class Point:
+
+ x: int
+ y: int
+
+# out:
+
+def foo():
+ print("All the newlines above me should be deleted!")
+
+
+if condition:
+ print("No newline above me!")
+
+ print("There is a newline above me, and that's OK!")
+
+
+class Point:
+ x: int
+ y: int
+```
It will also insert proper spacing before and after function definitions. It's one line
before and after inner functions and two lines before and after module-level functions
_Black_ does not format comment contents, but it enforces two spaces between code and a
comment on the same line, and a space before the comment text begins. Some types of
-comments that require specific spacing rules are respected: doc comments (`#: comment`),
-section comments with long runs of hashes, and Spyder cells. Non-breaking spaces after
-hashes are also preserved. Comments may sometimes be moved because of formatting
-changes, which can break tools that assign special meaning to them. See
-[AST before and after formatting](#ast-before-and-after-formatting) for more discussion.
+comments that require specific spacing rules are respected: shebangs (`#! comment`), doc
+comments (`#: comment`), section comments with long runs of hashes, and Spyder cells.
+Non-breaking spaces after hashes are also preserved. Comments may sometimes be moved
+because of formatting changes, which can break tools that assign special meaning to
+them. See [AST before and after formatting](#ast-before-and-after-formatting) for more
+discussion.
### Trailing commas
bracket pair into one item per line. Read more about this in the
[Pragmatism](#pragmatism) section below.
+(labels/strings)=
+
### Strings
_Black_ prefers double quotes (`"` and `"""`) over single quotes (`'` and `'''`). It
will replace the latter with the former as long as it does not result in more backslash
escapes than before.
-_Black_ also standardizes string prefixes, making them always lowercase. On top of that,
-if your code is already Python 3.6+ only or it's using the `unicode_literals` future
-import, _Black_ will remove `u` from the string prefix as it is meaningless in those
-scenarios.
+_Black_ also standardizes string prefixes. Prefix characters are made lowercase with the
+exception of [capital "R" prefixes](#rstrings-and-rstrings), unicode literal markers
+(`u`) are removed because they are meaningless in Python 3, and in the case of multiple
+characters "r" is put first as in spoken language: "raw f-string".
The main reason to standardize on a single form of quotes is aesthetics. Having one kind
of quotes everywhere reduces reader distraction. It will also enable a future version of
you can pass `--skip-string-normalization` on the command line. This is meant as an
adoption helper, avoid using this for new projects.
-(labels/experimental-string)=
-
-As an experimental option (can be enabled by `--experimental-string-processing`),
-_Black_ splits long strings (using parentheses where appropriate) and merges short ones.
-When split, parts of f-strings that don't need formatting are converted to plain
-strings. User-made splits are respected when they do not exceed the line length limit.
-Line continuation backslashes are converted into parenthesized strings. Unnecessary
-parentheses are stripped. Because the functionality is experimental, feedback and issue
-reports are highly encouraged!
-
_Black_ also processes docstrings. Firstly the indentation of docstrings is corrected
for both quotations and the text within, although relative indentation in the text is
preserved. Superfluous trailing whitespace on each line and unnecessary new lines at the
_Black_ standardizes most numeric literals to use lowercase letters for the syntactic
parts and uppercase letters for the digits themselves: `0xAB` instead of `0XAB` and
-`1e10` instead of `1E10`. Python 2 long literals are styled as `2L` instead of `2l` to
-avoid confusion between `l` and `1`.
+`1e10` instead of `1E10`.
### Line breaks & binary operators
[PEP 8](https://www.python.org/dev/peps/pep-0008/#should-a-line-break-before-or-after-a-binary-operator)
style guide, which emphasizes that this approach improves readability.
+Almost all operators will be surrounded by single spaces, the only exceptions are unary
+operators (`+`, `-`, and `~`), and power operators when both operands are simple. For
+powers, an operand is considered simple if it's only a NAME, numeric CONSTANT, or
+attribute access (chained attribute access is allowed), with or without a preceding
+unary operator.
+
+```python
+# For example, these won't be surrounded by whitespace
+a = x**y
+b = config.base**5.2
+c = config.base**runtime.config.exponent
+d = 2**5
+e = 2**~5
+
+# ... but these will be surrounded by whitespace
+f = 2 ** get_exponent()
+g = get_x() ** get_y()
+h = config['base'] ** 2
+```
+
### Slices
PEP 8
_Black_ enforces the above rules. There are additional guidelines for formatting `.pyi`
file that are not enforced yet but might be in a future version of the formatter:
-- all function bodies should be empty (contain `...` instead of the body);
-- do not use docstrings;
- prefer `...` over `pass`;
-- for arguments with a default, use `...` instead of the actual default;
- avoid using string literals in type annotations, stub files support forward references
natively (like Python 3.7 code with `from __future__ import annotations`);
- use variable annotations instead of type comments, even for stubs that target older
- versions of Python;
-- for arguments that default to `None`, use `Optional[]` explicitly;
-- use `float` instead of `Union[int, float]`.
+ versions of Python.
+
+### Line endings
+
+_Black_ will normalize line endings (`\n` or `\r\n`) based on the first line ending of
+the file.
## Pragmatism
_Black_ does make some exceptions to rules it otherwise holds. This section documents
what those exceptions are and why this is the case.
+(labels/magic-trailing-comma)=
+
### The magic trailing comma
_Black_ in general does not take existing formatting into account.
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.
+(labels/ast-changes)=
+
### 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:
+When run with `--safe` (the default), _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
+ difference, due to the various possibilities of forming docstrings, all real-world
+ uses of docstrings that we're aware of sanitize indentation and leading/trailing
whitespace anyway.
1. _Black_ manages optional parentheses for some statements. In the case of the `del`
projects / etc / vim.git / blobdiff