Known Deviations from Black
This document enumerates the known, intentional differences in code style between Black and Ruff's formatter.
For a list of unintentional deviations, see issue tracker.
Trailing end-of-line comments
Black's priority is to fit an entire statement on a line, even if it contains end-of-line comments. In such cases, Black collapses the statement, and moves the comment to the end of the collapsed statement:
# Input
while (
cond1 # almost always true
and cond2 # almost never true
):
print("Do something")
# Black
while cond1 and cond2: # almost always true # almost never true
print("Do something")
Ruff, like Prettier, expands any statement that contains trailing
end-of-line comments. For example, Ruff would avoid collapsing the while test in the snippet
above. This ensures that the comments remain close to their original position and retain their
original intent, at the cost of retaining additional vertical space.
This deviation only impacts unformatted code, in that Ruff's output should not deviate for code that has already been formatted by Black.
Pragma comments are ignored when computing line width
Pragma comments (# type, # noqa, # pyright, # pylint, etc.) are ignored when computing the width of a line.
This prevents Ruff from moving pragma comments around, thereby modifying their meaning and behavior:
See Ruff's pragma comment handling proposal for details.
This is similar to Pyink but a deviation from Black. Black avoids
splitting any lines that contain a # type comment (#997),
but otherwise avoids special-casing pragma comments.
As Ruff expands trailing end-of-line comments, Ruff will also avoid moving pragma comments in cases
like the following, where moving the # noqa to the end of the line causes it to suppress errors
on both first() and second():
# Input
[
first(), # noqa
second()
]
# Black
[first(), second()] # noqa
# Ruff
[
first(), # noqa
second(),
]
Line width vs. line length
Ruff uses the Unicode width of a line to determine if a line fits. Black uses Unicode width for strings, and character width for all other tokens. Ruff also uses Unicode width for identifiers and comments.
Parenthesizing long nested-expressions
Black 24 and newer parenthesizes long conditional expressions and type annotations in function parameters:
# Black
[
"____________________________",
"foo",
"bar",
(
"baz"
if some_really_looooooooong_variable
else "some other looooooooooooooong value"
),
]
def foo(
i: int,
x: (
Loooooooooooooooooooooooong
| Looooooooooooooooong
| Looooooooooooooooooooong
| Looooooong
),
*,
s: str,
) -> None:
pass
# Ruff
[
"____________________________",
"foo",
"bar",
"baz"
if some_really_looooooooong_variable
else "some other looooooooooooooong value",
]
def foo(
i: int,
x: Loooooooooooooooooooooooong
| Looooooooooooooooong
| Looooooooooooooooooooong
| Looooooong,
*,
s: str,
) -> None:
pass
We agree that Ruff's formatting (that matches Black's 23) is hard to read and needs improvement. But we aren't convinced that parenthesizing long nested expressions is the best solution, especially when considering expression formatting holistically. That's why we want to defer the decision until we've explored alternative nested expression formatting styles. See psf/Black#4123 for an in-depth explanation of our concerns and an outline of possible alternatives.
Call expressions with a single multiline string argument
Unlike Black, Ruff preserves the indentation of a single multiline-string argument in a call expression:
# Input
call(
""""
A multiline
string
"""
)
dedent(""""
A multiline
string
""")
# Black
call(
""""
A multiline
string
"""
)
dedent(
""""
A multiline
string
"""
)
# Ruff
call(
""""
A multiline
string
"""
)
dedent(""""
A multiline
string
""")
Black intended to ship a similar style change as part of the 2024 style that always removes the indent. It turned out that this change was too disruptive to justify the cases where it improved formatting. Ruff introduced the new heuristic of preserving the indent. We believe it's a good compromise that improves formatting but minimizes disruption for users.
Blank lines at the start of a block
Black 24 and newer allows blank lines at the start of a block, where Ruff always removes them:
Currently, we are concerned that allowing blank lines at the start of a block leads to unintentional blank lines when refactoring or moving code. However, we will consider adopting Black's formatting at a later point with an improved heuristic. The style change is tracked in #9745.
F-strings
Ruff formats expression parts in f-strings whereas Black does not:
# Input
f'test{inner + "nested_string"} including math {5 ** 3 + 10}'
# Black
f'test{inner + "nested_string"} including math {5 ** 3 + 10}'
# Ruff
f"test{inner + 'nested_string'} including math {5**3 + 10}"
For more details on the formatting style, refer to the f-string formatting section.
Implicit concatenated strings
Ruff merges implicitly concatenated strings if the entire string fits on a single line:
# Input
def test(max_history):
raise argparse.ArgumentTypeError(
f"The value of `--max-history {max_history}` " f"is not a positive integer."
)
# Black
def test(max_history):
raise argparse.ArgumentTypeError(
f"The value of `--max-history {max_history}` " f"is not a positive integer."
)
# Ruff
def test(max_history):
raise argparse.ArgumentTypeError(
f"The value of `--max-history {max_history}` is not a positive integer."
)
Black's unstable style applies the same formatting.
There are few rare cases where Ruff can't merge the implicitly concatenated strings automatically. In those cases, Ruff preserves if the implicit concatenated strings are formatted over multiple lines:
# Input
a = (
r"aaaaaaa"
"bbbbbbbbbbbb"
)
# Black
a = r"aaaaaaa" "bbbbbbbbbbbb"
# Ruff
a = (
r"aaaaaaa"
"bbbbbbbbbbbb"
)
This ensures compatibility with ISC001 (#8272).
assert statements
Unlike Black, Ruff prefers breaking the message over breaking the assertion, similar to how both Ruff and Black prefer breaking the assignment value over breaking the assignment target:
# Input
assert (
len(policy_types) >= priority + num_duplicates
), f"This tests needs at least {priority+num_duplicates} many types."
# Black
assert (
len(policy_types) >= priority + num_duplicates
), f"This tests needs at least {priority+num_duplicates} many types."
# Ruff
assert len(policy_types) >= priority + num_duplicates, (
f"This tests needs at least {priority + num_duplicates} many types."
)
global and nonlocal names are broken across multiple lines by continuations
If a global or nonlocal statement includes multiple names, and exceeds the configured line
width, Ruff will break them across multiple lines using continuations:
# Input
global analyze_featuremap_layer, analyze_featuremapcompression_layer, analyze_latencies_post, analyze_motions_layer, analyze_size_model
# Ruff
global \
analyze_featuremap_layer, \
analyze_featuremapcompression_layer, \
analyze_latencies_post, \
analyze_motions_layer, \
analyze_size_model
Trailing own-line comments on imports are not moved to the next line
Black enforces a single empty line between an import and a trailing own-line comment. Ruff leaves such comments in-place:
# Input
import os
# comment
import sys
# Black
import os
# comment
import sys
# Ruff
import os
# comment
import sys
Parentheses around awaited collections are not preserved
Black preserves parentheses around awaited collections:
Ruff will instead remove them:
This is more consistent to the formatting of other awaited expressions: Ruff and Black both
remove parentheses around, e.g., await (1), only retaining them when syntactically required,
as in, e.g., await (x := 1).
Implicit string concatenations in attribute accesses
Given the following unformatted code:
Internally, Black's logic will first expand the outermost print call:
Since the argument is still too long, Black will then split on the operator with the highest split precedence. In this case, Black splits on the implicit string concatenation, to produce the following Black-formatted code:
Ruff gives implicit concatenations a "lower" priority when breaking lines. As a result, Ruff would instead format the above as:
In general, Black splits implicit string concatenations over multiple lines more often than Ruff, even if those concatenations can fit on a single line. Ruff instead avoids splitting such concatenations unless doing so is necessary to fit within the configured line width.
Own-line comments on expressions don't cause the expression to expand
Given an expression like:
Black associates the comment with some_example_var, thus splitting it over two lines:
Ruff will instead associate the comment with the entire boolean expression, thus preserving the initial formatting:
Tuples are parenthesized when expanded
Ruff tends towards parenthesizing tuples (with a few exceptions), while Black tends to remove tuple parentheses more often.
In particular, Ruff will always insert parentheses around tuples that expand over multiple lines:
There's one exception here. In for loops, both Ruff and Black will avoid inserting unnecessary
parentheses:
# Input
for a, [b, d,] in c:
pass
# Black
for a, [
b,
d,
] in c:
pass
# Ruff
for a, [
b,
d,
] in c:
pass
Single-element tuples are always parenthesized
Ruff always inserts parentheses around single-element tuples, while Black will omit them in some cases:
Adding parentheses around single-element tuples adds visual distinction and helps avoid "accidental" tuples created by extraneous trailing commas (see, e.g., #17181).
Parentheses around call-chain assignment values are not preserved
Given:
def update_emission_strength():
(
get_rgbw_emission_node_tree(self)
.nodes["Emission"]
.inputs["Strength"]
.default_value
) = (self.emission_strength * 2)
Black will preserve the parentheses in (self.emission_strength * 2), whereas Ruff will remove
them.
Both Black and Ruff remove such parentheses in simpler assignments, like:
# Input
def update_emission_strength():
value = (self.emission_strength * 2)
# Black
def update_emission_strength():
value = self.emission_strength * 2
# Ruff
def update_emission_strength():
value = self.emission_strength * 2
Call chain calls break differently in some cases
Black occasionally breaks call chains differently than Ruff; in particular, Black occasionally expands the arguments for the last call in the chain, as in:
# Input
df.drop(
columns=["aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"]
).drop_duplicates().rename(
columns={
"a": "a",
}
).to_csv(path / "aaaaaa.csv", index=False)
# Black
df.drop(
columns=["aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"]
).drop_duplicates().rename(
columns={
"a": "a",
}
).to_csv(
path / "aaaaaa.csv", index=False
)
# Ruff
df.drop(
columns=["aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"]
).drop_duplicates().rename(
columns={
"a": "a",
}
).to_csv(path / "aaaaaa.csv", index=False)
Ruff will only expand the arguments if doing so is necessary to fit within the configured line width.
Note that Black does not apply this last-call argument breaking universally. For example, both Black and Ruff will format the following identically:
# Input
df.drop(
columns=["aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"]
).drop_duplicates(a).rename(
columns={
"a": "a",
}
).to_csv(
path / "aaaaaa.csv", index=False
).other(a)
# Black
df.drop(columns=["aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"]).drop_duplicates(a).rename(
columns={
"a": "a",
}
).to_csv(path / "aaaaaa.csv", index=False).other(a)
# Ruff
df.drop(columns=["aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"]).drop_duplicates(a).rename(
columns={
"a": "a",
}
).to_csv(path / "aaaaaa.csv", index=False).other(a)
Similarly, in some cases, Ruff will collapse composite binary expressions more aggressively than Black, if doing so allows the expression to fit within the configured line width:
# Black
assert AAAAAAAAAAAAAAAAAAAAAA.bbbbbb.fooo(
aaaaaaaaaaaa=aaaaaaaaaaaa
).ccccc() == (len(aaaaaaaaaa) + 1) * fooooooooooo * (
foooooo + 1
) * foooooo * len(
list(foo(bar(4, foo), foo))
)
# Ruff
assert AAAAAAAAAAAAAAAAAAAAAA.bbbbbb.fooo(
aaaaaaaaaaaa=aaaaaaaaaaaa
).ccccc() == (len(aaaaaaaaaa) + 1) * fooooooooooo * (
foooooo + 1
) * foooooo * len(list(foo(bar(4, foo), foo)))
Single with item targeting Python 3.8 or older
Unlike Black, Ruff uses the same layout for with statements with a single context manager as it does for while, if and other compound statements:
# Input
def run(data_path, model_uri):
with pyspark.sql.SparkSession.builder.config(
key="spark.python.worker.reuse", value=True
).config(key="spark.ui.enabled", value=False).master(
"local-cluster[2, 1, 1024]"
).getOrCreate():
# ignore spark log output
spark.sparkContext.setLogLevel("OFF")
print(score_model(spark, data_path, model_uri))
# Black
def run(data_path, model_uri):
with pyspark.sql.SparkSession.builder.config(
key="spark.python.worker.reuse", value=True
).config(key="spark.ui.enabled", value=False).master(
"local-cluster[2, 1, 1024]"
).getOrCreate():
# ignore spark log output
spark.sparkContext.setLogLevel("OFF")
print(score_model(spark, data_path, model_uri))
# Ruff
def run(data_path, model_uri):
with (
pyspark.sql.SparkSession.builder.config(
key="spark.python.worker.reuse", value=True
)
.config(key="spark.ui.enabled", value=False)
.master("local-cluster[2, 1, 1024]")
.getOrCreate()
):
# ignore spark log output
spark.sparkContext.setLogLevel("OFF")
print(score_model(spark, data_path, model_uri))
Ruff's formatting matches the formatting of other compound statements:
def test():
if (
pyspark.sql.SparkSession.builder.config(
key="spark.python.worker.reuse", value=True
)
.config(key="spark.ui.enabled", value=False)
.master("local-cluster[2, 1, 1024]")
.getOrCreate()
):
# ignore spark log output
spark.sparkContext.setLogLevel("OFF")
print(score_model(spark, data_path, model_uri))
The last context manager in a with statement may be collapsed onto a single line
When using a with statement with multiple unparenthesized context managers, Ruff may collapse the
last context manager onto a single line, if doing so allows the with statement to fit within the
configured line width.
Black, meanwhile, tends to break the last context manager slightly differently, as in the following example:
# Black
with tempfile.TemporaryDirectory() as d1:
symlink_path = Path(d1).joinpath("testsymlink")
with tempfile.TemporaryDirectory(dir=d1) as d2, tempfile.TemporaryDirectory(
dir=d1
) as d4, tempfile.TemporaryDirectory(dir=d2) as d3, tempfile.NamedTemporaryFile(
dir=d4
) as source_file, tempfile.NamedTemporaryFile(
dir=d3
) as lock_file:
pass
# Ruff
with tempfile.TemporaryDirectory() as d1:
symlink_path = Path(d1).joinpath("testsymlink")
with tempfile.TemporaryDirectory(dir=d1) as d2, tempfile.TemporaryDirectory(
dir=d1
) as d4, tempfile.TemporaryDirectory(dir=d2) as d3, tempfile.NamedTemporaryFile(
dir=d4
) as source_file, tempfile.NamedTemporaryFile(dir=d3) as lock_file:
pass
When targeting Python 3.9 or newer, parentheses will be inserted around the context managers to allow for clearer breaks across multiple lines, as in:
with tempfile.TemporaryDirectory() as d1:
symlink_path = Path(d1).joinpath("testsymlink")
with (
tempfile.TemporaryDirectory(dir=d1) as d2,
tempfile.TemporaryDirectory(dir=d1) as d4,
tempfile.TemporaryDirectory(dir=d2) as d3,
tempfile.NamedTemporaryFile(dir=d4) as source_file,
tempfile.NamedTemporaryFile(dir=d3) as lock_file,
):
pass
Preserving parentheses around single-element lists
Ruff preserves at least one parentheses around list elements, even if the list only contains a single element. The Black 2025 or newer, on the other hand, removes the parentheses for single-element lists if they aren't multiline and doing so does not change semantics: