Add optional parameters to a Python generator

gistfile1.py

# Add optional parameters to a decorator.
#
# Usage:
#
# Start with a normal decorator without parameters, e.g.
#
#   def decorator(f):
#      ...
# 
# Add additional parameters with default value and use @optionally_parametrized_decorator, e.g.
#   
#   @optionally_parametrized_decorator
#   def decorator(f, x=42, y=7):
#      ...
#
# Then you can use the resulting decorator without _and_ with parameters, e.g.
#
#   @decorator
#   def f(a, b, c):
#     ...
#
#   @decorator(x=1, y=2)
#   def f(a, b, c):
#     ...
#
def optionally_parametrized_decorator(orig_decorator):
    def decorator_or_decorator_func(orig_handler=None, **decorator_kwargs):
        def decorator_wrap(orig_handler):
            return orig_decorator(orig_handler, **decorator_kwargs)

        # Python decorator magic: when orig_handler!=None is passed, we know
        # that the decorator was called without parameters, i.e. like @with_valid_from_header.
        # If the decorator is used as with_valid_from_header(min_api_version=2), then
        # python will call this function without direct orig_handler parameter, but only with the
        # min_api_version.
        if orig_handler:
            return decorator_wrap(orig_handler)
        else:
            return decorator_wrap
    return decorator_or_decorator_func