For C++ Petru Marginean once invented the "scope guard" technique (elaborated on
by Andrei Alexandrescu, they published an article about it in DDJ) where all you
need to do to ensure some desired cleanup at the end of a scope, even when the
scope is exited via an exception, is to declare a ScopeGuard w/desired action.
The C++ ScopeGuard was/is for those situations where you don't have proper
classes with automatic cleanup, which happily is seldom the case in good C++
code, but languages like Java and Python don't support automatic cleanup and so
the use case for something like ScopeGuard is ever present.
For use with a 'with' statement and possibly suitable 'lambda' arguments:
<code>
class Cleanup:
def __init__( self ):
self._actions = []
def call( self, action ):
assert( is_callable( action ) )
self._actions.append( action )
def __enter__( self ):
return self
def __exit__( self, x_type, x_value, x_traceback ):
while( len( self._actions ) != 0 ):
try:
self._actions.pop()()
except BaseException as x:
raise AssertionError( "Cleanup: exception during cleanup" ) from
</code>
I guess the typical usage would be what I used it for, a case where the cleanup
action (namely, changing back to an original directory) apparently didn't fit
the standard library's support for 'with', like
with Cleanup as at_cleanup:
# blah blah
chdir( somewhere )
at_cleanup.call( lambda: chdir( original_dir ) )
# blah blah
Another use case might be where one otherwise would get into very deep nesting
of 'with' statements with every nested 'with' at the end, like a degenerate tree
that for all purposes is a list. Then the above, or some variant, can help to
/flatten/ the structure. To get rid of that silly & annoying nesting. :-)
Cheers,
- Alf (just sharing, it's not seriously tested code)
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