Python's closing package

Original

function as return value

Higher-order functions can also return a function as a result value, in addition to the ability to accept functions as parameters.

Let's implement the summation of a mutable parameter. In general, the function of summing is defined like this:

def calc_sum (*args):    = 0    for in  args:        = ax + n     return Ax

But what if you don't need to sum it right away, but in the later code, and then calculate it as needed? Instead of returning the result of a sum, you can return a function that sums:

def lazy_sum (*args):    def  sum ():        = 0        for in  args:            = ax + n        return  ax    return sum

When we call lazy_sum() , we return the SUM function instead of summing the result:

When the function is called f , the result of the sum is really computed:

In this example, we define the function in the function lazy_sum sum , and the intrinsic function sum can refer to lazy_sum the parameters and local variables of the external function, and when the lazy_sum function is returned sum , the relevant parameters and variables are stored in the returned function, which is called " The program structure of the closure (Closure) has great power.

Note again that when we call lazy_sum() , each call returns a new function, even if the same parameter is passed in:

f1()and f2() the results of the call are not affected.

Closed Package

Notice that the returned function references a local variable within its definition, args so when a function returns a function, its internal local variables are referenced by the new function, so it is not easy to implement them with a simple closure.

Another problem to be aware of is that the returned function is not executed immediately, but only when it is called. f() Let's look at an example:

def count ():     = []    for in range (1, 4):        def  F ():              return i*i        fs.append (f)    return= count ()

In the example above, each loop creates a new function and then returns the 3 functions that were created.

You may think that the call f1() , f2() and the f3() result should be 1 ,, but the 4 9 actual result is:

All of them 9 ! The reason is that the returned function refers to the variable i , but it is not executed immediately. When all 3 functions are returned, the variables they refer to are i already turned 3 , so the end result is 9 .

One thing to keep in mind when returning closures: The return function does not refer to any loop variables, or to subsequent variables that change.

What if you must refer to a loop variable? The method is to create a function that binds the current value of the loop variable with the parameter of the function, regardless of how the loop variable is subsequently changed, and the value that is bound to the function parameter remains the same:

def count ():     def F (j):         def g ():             return j*J        return  g    = []    for  in range (1, 4):        #  F (i) is executed immediately, so the current value of I is passed in F ()    return FS

And look at the results:

The disadvantage is that the code is long and can be shortened with a lambda function.

Python's closing package