flake8-comprehensions

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

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Requirements

Python 3.9 to 3.13 supported.

Installation

First, install with pip:

python -m pip install flake8-comprehensions

Second, if you define Flake8’s select setting, add the C4 prefix to it. Otherwise, the plugin should be active by default.

Rules

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

Rules:

• C400 Unnecessary generator - rewrite as a list comprehension.
• C401 Unnecessary generator - rewrite as a set comprehension.
• C402 Unnecessary generator - rewrite as a 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.

Rules:

• C403 Unnecessary list comprehension - rewrite as a set comprehension.
• C404 Unnecessary list comprehension - rewrite as a 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.

• C405 Unnecessary <list/tuple> literal - rewrite as a set literal.
• C406 Unnecessary <list/tuple> literal - rewrite as a 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

This rule was dropped in version 3.4.0, because it promoted an increase in laziness which could lead to bugs.

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>() - <advice>.

Rules:

• C409 Unnecessary <list/tuple> passed to tuple() - <advice>.
• C410 Unnecessary list passed to list() - <advice>.

Where <advice> is either:

• 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.

This rule was dropped in version 3.4.0, because it promoted an increase in laziness which could lead to bugs.

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 <dict/list/set> comprehension - rewrite using <dict/list/set>().

It's unnecessary to use a dict/list/set comprehension to build a data structure if the elements are unchanged. Wrap the iterable with dict(), list(), or set() instead. For example:

• Rewrite {a: b for a, b in iterable} as dict(iterable)
• Rewrite [x for x in iterable] as list(iterable)
• Rewrite {x for x in iterable} as set(iterable)

C417: Unnecessary map usage - rewrite using a generator expression/<list/set/dict> comprehension.

map(func, iterable) has great performance when func is a built-in function, and it makes sense if your function already has a name. But if your func is a lambda, it’s faster to use a generator expression or a comprehension, as it avoids the function call overhead. For example:

• Rewrite map(lambda x: x + 1, iterable) to (x + 1 for x in iterable)
• Rewrite map(lambda item: get_id(item), items) to (get_id(item) for item in items)
• Rewrite list(map(lambda num: num * 2, nums)) to [num * 2 for num in nums]
• Rewrite set(map(lambda num: num % 2 == 0, nums)) to {num % 2 == 0 for num in nums}
• Rewrite dict(map(lambda v: (v, v ** 2), values)) to {v : v ** 2 for v in values}

C418: Unnecessary <dict/dict comprehension> passed to dict() - remove the outer call to dict()

It's unnecessary to use a dict around a dict literal or dict comprehension, since either syntax already constructs a dict. For example:

• Rewrite dict({}) as {}
• Rewrite dict({"a": 1}) as {"a": 1}

C419 Unnecessary list comprehension in <any/all>() prevents short-circuiting - rewrite as a generator.

Using a list comprehension inside a call to any()/all() prevents short-circuiting when a True / False value is found. The whole list will be constructed before calling any()/all(), potentially wasting work.part-way. Rewrite to use a generator expression, which can stop part way. For example:

• Rewrite all([condition(x) for x in iterable]) as all(condition(x) for x in iterable)
• Rewrite any([condition(x) for x in iterable]) as any(condition(x) for x in iterable)

C420: Unnecessary dict comprehension - rewrite using dict.fromkeys().

It's unnecessary to use a dict comprehension to build a dict with all values set to the same constant. Use dict.fromkeys() instead, which is faster. For example:

• Rewrite {x: 1 for x in iterable} as dict.fromkeys(iterable, 1)
• Rewrite {x: None for x in iterable} as dict.fromkeys(iterable)