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https://github.com/adamchainz/flake8-comprehensions

❄️ A flake8 plugin to help you write better list/set/dict comprehensions.
https://github.com/adamchainz/flake8-comprehensions

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

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README 

        
=====================

flake8-comprehensions

=====================



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



----



**Linting a Django project?**

Check out my book `Boost Your Django DX `__ which covers Flake8 and many other code quality tools.



----



Requirements

============



Python 3.8 to 3.12 supported.



Installation

============



First, install with ``pip``:



.. code-block:: sh



     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 ```` 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 ```` 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 ```` literal - rewrite as a ```` literal.

-------------------------------------------------------------------------------------



* C405 Unnecessary ```` literal - rewrite as a set literal.

* C406 Unnecessary ```` 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 ```` comprehension - ```` 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 ```` 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 ```` passed to ````\() - ````.

------------------------------------------------------------------------------------



Rules:



* C409 Unnecessary ```` passed to tuple() - ````.

* C410 Unnecessary list passed to list() - ````.



Where ```` is either:



* remove the outer call to ````\()

* rewrite as a ```` 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 ```` 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 ```` 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 ```` call within ````\().

--------------------------------------------------------------------------------------------------



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 ````\().

-------------------------------------------------------------------------------------



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 ```` comprehension - rewrite using ````\().

-------------------------------------------------------------------------------------------



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/```` 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 ```` 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 ````\() 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)``