"""

A simple class to manage a piece of global science state. See

:ref:`config-developer` for more details.

"""

__all__ = ['ScienceState']

class ScienceState:

"""

Science state subclasses are used to manage global items that can

affect science results. Subclasses will generally override

`validate` to convert from any of the acceptable inputs (such as

strings) to the appropriate internal objects, and set an initial

value to the ``_value`` member so it has a default.

Examples

--------

::

class MyState(ScienceState):

@classmethod

def validate(cls, value):

if value not in ('A', 'B', 'C'):

raise ValueError("Must be one of A, B, C")

return value

"""

def __init__(self):

raise RuntimeError(

"This class is a singleton. Do not instantiate.")

@classmethod

def get(cls):

"""

Get the current science state value.

"""

return cls.validate(cls._value)

@classmethod

def set(cls, value):

"""

Set the current science state value.

"""

class _Context:

def __init__(self, parent, value):

self._value = value

self._parent = parent

def __enter__(self):

pass

def __exit__(self, type, value, tb):

self._parent._value = self._value

def __repr__(self):

return ('<ScienceState {0}: {1!r}>'

.format(self._parent.__name__, self._parent._value))

ctx = _Context(cls, cls._value)

value = cls.validate(value)

cls._value = value

return ctx

@classmethod

def validate(cls, value):

"""

Validate the value and convert it to its native type, if

necessary.

"""

return value