Book Excerpt: The Python Standard Library by Example
Acquiring Function Properties for Decorators
Updating the properties of a wrapped callable is especially useful when used in a decorator, since the transformed function ends up with properties of the original “bare” function.
import functools def show_details(name, f): """Show details of a callable object.""" print ’%s:’ % name print ’ object:’,f print ’ __name__:’, try: print f.__name__ except AttributeError: print ’(no __name__)’ print ’ __doc__’, repr(f.__doc__) print return def simple_decorator(f): @functools.wraps(f) def decorated(a=’decorated defaults’, b=1): print ’ decorated:’, (a, b) print ’’, f(a, b=b) return return decorated def myfunc(a, b=2): "myfunc() is not complicated" print ’ myfunc:’, (a,b) return # The raw function show_details(’myfunc’, myfunc) myfunc(’unwrapped, default b’) myfunc(’unwrapped, passing b’, 3) print # Wrap explicitly wrapped_myfunc = simple_decorator(myfunc) show_details(’wrapped_myfunc’, wrapped_myfunc) wrapped_myfunc() wrapped_myfunc(’args to wrapped’, 4) print # Wrap with decorator syntax @simple_decorator def decorated_myfunc(a, b): myfunc(a, b) return show_details(’decorated_myfunc’, decorated_myfunc) decorated_myfunc() decorated_myfunc(’args to decorated’, 4)
functools provides a decorator, wraps(), that applies update_wrapper() to the decorated function.
$ python functools_wraps.py myfunc: object: <function myfunc at 0x100da3488> __name__: myfunc __doc__ ’myfunc() is not complicated’ myfunc: (’unwrapped, default b’, 2) myfunc: (’unwrapped, passing b’, 3) wrapped_myfunc: object: <function myfunc at 0x100da3500> __name__: myfunc __doc__ ’myfunc() is not complicated’ decorated: (’decorated defaults’, 1) myfunc: (’decorated defaults’, 1) decorated: (’args to wrapped’, 4) myfunc: (’args to wrapped’, 4) decorated_myfunc: object: <function decorated_myfunc at 0x100da35f0> __name__: decorated_myfunc __doc__ None decorated: (’decorated defaults’, 1) myfunc: (’decorated defaults’, 1) decorated: (’args to decorated’, 4) myfunc: (’args to decorated’, 4)
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