Beautiful Python Decorators

Question

How do I beautifully write a decorator?

In particular, problems include: compatibility with other decorators, saving signatures, etc.

I would like to avoid dependency on the decorator module if possible, but if there were sufficient advantages, I would consider it.

Related

• Keeping signatures of decorated functions is a much more specific issue. The answer here is to use a third-party decorator module that annotates the decorator using @ decorator.decorator

Answer 1

. , , , .

decorator, , functools.wraps() ( 2.5).

, , . *args **kw, . , , :

def keep_none(func):
    def _exec(*args, **kw):
        return None if args[0] is None else func(*args, **kw)

    return _exec

... ...

def keep_none(func):
    """Wraps a function which expects a value as the first argument, and
    ensures the function won't get called with *None*.  If it is, this 
    will return *None*.

    >>> def f(x):
    ...     return x + 5
    >>> f(1)
    6
    >>> f(None) is None
    Traceback (most recent call last):
        ...
    TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
    >>> f = keep_none(f)
    >>> f(1)
    6
    >>> f(None) is None
    True"""

    @wraps(func)
    def _exec(value, *args, **kw):
        return None if value is None else func(value, *args, **kw)

    return _exec

Answer 2

functools . .

doc.

>>> from functools import wraps
>>> def my_decorator(f):
...     @wraps(f)
...     def wrapper(*args, **kwds):
...         print 'Calling decorated function'
...         return f(*args, **kwds)
...     return wrapper
...
>>> @my_decorator
... def example():
...     """Docstring"""
...     print 'Called example function'
...
>>> example()
Calling decorated function
Called example function
>>> example.__name__
'example'
>>> example.__doc__
'Docstring'