Comparison of closures in Python and Javascript, pythonjavascript

Comparison of closures in Python and Javascript, pythonjavascript

Similar to scripting languages, python and Javascript have similar variable scopes. Unlike php, all internal variables of a function are isolated from the outside. That is to say, to process external data of a function, parameters must be used to pass in the data to be processed (the global keyword is not discussed here), while python and Javascript are different. If a function declares a variable, it will search online step by step, until a value is returned or undefined.

In this case, the closure of python should be very simple. Like javascript, we write similar code:

def func1():    a = 1    def func2():        a = a + 1        return a    return func2re=func1()print re()print re()

However, the actual situation is that the result does not show 2 and 3 as we expected. Instead, the following error occurs: "UnboundLocalError: local variable 'A' referenced before assignment "(the local variable a is referenced before being assigned a value ). Why is there such a problem? Let's first look at how js implements this closure:

<script> function func1(){ var a=1;  function func2(){  a=a+1;  return a;  } return func2; }re=func1();console.log(re());console.log(re());</script>

As expected, enter 2 and 3. Note the first line of the program. If I add a var above, what is the result of the program running? The final result is that two "NaN" are entered. on Firefox's developer platform, the following description about var is found:

Declares a variable, optionally initializing it to a value.
The scope of a variable declared with var is the enclosing function or, for variables declared outside a function, the global scope (which is bound to the global object ).

Var is used to declare local variables. In the above example, if var a = a + 1 is used, then a is already a local variable in func2, instead of inheriting from func1, the result of NaN is displayed.

Let's go back to the closure of python. The error message means that a is a local variable. In fact, python requires that all the variables on the left of the value assignment statement are local variables, this a is on the left of the equal sign, so it becomes a local variable. As a result, I cannot access a in func1. How can this problem be solved? If it is above python3.0, you can use nonloacal a to specify a as not a local variable before a = a + 1. The nonloacal keyword is not supported in versions earlier than 3.0. We can do this:

def func1():    a = [1]    def func2():        a[0] = a[0] + 1        return a[0]    return func2re=func1()print re()print re()

As expected, 2 and 3 are printed. From the example of python and Javascript closures, you need to understand the similarities and differences between the variable scopes in python and js variables declaration.