flake8-comprehensions

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

Requirements

Python 3.6 to 3.9 supported.

Installation

First, install with pip:

python -m pip install flake8-comprehensions

Second, check that flake8 lists the plugin in its version line:

$ 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

Third, add the C4 prefix to your select list. For example, if you have your configuration in setup.cfg:

[flake8]
select = E,F,W,C4

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Rules

C400-402: Unnecessary generator - rewrite as a <list/set/dict> comprehension.

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 - rewrite as a <set/dict> 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: Unnecessary <list/tuple> literal - rewrite as a <set/dict> literal.

It's unnecessary to use a list or tuple literal within a call to set or dict. For example:

• Rewrite set([1, 2]) as {1, 2}
• Rewrite set((1, 2)) as {1, 2}
• Rewrite set([]) as set()
• Rewrite dict([(1, 2)]) as {1: 2}
• Rewrite 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 dict(a=1, b=2) as {"a": 1, "b": 2}
• Rewrite list() as []
• Rewrite tuple() as ()

C409-410: Unnecessary <list/tuple> passed to <list/tuple>() - (remove the outer call to <list/tuple>``()/rewrite as a ``<list/tuple> literal).

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

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

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

It's unnecessary to use a list around a 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', as it 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 unnecessary 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 double-cast or double-process iterables by wrapping the listed functions within list/set/sorted/tuple. 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 when passing it into one of the listed functions will change the order again. 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)