Created
February 17, 2014 11:20
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Class switching context object to prevent applications for assigning to protected properties. Uses class swizzling.
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"Program execution context" | |
import warnings | |
class SwizzleContext(object): | |
"""Type switching context object base class | |
Supports the context manager protocol to switch the instance from a | |
MutableContext to the protected Context. | |
""" | |
_protected = { | |
'_protected': None, | |
'opts_current': None, | |
'opts_next': tuple(), | |
'opts_previous': tuple(), | |
'return_values': tuple() | |
} | |
def __enter__(self): | |
self.__class__ = Context | |
return self | |
def __exit__(self, exc_type, exc_value, traceback): | |
self.__class__ = MutableContext | |
@property | |
def last_return(self): | |
try: | |
return self.return_values[-1] | |
except IndexError: | |
return None | |
@property | |
def return_value(self): | |
warnings.warn( | |
"The 'return_value' property will be removed in future versions. " | |
"Use 'last_return' instead.", | |
DeprecationWarning, stacklevel=2 | |
) | |
return self.last_return | |
class MutableContext(SwizzleContext): | |
"""Mutable context object | |
All properties may be assigned to. Constructor ensures all protected | |
properties are initialised correctly. | |
""" | |
def __init__(self): | |
for name in self._protected: | |
if name.startswith('_'): | |
continue | |
setattr(self, name, self._protected[name]) | |
class Context(SwizzleContext): | |
"""Protected context object | |
Attempts to assign to protected properties will raise an AttributeError. | |
""" | |
def __setattr__(self, name, value): | |
if name in self._protected: | |
msg = "attempt to assign value to to immutable attribute '{0}'".format(name) | |
raise AttributeError(msg) | |
object.__setattr__(self, name, value) | |
context = MutableContext() |
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