 which case it's invoked when the default is needed
                without any arguments.
:param callback: A function to further process or validate the value
    after type conversion. It is called as ``f(ctx, param, value)``
    and must return the value. It is called for all sources,
    including prompts.
:param nargs: the number of arguments to match.  If not ``1`` the return
              value is a tuple instead of single value.  The default for
              nargs is ``1`` (except if the type is a tuple, then it's
              the arity of the tuple). If ``nargs=-1``, all remaining
              parameters are collected.
:param metavar: how the value is represented in the help page.
:param expose_value: if this is `True` then the value is passed onwards
                     to the command callback and stored on the context,
                     otherwise it's skipped.
:param is_eager: eager values are processed before non eager ones.  This
                 should not be set for arguments or it will inverse the
                 order of processing.
:param envvar: a string or list of strings that are environment variables
               that should be checked.
:param shell_complete: A function that returns custom shell
    completions. Used instead of the param's type completion if
    given. Takes ``ctx, param, incomplete`` and must return a list
    of :class:`~click.shell_completion.CompletionItem` or a list of
    strings.

.. versionchanged:: 8.0
    ``process_value`` validates required parameters and bounded
    ``nargs``, and invokes the parameter callback before returning
    the value. This allows the callback to validate prompts.
    ``full_process_value`` is removed.

.. versionchanged:: 8.0
    ``autocompletion`` is renamed to ``shell_complete`` and has new
    semantics described above. The old name is deprecated and will
    be removed in 8.1, until then it will be wrapped to match the
    new requirements.

.. versionchanged:: 8.0
    For ``multiple=True, nargs>1``, the default must be a list of
    tuples.

.. versionchanged:: 8.0
    Setting a default is no longer required for ``nargs>1``, it will
    default to ``None``. ``multiple=True`` or ``nargs=-1`` will
    default to ``()``.

.. versionchanged:: 7.1
    Empty environment variables are ignored rather than taking the
    empty string value. This makes it possible for scripts to clear
    variables if they can't unset them.

.. versionchanged:: 2.0
    Changed signature for parameter callback to also be passed the
    parameter. The old callback format will still work, but it will
    raise a warning to give you a chance to migrate the code easier.
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