A "scopeguard" for Python
in [Python]
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From: Alf P. Steinbach on 3 Mar 2010 10:10 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)
From: Mike Kent on 3 Mar 2010 10:39 What's the compelling use case for this vs. a simple try/finally? original_dir = os.getcwd() try: os.chdir(somewhere) # Do other stuff finally: os.chdir(original_dir) # Do other cleanup
From: Alf P. Steinbach on 3 Mar 2010 10:56 * Mike Kent: > What's the compelling use case for this vs. a simple try/finally? if you thought about it you would mean a simple "try/else". "finally" is always executed. which is incorrect for cleanup by the way, that's one advantage: a "with Cleanup" is difficult to get wrong, while a "try" is easy to get wrong, as you did here --- another general advantage is as for the 'with' statement generally > original_dir = os.getcwd() > try: > os.chdir(somewhere) > # Do other stuff also, the "do other stuff" can be a lot of code and also, with more than one action the try-else introduces a lot of nesting > finally: > os.chdir(original_dir) > # Do other cleanup cheers & hth., - alf
From: Robert Kern on 3 Mar 2010 12:00 On 2010-03-03 09:39 AM, Mike Kent wrote: > What's the compelling use case for this vs. a simple try/finally? > > original_dir = os.getcwd() > try: > os.chdir(somewhere) > # Do other stuff > finally: > os.chdir(original_dir) > # Do other cleanup A custom-written context manager looks nicer and can be more readable. from contextlib import contextmanager import os @contextmanager def pushd(path): original_dir = os.getcwd() os.chdir(path) try: yield finally: os.chdir(original_dir) with pushd(somewhere): ... I don't think a general purpose ScopeGuard context manager has any such benefits over the try: finally:, though. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco
From: Alf P. Steinbach on 3 Mar 2010 12:09
* Robert Kern: > On 2010-03-03 09:39 AM, Mike Kent wrote: >> What's the compelling use case for this vs. a simple try/finally? >> >> original_dir = os.getcwd() >> try: >> os.chdir(somewhere) >> # Do other stuff >> finally: >> os.chdir(original_dir) >> # Do other cleanup > > A custom-written context manager looks nicer and can be more readable. > > from contextlib import contextmanager > import os > > @contextmanager > def pushd(path): > original_dir = os.getcwd() > os.chdir(path) > try: > yield > finally: > os.chdir(original_dir) > > > with pushd(somewhere): > ... > > > I don't think a general purpose ScopeGuard context manager has any such > benefits over the try: finally:, though. I don't think that's a matter of opinion, since one is correct while the other is incorrect. Cheers, - ALf |