# What's New in Pylint 2.4

Release: | 2.4
Date: | 2019-09-24

## Summary -- Release highlights

## New checkers

- `import-outside-toplevel`

  This check warns when modules are imported from places other than a module toplevel, e.g. inside a function or a class.

- Added a new check, `consider-using-sys-exit`

  This check is emitted when we detect that a quit() or exit() is invoked instead of sys.exit(), which is the preferred way of exiting in program.

  Close #2925

- Added a new check, `arguments-out-of-order`

  This check warns if you have arguments with names that match those in a function's signature but you are passing them in to the function in a different order.

  Close #2975

- Added new checks, `no-else-break` and `no-else-continue`

  These checks highlight unnecessary `else` and `elif` blocks after `break` and `continue` statements.

  Close #2327

- Added unnecessary-comprehension that detects unnecessary comprehensions.

  This check is emitted when `pylint` finds list-, set- or dict-comprehensions, that are unnecessary and can be rewritten with the list-, set- or dict-constructors.

  Close #2905

- Added a new check, `invalid-overridden-method`

  This check is emitted when we detect that a method is overridden as a property or a property is overridden as a method. This can indicate a bug in the application code that will trigger a runtime error.

  Close #2670

- Added a new check, `redeclared-assigned-name`

  This check is emitted when `pylint` detects that a name was assigned one or multiple times in the same assignment, which indicate a potential bug.

  Close #2898

- Added a new check, `self-assigning-variable`

  This check is emitted when we detect that a variable is assigned to itself, which might indicate a potential bug in the code application.

  For example, the following would raise this warning:

      def new_a(attr, attr2):
        a_inst = Aclass()
        a_inst.attr2 = attr2
        # should be: a_inst.attr = attr, but have a typo
        attr = attr
        return a_inst

  Close #2930

- Added a new check `property-with-parameters` which detects when a property has more than a single argument.

  Close #3006

- Added subprocess-run-check to handle subprocess.run without explicitly set check keyword.

  Close #2848

- We added a new check message `dict-iter-missing-items`. This is emitted when trying to iterate through a dict in a for loop without calling its .items() method.

  Closes #2761

- We added a new check message `missing-parentheses-for-call-in-test`. This is emitted in case a call to a function is made inside a test but it misses parentheses.

- A new check `class-variable-slots-conflict` was added.

  This check is emitted when `pylint` finds a class variable that conflicts with a slot name, which would raise a `ValueError` at runtime.

  For example, the following would raise an error:

      class A:
          __slots__ = ('first', 'second')
          first = 1

- A new check `preferred-module` was added.

  This check is emitted when `pylint` finds an imported module that has a preferred replacement listed in `preferred-modules`.

  For example, you can set the preferred modules as `xml:defusedxml,json:ujson` to make `pylint` suggest using `defusedxml` instead of `xml` and `ujson` rather than `json`.

- A new extension `broad_try_clause` was added.

  This extension enforces a configurable maximum number of statements inside of a try clause. This facilitates enforcing PEP 8's guidelines about try / except statements and the amount of code in the try clause.

  You can enable this extension using `--load-plugins=pylint.extensions.broad_try_clause` and you can configure the amount of statements in a try statement using `--max-try-statements`.

## Other Changes

- Don't emit `protected-access` when a single underscore prefixed attribute is used inside a special method

  Close #1802

- `len-as-condition` now only fires when a `len(x)` call is made without an explicit comparison.

  The message and description accompanying this checker has been changed reflect this new behavior, by explicitly asking to either rely on the fact that empty sequence are false or to compare the length with a scalar.

  OK:

      if len(x) == 0:
        pass

      while not len(x) == 0:
        pass

      assert len(x) > 5, message

  KO:

      if not len(x):
        pass

      while len(x) and other_cond:
        pass

      assert len(x), message

- A file is now read from stdin if the `--from-stdin` flag is used on the command line. In addition to the `--from-stdin` flag a (single) file name needs to be specified on the command line, which is needed for the report.

- The checker for ungrouped imports is now more permissive.

The import can now be sorted alphabetically by import style. This makes pylint compatible with isort.

The following imports do not trigger an `ungrouped-imports` anymore

    import unittest
    import zipfile
    from unittest import TestCase
    from unittest.mock import MagicMock

- The checker for missing return documentation is now more flexible.

The following does not trigger a `missing-return-doc` anymore

    def my_func(self):
        """This is a docstring.

        Returns
        -------
        :obj:`list` of :obj:`str`
            List of strings
        """
        return ["hi", "bye"] #@

- `signature-mutators` CLI and config option was added.

With this option, users can choose to ignore too-many-function-args, unexpected-keyword-arg, and no-value-for-parameter for functions decorated with decorators that change the signature of a decorated function.

For example a test may want to make use of hypothesis. Adding hypothesis.extra.numpy.arrays to signature_mutators would mean that no-value-for-parameter would not be raised for:

    @given(img=arrays(dtype=np.float32, shape=(3, 3, 3, 3)))
    def test_image(img):
        ...

- Allow the option of f-strings as a valid logging string formatting method.

logging-fstring--interpolation has been merged into logging-format-interpolation to allow the logging-format-style option to control which logging string format style is valid. To allow this, a new fstr value is valid for the logging-format-style option.

- `--list-msgs-enabled` command was added.

When enabling/disabling several messages and groups in a config file, it can be unclear which messages are actually enabled and which are disabled. This new command produces the final resolved lists of enabled/disabled messages, sorted by symbol but with the ID provided for use with `--help-msg`.