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0 pointstibordp3y ago0 comments

It's not that it's hard, it's just that it is not inline, so it requires a context switch because the CM is defined outside, even when it's doing something specific.

The most common problem that defer is trying to solve is cleanup when the function returns early (ususally because of an error). Writing the cleanup code inline before the early return results in code duplication.

C#/Java/Javascript have try/finally for this, C has the "goto cleanup" idiom, and C++ and Rust have the guard objects. Go and Alumina have defer.

0 comments

tzot3y ago

There's `contextlib.closing` for objects that do not support the context manager protocol and they should be closed.

And then one can simulate defer in the spirit of the `atexit` module with a single context manager (say `finalizer`), defined only once, which could be used as:

    with finalizer() as defer:
        ...
        req = requests.get('https://w3.org/')
        defer(req.close)  # just like contextlib.closing
        ...
        a_dict['key'] = value
        defer(a_dict.__delitem__, 'key')
        ...
        defer(print, "all defers ran", file=sys.stderr)
        ...

The `__call__` of finalizer adds callables with their *args and **kwargs to a fifo or a stack, and its `__exit__` will call them in sequence.

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