What is an anonymous function? What are the uses of Python anonymous functions?

When we pass in a function, sometimes there is no need to explicitly define the function. It is more convenient to pass in the anonymous function directly.

In Python, there is limited support for anonymous functions. Still taking the map() function as an example, when calculating f(x)=x2, in addition to defining a function of f(x), you can also directly pass in an anonymous function:

>>> list(map(lambda x: x * x, [1, 2, 3, 4, 5, 6, 7, 8, 9]))
[1, 4, 9, 16, 25, 36, 49, 64, 81]

As can be seen from comparison, Anonymous function lambda x: x * x is actually:

def f(x):
    return x * x

The keyword lambda represents an anonymous function, and the x before the colon represents the function parameter.

Anonymous functions have a limitation, that is, they can only have one expression. There is no need to write return. The return value is the result of the expression.

There is an advantage to using anonymous functions, because the function has no name, so you don’t have to worry about function name conflicts. In addition, the anonymous function is also a function object. You can also assign the anonymous function to a variable and then use the variable to call the function:

>>> f = lambda x: x * x
>>> f
<function <lambda> at 0x101c6ef28>
>>> f(5)25

Similarly, you can also return the anonymous function as a return value, such as:

def build(x, y):
    return lambda: x * x + y * y

Encapsulates details to improve security and controllability. It is often used outside functions in the global scope, thereby limiting the addition of too many variables and functions to the global scope.
Using block-level scope in the global scope can reduce the problem of closures occupying memory, because there is no reference to the anonymous function, and its scope chain can be destroyed immediately as long as the function is executed.

Imitate block-level (private) scope:

function box(){
    for(var i=0;i<5;i++){  //块级作用域(js无)
    }   
     var i //即便重新声明，也不会影响之前的值
    alert(i);//5
 }
box();

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