Example of a naïve decorator function

naive_decorator.py

def prefixer(fn, prefix):
    '''Return a wrapper which prefix first (string) argument with prefix'''
    def wrapper(output, *args, **kwargs):
        output = '%s%s' % (prefix, output)
        return fn(output, *args, **kwargs)
    return wrapper

## Example of a decorator WITHOUT using the @wrapper syntactic sugar
##    (to save ref to print if we want to use it later: old_print = print)
print = prefixer(print, 'Prefixed: ')
## Now the previous (builtin) print function is "wrapped" with our prefixer

## Tests: 
print('foo')
## >>> Prefixed: foo

print('foo', 'bar', file=sys.stderr)
## >>> Prefixed: foobar ... but going to stderr
## Notice that the variable argument handling and the keyword argument (**kwargs) handling
## ... of the original function have been preserved.  However the side-effect is that the
## first argument will to the wrapped function will be converted to a string and modified
## *before* it's passed into the original function.

## It's "naïve" because we didn't preserve certain metadata about the function:
print.__name__
## >>> wrapped

oprint.__name__
## >>> print

## functools.wraps is a decorator which will handle these details
## to use it in the example above we'd insert:
## @functools.wraps
## ... on a line just before def wrapper(....) ... on line 4 of this example