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 eventually 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 will use parentheses instead 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.

Removing newlines in the beginning of code blocks#

Black will remove newlines in the beginning of new code blocks, i.e. when the indentation level is increased. For example:

def my_func():

    print("The line above me will be deleted!")

will be changed to:

def my_func():
    print("The line above me will be deleted!")

This new feature will be applied to all code blocks: def, class, if, for, while, with, case and match.

Improved parentheses management#

Black will format parentheses around return annotations similarly to other sets of parentheses. For example:

def foo() -> (int):
    ...

def foo() -> looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong:
    ...

will be changed to:

def foo() -> int:
    ...


def foo() -> (
    looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong
):
    ...

And, extra parentheses in await expressions and with statements are removed. For example:

with ((open("bla.txt")) as f, open("x")):
    ...

async def main():
    await (asyncio.sleep(1))

will be changed to:

with open("bla.txt") as f, open("x"):
    ...


async def main():
    await asyncio.sleep(1)