Re: [Tutor] "TypeError: 'int' object is not callable"??

[Kent Johnson]

A callable is something that can be called with functional notation. It can be a function, a class, 
or in some cases a class instance. In general, any object that has a __call__() special method is 
callable. The callable() built-in tells you if an object is callable, though you can also just try 
it. For example,

the built-in len is callable and has a __call__ attribute:
>>> callable(len)
True
>>> dir(len)
['__call__', ... ]
1 is not callable and does not have a __call__ attribute:
>>> callable(1)
False
>>> dir(1)
[ <a long list that doesn't include __call__>]
As you discovered, trying to call 1 as a function doesn't work:
>>> 1()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: 'int' object is not callable
A user-defined function is callable and has a __call__ attribute:
>>> def f(): pass
...
>>> callable(f)
True
>>> dir(f)
['__call__', ... ]
>>>
A class is callable (you call it to create an instance):
>>> class C:
...   pass
...
>>> callable(C)
True
Instances of a class, in general, are not callable:
>>> c=C()
>>> callable(c)
False
You can make a class whose instances are callable by defining a __call__ method on the class. This 
allows you to make a class whose instances behave like functions, which is sometimes handy. (This 
behaviour is built-in to Python - __call__() is called a special method. There are many special 
methods that let you customize the behaviour of a class.)

>>> class D:
...   def __call__(self, x):
...     print 'x =', x
...
>>> d=D()  # Calling the class creates an instance
>>> callable(d)
True
>>> d(3)  # Calling the instance ends up in the __call__() method of the class
x = 3
Kent
Dick Moores wrote:
I got this error msg for this line of code:
n = -(2(a**3.0)/27.0 - a*b/3.0 + c)
(where a = 1, b = 2, c = 3)
And was baffled until I realized the line should be
n = -(2*(a**3.0)/27.0 - a*b/3.0 + c)
But I still don't understand what "callable" means. Can someone help?
Thanks,
Dick Moores
[EMAIL PROTECTED]
_______________________________________________
Tutor maillist  -  [EMAIL PROTECTED]
http://mail.python.org/mailman/listinfo/tutor
_______________________________________________
Tutor maillist  -  [EMAIL PROTECTED]
http://mail.python.org/mailman/listinfo/tutor