Decorators with parameters?

Even if this question was already answered and accepted, I’d like to show an idea which is IMHO quite elegant. The solution proposed by t.dubrownik shows a pattern which is always the same: you need the three-layered wrapper regardless of what the decorator does.

So I thought this is a job for a meta-decorator, that is a decorator for decorators. As a decorator is a function, it actually works as a regular decorator with arguments:

def parametrized(dec):
	def layer(*args, **kwargs):
		def repl(f):
			return dec(f, *args, **kwargs)
		return repl
	return layer

This can be applied to a regular decorator in order to add parameters. So for instance, say we have the decorator which doubles the result of a function:

def double(f):
	def aux(*xs, **kws):
		return 2 * f(*xs, **kws)
	return aux
 
@double
def function(a):
	return 10 + a print function(3)
	# Prints 26, namely 2 * (10 + 3)

With @parametrized we can build a generic @multiply decorator having a parameter

@parametrized
def multiply(f, n):
	def aux(*xs, **kws):
		return n * f(*xs, **kws)
	return aux @multiply(2)
 
def function(a):
	return 10 + a print function(3) # Prints 26
 
@multiply(3)
def function_again(a):
	return 10 + a print function(3) # Keeps printing 26
 
print function_again(3) # Prints 39, namely 3 * (10 + 3)

Conventionally the first parameter of a parametrized decorator is the function, while the remaining arguments will correspond to the parameter of the parametrized decorator.

An interesting usage example could be a type-safe assertive decorator:

import itertools as it
 
@parametrized
def types(f, *types):
	def rep(*args):
		for a, t, n in zip(args, types, it.count()):
			if type(a) is not t:
				raise TypeError('Value %d has not type %s. %s instead' % (n, t, type(a)) )
		return f(*args)
	return rep @types(str, int) # arg1 is str, arg2 is int
 
def string_multiply(text, times):
	return text * times print(string_multiply('hello', 3))
	# prints hellohellohello
 
print(string_multiply(3, 3)) # Fails miserably with TypeError

Decorators with parameters?