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The (future of the) Black code 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. Because the functionality is experimental, feedback and issue reports are highly encouraged!

In the past, the preview style included some features with known bugs, so that we were unable to move these features to the stable style. Therefore, such features are now moved to the --unstable style. All features in the --preview style are expected to make it to next year's stable style; features in the --unstable style will be stabilized only if issues with them are fixed. If bugs are discovered in a --preview feature, it is demoted to the --unstable style. To avoid thrash when a feature is demoted from the --preview to the --unstable style, users can use the --enable-unstable-feature flag to enable specific unstable features.

Currently, the following features are included in the preview style:

  • hex_codes_in_unicode_sequences: normalize casing of Unicode escape characters in strings
  • unify_docstring_detection: fix inconsistencies in whether certain strings are detected as docstrings
  • hug_parens_with_braces_and_square_brackets: more compact formatting of nested brackets (see below)
  • no_normalize_fmt_skip_whitespace: whitespace before # fmt: skip comments is no longer normalized

(labels/unstable-features)=

The unstable style additionally includes the following features:

  • string_processing: split long string literals and related changes (see below)
  • wrap_long_dict_values_in_parens: add parentheses to long values in dictionaries (see below)
  • multiline_string_handling: more compact formatting of expressions involving multiline strings (see below)

(labels/hug-parens)=

Improved multiline dictionary and list indentation for sole function parameter

For better readability and less verticality, Black now pairs parentheses ("(", ")") with braces ("{", "}") and square brackets ("[", "]") on the same line. For example:

foo(
    [
        1,
        2,
        3,
    ]
)

nested_array = [
    [
        1,
        2,
        3,
    ]
]

will be changed to:

foo([
    1,
    2,
    3,
])

nested_array = [[
    1,
    2,
    3,
]]

This also applies to list and dictionary unpacking:

foo(
    *[
        a_long_function_name(a_long_variable_name)
        for a_long_variable_name in some_generator
    ]
)

will become:

foo(*[
    a_long_function_name(a_long_variable_name)
    for a_long_variable_name in some_generator
])

You can use a magic trailing comma to avoid this compacting behavior; by default, Black will not reformat the following code:

foo(
    [
        1,
        2,
        3,
    ],
)

(labels/string-processing)=

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.

(labels/wrap-long-dict-values)=

Improved parentheses management in dicts

For dict literals with long values, they are now wrapped in parentheses. Unnecessary parentheses are now removed. For example:

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:

my_dict = {
    "a key in my dict": (
        a_very_long_variable * and_a_very_long_function_call() / 100000.0
    ),
    "another key": short_value,
}

(labels/multiline-string-handling)=

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

textwrap.dedent(
    """\
    This is a
    multiline string
"""
)

to:

textwrap.dedent("""\
    This is a
    multiline string
""")

And:

MULTILINE = """
foobar
""".replace(
    "\n", ""
)

to:

MULTILINE = """
foobar
""".replace("\n", "")

Implicit multiline strings are special, because they can have inline comments. Strings without comments are merged, for example

s = (
    "An "
    "implicit "
    "multiline "
    "string"
)

becomes

s = "An implicit multiline string"

A comment on any line of the string (or between two string lines) will block the merging, so

s = (
    "An "  # Important comment concerning just this line
    "implicit "
    "multiline "
    "string"
)

and

s = (
    "An "
    "implicit "
    # Comment in between
    "multiline "
    "string"
)

will not be merged. Having the comment after or before the string lines (but still inside the parens) will merge the string. For example

s = (  # Top comment
    "An "
    "implicit "
    "multiline "
    "string"
    # Bottom comment
)

becomes

s = (  # Top comment
    "An implicit multiline string"
    # Bottom comment
)

Potential future changes

This section lists changes that we may want to make in the future, but that aren't implemented yet.

Using backslashes for with statements

Backslashes are bad and should be never be used however there is one exception: with statements using multiple context managers. Before Python 3.9 Python's grammar does not allow organizing parentheses around the series of context managers.

We don't want formatting like:

with make_context_manager1() as cm1, make_context_manager2() as cm2, make_context_manager3() as cm3, make_context_manager4() as cm4:
    ...  # nothing to split on - line too long

So Black will, when we implement this, format it like this:

with \
     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

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.

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:

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)=