Re: Why won't this decorator work?
- From: ChasBrown <cbrown@xxxxxxxxxxxxxxxxx>
- Date: Sat, 2 Jul 2011 12:16:36 -0700 (PDT)
On Jul 2, 11:08 am, John Salerno <johnj...@xxxxxxxxx> wrote:
On Jul 2, 12:33 pm, MRAB <pyt...@xxxxxxxxxxxxxxxxxxx> wrote:
On 02/07/2011 17:56, John Salerno wrote:
I thought I had finally grasped decorators, but the error I'm getting
('str' type is not callable) is confusing me.
def move(roll):
return 'You moved to space {0}.'.format(roll)
<snip>
A decorator should return a callable.
Well, should typically return a callable, but there are exceptions...
This:
@move
def roll_die():
return random.randint(1, 6)
is equivalent to this:
def roll_die():
return random.randint(1, 6)
roll_die = move(roll_die)
You should be defining a function (a callable) and then passing it to a
decorator which returns a callable.
As it is, you're defining a function and then passing it to a decorator
which is returning a string. Strings aren't callable.
But why does the documentation say "The return value of the decorator
need not be callable"?
Well, because it need not be callable: i.e., if the return value is
not callable, that is perfectly legal. You very rarely want to return
something else, but an example of this is the @property decorator: it
returns a property object, which is not callable.
And why, if I remove the decorator and just
leave the two functions as if, does the call to move(roll_die()) work?
Isn't that what the decorator syntax is essentially doing?
No; instead it's doing the similar looking but quite different
move(roll_die)()
As you wrote it, move(roll_die) returns the string 'You moved to space
<function roll_die>.' which is not callable. You typically want
instead something like:
def move(roll):
# note that roll is a function, not a number
def wrapper():
result = roll()
print 'You moved to space {0}.'.format(result)
return result
return wrapper # which is a function
Now move returns a callable (the function 'wrapper') so move(roll_die)
is callable, and move(roll_die)() is legal.
Cheers - Chas
.
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- From: John Salerno
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