old_contents: # This code is part of Qiskit.
  #
  # (C) Copyright IBM 2017, 2020.
  #
  # This code is licensed under the Apache License, Version 2.0. You may
  # obtain a copy of this license in the LICENSE.txt file in the root directory
  # of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
  #
  # Any modifications or derivative works of this code must retain this
  # copyright notice, and modified files need to carry a notice indicating
  # that they have been altered from the originals.
  
  """Exceptions for errors raised by Qiskit."""
  
  from typing import Optional
  
  
  class QiskitError(Exception):
      """Base class for errors raised by Qiskit."""
  
      def __init__(self, *message):
          """Set the error message."""
          super().__init__(' '.join(message))
          self.message = ' '.join(message)
  
      def __str__(self):
          """Return the message."""
          return repr(self.message)
  
  
  class QiskitIndexError(QiskitError, IndexError):
      """Raised when a sequence subscript is out of range."""
      pass
  
  
  class QiskitUserConfigError(QiskitError):
      """Raised when an error is encountered reading a user config file."""
      message = "User config invalid"
  
  
  class AquaError(QiskitError):
      """Base class for errors raised by Aqua."""
  
      def __init__(self, *message) -> None:
          """Set the error message."""
          super().__init__(' '.join(message))
          self.message = ' '.join(message)
  
      def __str__(self) -> str:
          """Return the message."""
          return repr(self.message)
  
  
  class MissingOptionalLibraryError(AquaError):
      """Raised when an optional library is missing."""
  
      def __init__(self,
                   libname: str,
                   name: str,
                   pip_install: Optional[str] = None,
                   msg: Optional[str] = None) -> None:
          """Set the error message.
          Args:
              libname: Name of missing library
              name: Name of class, function, module that uses this library
              pip_install: pip install command, if any
              msg: Descriptive message, if any
          """
          message = ["The '{}' library is required to use '{}'.".format(libname, name)]
          if pip_install:
              message.append("You can install it with '{}'.".format(pip_install))
          if msg:
              message.append(' {}.'.format(msg))
  
          super().__init__(' '.join(message))
          self.message = ' '.join(message)
  
      def __str__(self) -> str:
          """Return the message."""
          return repr(self.message)
new_contents: # This code is part of Qiskit.
  #
  # (C) Copyright IBM 2017, 2020.
  #
  # This code is licensed under the Apache License, Version 2.0. You may
  # obtain a copy of this license in the LICENSE.txt file in the root directory
  # of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
  #
  # Any modifications or derivative works of this code must retain this
  # copyright notice, and modified files need to carry a notice indicating
  # that they have been altered from the originals.
  
  """Exceptions for errors raised by Qiskit."""
  
  from typing import Optional
  
  
  class QiskitError(Exception):
      """Base class for errors raised by Qiskit."""
  
      def __init__(self, *message):
          """Set the error message."""
          super().__init__(' '.join(message))
          self.message = ' '.join(message)
  
      def __str__(self):
          """Return the message."""
          return repr(self.message)
  
  
  class QiskitIndexError(QiskitError, IndexError):
      """Raised when a sequence subscript is out of range."""
      pass
  
  
  class QiskitUserConfigError(QiskitError):
      """Raised when an error is encountered reading a user config file."""
      message = "User config invalid"
  
  
  class AquaError(QiskitError):
      """Base class for errors raised by Aqua."""
  
      def __init__(self, *message) -> None:
          """Set the error message."""
          super().__init__(' '.join(message))
          self.message = ' '.join(message)
  
      def __str__(self) -> str:
          """Return the message."""
          return repr(self.message)
  
  
  class MissingOptionalLibraryError(AquaError):
      """Raised when an optional library is missing."""
  
      def __init__(self,
                   libname: str,
                   name: str,
                   pip_install: Optional[str] = None,
                   msg: Optional[str] = None) -> None:
          """Set the error message.
          Args:
              libname: Name of missing library
              name: Name of class, function, module that uses this library
              pip_install: pip install command, if any
              msg: Descriptive message, if any
          """
          message = ["The '{}' library is required to use '{}'.".format(libname, name)]
          if pip_install:
              message.append("You can install it with '{}'.".format(pip_install))
          if msg:
              message.append(' {}.'.format(msg))
  
          super().__init__(' '.join(message))
          self.message = ' '.join(message)
  
      def __str__(self) -> str:
          """Return the message."""
          return repr(self.message)
  
subject: fix lint