Question

I’ve made myself a simple event system in python, and I found that the way I fired the events was pretty much the same every time: either at the end of a call, or before it. It felt like this would be a nice thing to have as a decorator. Here’s the code I’m using:

from functools import wraps

def fires(event):
    """
    Returns a decorater that causes an `Event` to fire immediately before the
    decorated function is called
    """
    def beforeDecorator(f):
        """Fires the event before the function executes"""
        @wraps(f)
        def wrapped(*args, **kargs):
            event.fire(*args, **kargs)
            return f(*args, **kargs)
        return wrapped

    def afterDecorator(f):
        """Fires the event after the function executes"""
        @wraps(f)
        def wrapped(*args, **kargs):
            result = f(*args, **kargs)
            event.fire(*args, **kargs)
            return result
        return wrapped

    # Should allow more explicit `@fires(event).uponCompletion` and
    # `@fires(event).whenCalled`
    afterDecorator.onceComplete = afterDecorator
    afterDecorator.whenCalled = afterDecorator

    return afterDecorator

With this code, I can successfully write this:

@fires(myEvent)
def foo(y):
    return y*y

print func(2)

And everything works. The problem comes when I try to write this:

@fires(myEvent).onceComplete
def foo(y):
    return y*y

print func(2)

That gives me a syntax error. Is there some special syntax for complex decorators? Does the parser stop after the first set of parentheses?

Answer 1

Nope, according to the grammar specification, not possible:

funcdef        ::=  [decorators] "def" funcname "(" [parameter_list] ")" ["->" expression] ":" suite
decorators     ::=  decorator+
decorator      ::=  "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINE
dotted_name    ::=  identifier ("." identifier)*
parameter_list ::=  (defparameter ",")*
                    (  "*" [parameter] ("," defparameter)*
                    [, "**" parameter]
                    | "**" parameter
                    | defparameter [","] )
parameter      ::=  identifier [":" expression]
defparameter   ::=  parameter ["=" expression]
funcname       ::=  identifier

Decorators must have their parentheses at the end