flake8-comprehensions 3.2.1

A flake8 plugin to help you write better list/set/dict comprehensions.

flake8-comprehensions

A flake8 plugin that helps you write better list/set/dict comprehensions.

Installation

Install from pip with:

pip install flake8-comprehensions

Python 3.5 to 3.8 supported.

When installed it will automatically be run as part of flake8; you can check it is being picked up with:

$ flake8 --version
3.7.8 (flake8-comprehensions: 3.0.0, mccabe: 0.6.1, pycodestyle: 2.5.0, pyflakes: 2.1.1) CPython 3.8.0 on Linux

Rules

Code	Rule
C400	Unnecessary generator - rewrite as a list comprehension.
C401	Unnecessary generator - rewrite as a set comprehension.
C402	Unnecessary generator - rewrite as a dict comprehension.
C403	Unnecessary list comprehension - rewrite as a set comprehension.
C404	Unnecessary list comprehension - rewrite as a dict comprehension.
C405	Unnecessary (list/tuple) literal - rewrite as a set literal.
C406	Unnecessary (list/tuple) literal - rewrite as a dict literal.
C407	Unnecessary (dict/list) comprehension - ‘<builtin>’ can take a generator.
C408	Unnecessary (dict/list/tuple) call - rewrite as a literal.
C409	Unnecessary (list/tuple) passed to tuple() - (remove the outer call to tuple()/rewrite as a tuple literal).
C410	Unnecessary (list/tuple) passed to list() - (remove the outer call to list()/rewrite as a list literal).
C411	Unnecessary list call - remove the outer call to list().
C412	Unnecessary (dict/list/set) comprehension - ‘in’ can take a generator.
C413	Unnecessary list call around sorted().
C413	Unnecessary reversed call around sorted() - (use sorted(…, reverse=(True/False))/toggle reverse argument to sorted()).
C414	Unnecessary (list/reversed/set/sorted/tuple) call within list/set/sorted/tuple().
C415	Unnecessary subscript reversal of iterable within reversed/set/sorted().
C416	Unnecessary (list/set) comprehension - rewrite using list/set().

Examples

C400-402: Unnecessary generator

It’s unnecessary to use list, set, or dict around a generator expression, since there are equivalent comprehensions for these types. For example:

• Rewrite list(f(x) for x in foo) as [f(x) for x in foo]

• Rewrite set(f(x) for x in foo) as {f(x) for x in foo}

• Rewrite dict((x, f(x)) for x in foo) as {x: f(x) for x in foo}

C403-404: Unnecessary list comprehension

It’s unnecessary to use a list comprehension inside a call to set or dict, since there are equivalent comprehensions for these types. For example:

• Rewrite set([f(x) for x in foo]) as {f(x) for x in foo}

• Rewrite dict([(x, f(x)) for x in foo]) as {x: f(x) for x in foo}

C405-406,C409-410: Unnecessary list/tuple literal

It’s unnecessary to use a list or tuple literal within a call to tuple, list, set, or dict since there is literal syntax for these types. For example:

• Rewrite tuple([1, 2]) or tuple((1, 2)) as (1, 2)

• Rewrite tuple([]) as ()

• Rewrite list([1, 2]) or list((1, 2)) as [1, 2]

• Rewrite list([]) as []

• Rewrite set([1, 2]) or set((1, 2)) as {1, 2}

• Rewrite set([]) as set()

• Rewrite dict([(1, 2)]) or dict(((1, 2),)) as {1: 2}

• Rewrite dict([]) as {}

C407: Unnecessary (dict/list) comprehension - ‘<builtin>’ can take a generator

It’s unnecessary to pass a list comprehension to some builtins that can take generators instead. For example:

• Rewrite sum({x ** 2 for x in range(10)}) as sum(x ** 2 for x in range(10))

• Rewrite all([foo.bar for foo in foos]) as all(foo.bar for foo in foos)

• Rewrite filter(lambda x: x % 2 == 0, [x ** 3 for x in range(10)]) as filter(lambda x: x % 2 == 0, (x ** 3 for x in range(10)))

The list of builtins that are checked for are:

• all

• any

• enumerate

• filter

• frozenset

• map

• max

• min

• sorted

• sum

• tuple

C408: Unnecessary (dict/list/tuple) call - rewrite as a literal.

It’s slower to call e.g. dict() than using the empty literal, because the name dict must be looked up in the global scope in case it has been rebound. Same for the other two basic types here. For example:

• Rewrite dict() as {}

• Rewrite list() as []

• Rewrite tuple() as ()

C411: Unnecessary list call - remove the outer call to list().

It’s unnecessary to use a list around list comprehension, since it is equivalent without it. For example:

• Rewrite list([f(x) for x in foo]) as [f(x) for x in foo]

C412: Unnecessary (dict/list/set) comprehension - ‘in’ can take a generator.

It’s unnecessary to pass a dict/list/set comprehension to ‘in’ that can take a generator instead. For example:

• Rewrite y in [f(x) for x in foo] as y in (f(x) for x in foo)

• Rewrite y in {x ** 2 for x in foo} as y in (x ** 2 for x in foo)

C413: Unnecessary list/reversed call around sorted().

It’s unnecessary to use list() around sorted() as it already returns a list. It is also suboptimal to use reversed() around sorted() as the latter has a reverse argument. For example:

• Rewrite list(sorted([2, 3, 1])) as sorted([2, 3, 1])

• Rewrite reversed(sorted([2, 3, 1])) as sorted([2, 3, 1], reverse=True)

• Rewrite reversed(sorted([2, 3, 1], reverse=True)) as sorted([2, 3, 1])

C414: Unnecessary (list/reversed/set/sorted/tuple) call within list/set/sorted/tuple().

It’s unnecessary to change the type of the iterable or change the order of elements within certain other function calls that will themselves define the order of the iterable or the type that is output. For example:

• Rewrite list(list(iterable)) as list(iterable)

• Rewrite list(tuple(iterable)) as list(iterable)

• Rewrite tuple(list(iterable)) as tuple(iterable)

• Rewrite tuple(tuple(iterable)) as tuple(iterable)

• Rewrite set(set(iterable)) as set(iterable)

• Rewrite set(list(iterable)) as set(iterable)

• Rewrite set(tuple(iterable)) as set(iterable)

• Rewrite set(sorted(iterable)) as set(iterable)

• Rewrite set(reversed(iterable)) as set(iterable)

• Rewrite sorted(list(iterable)) as sorted(iterable)

• Rewrite sorted(tuple(iterable)) as sorted(iterable)

• Rewrite sorted(sorted(iterable)) as sorted(iterable)

• Rewrite sorted(reversed(iterable)) as sorted(iterable)

C415: Unnecessary subscript reversal of iterable within reversed/set/sorted().

It’s unnecessary to reverse the order of an iterable using a [::-1] before passing it into set() which will randomize the order, sorted() which will return a new sorted list, or reversed() which will effectively return the original iterable. For example:

• Rewrite set(iterable[::-1]) as set(iterable)

• Rewrite sorted(iterable[::-1]) as sorted(iterable, reverse=True)

• Rewrite reversed(iterable[::-1]) as iterable

C416: Unnecessary (list/set) comprehension - rewrite using list/set().

It’s unnecessary to use a list comprehension if the elements are unchanged. The iterable should be wrapped in list() or set() instead. For example:

• Rewrite [x for x in iterable] as list(iterable)

• Rewrite {x for x in iterable} as set(iterable)

History

3.2.1 (2020-01-21)

• Remove check for set comprehensions in rule C407 as it would change the results for certain builtins such as sum().

3.2.0 (2020-01-20)

• Add filter and map to rule C407.

• Check for dict and set comprehensions in rules C407 and C412.

3.1.4 (2019-11-20)

• Remove the tuple/unpacking check from C416 to prevent false positives where the type of the iterable is changed from some iterable to a tuple.

3.1.3 (2019-11-19)

• Ensure the fix for false positives in C416 rule for asynchronous comprehensions runs on Python 3.6 too.

3.1.2 (2019-11-18)

• Fix false positives in C416 rule for list comprehensions returning tuples.

3.1.1 (2019-11-16)

• Fix false positives in C416 rule for asynchronous comprehensions.

3.1.0 (2019-11-15)

• Update Python support to 3.5-3.8.

• Fix false positives for C404 for list comprehensions not directly creating tuples.

• Add C413 rule that checks for unnecessary use of list() or reversed() around sorted().

• Add C414 rule that checks for unnecessary use of the following:

  • list(), reversed(), sorted(), or tuple() within set or sorted()

  • list() or tuple() within list() or tuple()

  • set() within set

• Add C415 rule that checks for unnecessary reversal of an iterable via subscript within reversed(), set(), or sorted().

• Add C416 rule that checks for unnecessary list or set comprehensions that can be rewritten using list() or set().

3.0.1 (2019-10-28)

• Fix version display on flake8 --version (removing dependency on cached-property). Thanks to Jon Dufresne.

3.0.0 (2019-10-25)

• Update Flake8 support to 3.0+ only. 3.0.0 was released in 2016 and the plugin hasn’t been tested with it since.

2.3.0 (2019-10-25)

• Converted setuptools metadata to configuration file. This meant removing the __version__ attribute from the package. If you want to inspect the installed version, use importlib.metadata.version("flake8-comprehensions") (docs / backport).

• Add dependencies on cached-property and importlib-metadata.

• Fix false negatives in C407 for cases when enumerate and sum() are passed more than one argument.

2.2.0 (2019-08-12)

• Update Python support to 3.5-3.7, as 3.4 has reached its end of life.

• C412 rule that complains about using list comprehension with in.

2.1.0 (2019-03-01)

• Add missing builtin enumerate to C407.

2.0.0 (2019-02-02)

• Drop Python 2 support, only Python 3.4+ is supported now.

1.4.1 (2017-05-17)

• Fix false positives in C408 for calls using *args or **kwargs.

1.4.0 (2017-05-14)

• Plugin now reserves the full C4XX code space rather than just C40X

• C408 rule that complains about using tuple(), list(), or dict() instead of a literal.

• C409 and C410 rules that complain about an unnecessary list or tuple that could be rewritten as a literal.

• C411 rule that complains about using list comprehension inside a list() call.

1.3.0 (2017-05-01)

• Don’t allow installation with Flake8 3.2.0 which doesn’t enable the plugin. This bug was fixed in Flake8 3.2.1.

• Prevent false positives of C402 from generators of expressions that aren’t two-tuples.

• C405 and C406 now also complain about unnecessary tuple literals.

1.2.1 (2016-06-27)

• C407 rule that complains about unnecessary list comprehensions inside builtins that can work on generators.

1.2.0 (2016-07-11)

• Split all rule codes by type. This allows granular selection of the rules in flake8 configuration.

1.1.1 (2016-04-06)

• Fix crash on method calls

1.1.0 (2016-04-06)

• C401 rule that complains about unnecessary list comprehensions inside calls to set() or dict().

• C402 rule that complains about unnecessary list literals inside calls to set() or dict().

1.0.0 (2016-04-05)

• C400 rule that complains about an unnecessary usage of a generator when a list/set/dict comprehension would do.