Run pyupgrade on blib2to3 and src (#3771)

[etc/vim.git] / docs / the_black_code_style / future_style.md

diff --git a/docs/the_black_code_style/future_style.md b/docs/the_black_code_style/future_style.md
index a7676090553a319a2f4ef003bed71634f0b17d3a..861bb64bff4d10016fde5db5230d887fbca364ec 100644 (file)
--- a/docs/the_black_code_style/future_style.md
+++ b/docs/the_black_code_style/future_style.md
@@ -19,24 +19,144 @@ with make_context_manager1() as cm1, make_context_manager2() as cm2, make_contex
     ...  # nothing to split on - line too long
 ```
 
     ...  # nothing to split on - line too long
 ```
 

-So _Black_ will eventually format it like this:
+So _Black_ will, when we implement this, format it like this:

 
 ```py3
 with \
 
 ```py3
 with \

-     make_context_manager(1) as cm1, \
-     make_context_manager(2) as cm2, \
-     make_context_manager(3) as cm3, \
-     make_context_manager(4) as cm4 \
+     make_context_manager1() as cm1, \
+     make_context_manager2() as cm2, \
+     make_context_manager3() as cm3, \
+     make_context_manager4() as cm4 \

 :
     ...  # backslashes and an ugly stranded colon
 ```
 
 :
     ...  # backslashes and an ugly stranded colon
 ```
 

-Although when the target version is Python 3.9 or higher, _Black_ will use parentheses
-instead since they're allowed in Python 3.9 and higher.
+Although when the target version is Python 3.9 or higher, _Black_ uses parentheses
+instead in `--preview` mode (see below) since they're allowed in Python 3.9 and higher.

 
 

-## Improved string processing
+An alternative to consider if the backslashes in the above formatting are undesirable is
+to use {external:py:obj}`contextlib.ExitStack` to combine context managers in the
+following way:

 
 

-Currently, _Black_ does not split long strings to fit the line length limit. Currently,
-there is [an experimental option](labels/experimental-string) to enable splitting
-strings. We plan to enable this option by default once it is fully stable. This is
+```python
+with contextlib.ExitStack() as exit_stack:
+    cm1 = exit_stack.enter_context(make_context_manager1())
+    cm2 = exit_stack.enter_context(make_context_manager2())
+    cm3 = exit_stack.enter_context(make_context_manager3())
+    cm4 = exit_stack.enter_context(make_context_manager4())
+    ...
+```
+
+(labels/preview-style)=
+
+## Preview style
+
+Experimental, potentially disruptive style changes are gathered under the `--preview`
+CLI flag. At the end of each year, these changes may be adopted into the default style,
+as described in [The Black Code Style](index.md). Because the functionality is
+experimental, feedback and issue reports are highly encouraged!
+
+### Improved string processing
+
+_Black_ will split long string literals and merge short ones. Parentheses are used where
+appropriate. 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. The stability and status of this feature is

 tracked in [this issue](https://github.com/psf/black/issues/2188).
 tracked in [this issue](https://github.com/psf/black/issues/2188).

+
+### Improved line breaks
+
+For assignment expressions, _Black_ now prefers to split and wrap the right side of the
+assignment instead of left side. For example:
+
+```python
+some_dict[
+    "with_a_long_key"
+] = some_looooooooong_module.some_looooooooooooooong_function_name(
+    first_argument, second_argument, third_argument
+)
+```
+
+will be changed to:
+
+```python
+some_dict["with_a_long_key"] = (
+    some_looooooooong_module.some_looooooooooooooong_function_name(
+        first_argument, second_argument, third_argument
+    )
+)
+```
+
+### Improved parentheses management
+
+For dict literals with long values, they are now wrapped in parentheses. Unnecessary
+parentheses are now removed. For example:
+
+```python
+my_dict = {
+    "a key in my dict": a_very_long_variable
+    * and_a_very_long_function_call()
+    / 100000.0,
+    "another key": (short_value),
+}
+```
+
+will be changed to:
+
+```python
+my_dict = {
+    "a key in my dict": (
+        a_very_long_variable * and_a_very_long_function_call() / 100000.0
+    ),
+    "another key": short_value,
+}
+```
+
+### Improved multiline string handling
+
+_Black_ is smarter when formatting multiline strings, especially in function arguments,
+to avoid introducing extra line breaks. Previously, it would always consider multiline
+strings as not fitting on a single line. With this new feature, _Black_ looks at the
+context around the multiline string to decide if it should be inlined or split to a
+separate line. For example, when a multiline string is passed to a function, _Black_
+will only split the multiline string if a line is too long or if multiple arguments are
+being passed.
+
+For example, _Black_ will reformat
+
+```python
+textwrap.dedent(
+    """\
+    This is a
+    multiline string
+"""
+)
+```
+
+to:
+
+```python
+textwrap.dedent("""\
+    This is a
+    multiline string
+""")
+```
+
+And:
+
+```python
+MULTILINE = """
+foobar
+""".replace(
+    "\n", ""
+)
+```
+
+to:
+
+```python
+MULTILINE = """
+foobar
+""".replace("\n", "")
+```