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flake8-comprehensions

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

Installation

Install from pip with:

python -m 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 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 these types than to construct them with literals, e.g. dict() is slower than {}, because the name dict must be looked up in the global scope in case it has been rebound, and it includes the overhead of a function call. Same for the other two builtin 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)