# What’s New In Pylint 1.8

Release: | 1.8
Date: | 2017-12-15

## Summary – Release highlights

- None so far

## New checkers

- A new check was added, `shallow-copy-environ`.

  This warning message is emitted when shallow copy of os.environ is created. Shallow copy of os.environ doesn’t work as people may expect. os.environ is not a dict object but rather a proxy object, so any changes made on copy may have unexpected effects on os.environ

  Instead of copy.copy(os.environ) method os.environ.copy() should be used.

  See https://bugs.python.org/issue15373 for details.

      import copy
      import os
      wrong_env_copy = copy.copy(os.environ)  # will emit pylint warning
      wrong_env_copy['ENV_VAR'] = 'new_value'  # changes os.environ
      assert os.environ['ENV_VAR'] == 'new_value'

      good_env_copy = os.environ.copy()  # the right way
      good_env_copy['ENV_VAR'] = 'different_value'  # doesn't change os.environ
      assert os.environ['ENV_VAR'] == 'new_value'

- A new check was added, `keyword-arg-before-vararg`.

  This warning message is emitted when a function is defined with a keyword argument appearing before variable-length positional arguments (*args). This may lead to args list getting modified if keyword argument’s value is not provided in the function call assuming it will take default value provided in the definition.

      def foo(a, b=3, *args):
          print(a, b, args)

      # Case1: a=0, b=2, args=(4,5)
      foo(0,2,4,5) # 0 2 (4,5) ==> Observed values are same as expected values

      # Case2: a=0, b=<default_value>, args=(4,5)
      foo(0,4,5) # 0 4 (5,) ==> args list got modified as well as the observed value of b

      # Case3: Syntax Error if tried as follows
      foo(0,b=2,4,5) # syntax error

- A new check was added, `simplify-boolean-expression`.

  This message is emitted when `consider-using-ternary` check would emit not equivalent code, due to truthy element being falsy in boolean context.

      value = condition and False or other_value

  This flawed construct may be simplified to:

      value = other_value

- A new check was added, `bad-thread-instantiation`.

  This message is emitted when the threading.Thread class does not receive the target argument, but receives just one argument, which is by default the group parameter.

  In the following example, the instantiation will fail, which is definitely not desired:

      import threading
      threading.Thread(lambda: print(1)) # Oups, this is the group parameter

- A new Python 3 checker was added to warn about accessing functions that have been removed from the itertools module `izip`, `imap`, `iflter`, `izip_longest`, and `ifilterfalse`.

      from itertools import izip
      print(list(izip([1, 2], [3])))

  Instead use `six.moves` to import a Python 2 and Python 3 compatible function:

      from six.moves import zip
      print(list(zip([1, 2], [3])))

- A new Python 3 checker was added to warn about accessing deprecated fields from the types module like `ListType` or `IntType`

      from types import ListType
      print(isinstance([], ListType))

  Instead use the declarations in the builtin namespace:

      print(isinstance([], list))

- A new Python 3 checker was added to warn about declaring a `next` method that would have implemented the `Iterator` protocol in Python 2 but is now a normal method in Python 3.

      class Foo(object):
          def next(self):
              return 42

  Instead implement a `__next__` method and use `six.Iterator` as a base class or alias `next` to `__next__`:

      class Foo(object):
          def __next__(self):
              return 42
          next = __next__

- Three new Python 3 checkers were added to warn about using dictionary methods in non-iterating contexts.

  For example, the following are returning iterators in Python 3:

      .. code-block:: python

  > d = {} d.keys()[0] d.items()[0] d.values() + d.keys()

- A new Python 3 porting check was added, `non-ascii-bytes-literals`

  This message is emitted whenever we detect that a bytes string contain non-ASCII characters, which results in a SyntaxError on Python 3.

- A new warning, `raising-format-tuple`, will catch situations where the intent was likely raising an exception with a formatted message string, but the actual code did omit the formatting and instead passes template string and value parameters as separate arguments to the exception constructor. So it detects things like

      raise SomeError('message about %s', foo)
      raise SomeError('message about {}', foo)

  which likely were meant instead as

      raise SomeError('message about %s' % foo)
      raise SomeError('message about {}'.format(foo))

  This warning can be ignored on projects which deliberately use lazy formatting of messages in all user-facing exception handlers.

- Following the recommendations of PEP479 ,a new Python 3.0 checker was added to warn about raising a `StopIteration` inside a generator. Raising a `StopIteration` inside a generator may be due a direct call to `raise StopIteration`:

      def gen_stopiter():
          yield 1
          yield 2
          yield 3
          raise StopIteration

  Instead use a simple `return` statement

      def gen_stopiter():
          yield 1
          yield 2
          yield 3
          return

  Raising a `StopIteration` may also be due to the call to `next` function with a generator as argument:

      def gen_next_raises_stopiter():
          g = gen_ok()
          while True:
              yield next(g)

  In this case, surround the call to `next` with a try/except block:

      def gen_next_raises_stopiter():
          g = gen_ok()
          while True:
              try:
                  yield next(g)
              except StopIteration:
                  return

  The check about raising a StopIteration inside a generator is also valid if the exception raised inherit from StopIteration. Close #1385

- A new Python checker was added to warn about using a `+` operator inside call of logging methods when one of the operands is a literal string:

      import logging
      var = "123"
      logging.log(logging.INFO, "Var: " + var)

  Instead use formatted string and positional arguments :

      import logging
      var = "123"
      logging.log(logging.INFO, "Var: %s", var)

- A new Python checker was added to warn about `inconsistent-return-statements`. A function or a method has inconsistent return statements if it returns both explicit and implicit values :

      def mix_implicit_explicit_returns(arg):
          if arg < 10:
              return True
          elif arg < 20:
              return

  According to PEP8, if any return statement returns an expression, any return statements where no value is returned should explicitly state this as return None, and an explicit return statement should be present at the end of the function (if reachable). Thus, the previous function should be written:

      def mix_implicit_explicit_returns(arg):
          if arg < 10:
              return True
          elif arg < 20:
              return None

  Close #1267

## Other Changes

- Fixing u’’ string in superfluous-parens message.

- Configuration options of invalid name checker are significantly redesigned. Predefined rules for common naming styles were introduced. For typical setups, user friendly options like `--function-naming-style=camelCase` may be used in place of hand-written regular expressions. Default linter config enforce PEP8-compatible naming style. See documentation for details.

- Raise meaningful exception in case of invalid reporter class (output format) being selected.

- The docparams extension now allows a property docstring to document both the property and the setter. Therefore setters can also have no docstring.

- The docparams extension now understands property type syntax.

      class Foo(object):
          @property
          def foo(self):
              """My Sphinx style docstring description.

              :type: int
              """
              return 10

      class Foo(object):
          @property
          def foo(self):
              """int: My Numpy and Google docstring style description."""
              return 10

- In case of `--output-format=json`, the dictionary returned holds a new key-value pair. The key is `message-id` and the value the message id.

- Spelling checker has a new configuration parameter `max-spelling-suggestions`, which affects maximum count of suggestions included in emitted message.

- The invalid-name check contains the name of the template that caused the failure.

  For the given code, pylint used to emit `invalid-name` in the form `Invalid constant name var`, without offering any context why `var` is not such a good name.

  With this change, it is now more clear what should be improved for a name to be accepted according to its corresponding template.

- New configuration flag, `suggestion-mode` was introduced. When enabled, pylint would attempt to emit user-friendly suggestions instead of spurious errors for some known false-positive scenarios. Flag is enabled by default.

- `superfluous-parens` is no longer wrongly emitted for logical statements involving `in` operator (see example below for what used to be false-postive).

      foo = None
      if 'bar' in (foo or {}):
        pass

- Redefinition of dummy function is now possible. `function-redefined` message won’t be emitted anymore when dummy functions are redefined.

- `missing-param-doc` and `missing-type-doc` are no longer emitted when `Args` and `Keyword Args` are mixed in Google docstring.

- Fix of false positive `useless-super-delegation` message when parameters default values are different from those used in the base class.

- Fix of false positive `useless-else-on-loop` message when break statements are deeply nested inside loop.

- The Python 3 porting checker no longer emits multiple no-absolute-import per file.

- The Python 3 porting checker respects disabled checkers found in the config file.

- Modules, classes, or methods consist of compound statements that exceed the `docstring-min-length` are now correctly emitting missing-docstring

- Fix no `wrong-import-order` message emitted on ordering of first and third party libraries. With this fix, pylint distinguishes first and third party modules when checking import order.

- Fix the ignored `pylint disable=fixme` directives for comments following the last statement in a file.

- Fix `line-too-long` message deactivated by wrong disable directive. The directive `disable=fixme` doesn’t deactivate anymore the emission of `line-too-long` message for long commented lines.

- If the rcfile specified on the command line doesn’t exist, then an IOError exception is raised.

- Fix a false positive inconsistent-return-statements message when if statement is inside try/except. (backport from 2.0)

- Fix a false positive inconsistent-return-statements message when while loop are used. (backport from 2.0)

- Fix unused-argument false positives with overshadowed variable in dictionary comprehension. (backport from 2.0)

- Fixing false positive `inconsistent-return-statements` when never returning functions are used (i.e such as sys.exit). (backport from 2.0)

- Fix false positive `inconsistent-return-statements` message when a function is defined under an if statement. (backport from 2.0)

- Fix false positive `inconsistent-return-statements` message by avoiding useless exception inference if the exception is not handled. (backport from 2.0)

- Fix false positive `undefined-variable` for lambda arguments in

    class definitions

- Fix false-positive `undefined-variable` for generated comprehension variables in function default arguments