The (future of the) Black code style

Warning

Changes to this document often aren’t tied and don’t relate to releases of Black. It’s recommended that you read the latest version available.

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 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())
    ...

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!

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.

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:

some_dict[
    "with_a_long_key"
] = some_looooooooong_module.some_looooooooooooooong_function_name(
    first_argument, second_argument, third_argument
)

will be changed to:

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:

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,
}

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", "")