Confused with Functions and decorators

[Wojciech Giel]

On 19/07/14 11:52, Jerry lu wrote:
> Ok so i am trying to learn this and i do not understand some of it. I also tried to searched the web but i couldnt find any answers.
>
> 1. I dont understand when i will need to use a function that returns another function.
> eg
>                     def outer():
>                           def inner():
>                                 x = 5
>                            return inner
>      
>       why not just use inner and forget about the whole outer function?
function compositing is one possible use. Functions are first class 
objects so they can be passed as arguments to other functions. in math 
you can define function composition  f(x)  * g(x) by saying that (f * g 
)(x) = f(g(x)).
so composition of two functions gives you a new function with behaviour 
the same as applying fist function to the output of the second.

ex.
you are give two functions f and g to construct their composition:

 >>> def outer(f, g):
...     def inner(x):
...         return f(g(x))
...     return inner
...
 >>> def f(x):
...     print("f ({}) called".format(x))
...     return x
...
 >>> def g(x):
...     print("g ({}) called".format(x))
...     return x
...
 >>> h = outer(f, g)
 >>> h("test")
g (test) called
f (test) called
'test'

cheers
Wojciech