A flake8 plugin that helps you write better list/set/dict comprehensions.
Python 3.7 to 3.11 supported.
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.
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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}
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}
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([])
asset()
- Rewrite
dict([(1, 2)])
as{1: 2}
- Rewrite
dict(((1, 2),))
as{1: 2}
- Rewrite
dict([])
as{}
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[]
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]
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]))
assorted([2, 3, 1])
- Rewrite
reversed(sorted([2, 3, 1]))
assorted([2, 3, 1], reverse=True)
- Rewrite
reversed(sorted([2, 3, 1], reverse=True))
assorted([2, 3, 1])
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))
aslist(iterable)
- Rewrite
list(tuple(iterable))
aslist(iterable)
- Rewrite
tuple(list(iterable))
astuple(iterable)
- Rewrite
tuple(tuple(iterable))
astuple(iterable)
- Rewrite
set(set(iterable))
asset(iterable)
- Rewrite
set(list(iterable))
asset(iterable)
- Rewrite
set(tuple(iterable))
asset(iterable)
- Rewrite
set(sorted(iterable))
asset(iterable)
- Rewrite
set(reversed(iterable))
asset(iterable)
- Rewrite
sorted(list(iterable))
assorted(iterable)
- Rewrite
sorted(tuple(iterable))
assorted(iterable)
- Rewrite
sorted(sorted(iterable))
assorted(iterable)
- Rewrite
sorted(reversed(iterable))
assorted(iterable)
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])
asset(iterable)
- Rewrite
sorted(iterable)[::-1]
assorted(iterable, reverse=True)
- Rewrite
reversed(iterable[::-1])
asiterable
C416: Unnecessary <dict/list/set>
comprehension - rewrite using <dict/list/set>
(). ---------------------------------------------------------------------------------
It's unnecessary to use a list comprehension if the elements are unchanged. The iterable should be wrapped in dict()
, list()
, or set()
instead. For example:
- Rewrite
{a:b for a, b in iterable}
asdict(iterable)
- Rewrite
[x for x in iterable]
aslist(iterable)
- Rewrite
{x for x in iterable}
asset(iterable)
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}