How can I implement Javascript closures in Python?

Question

I am trying to implement Javascript closures in Python 2.7.

Consider the following code:

var = {'upper_scope':None}

def f1():
    print var

def f2():
    var = {'upper_scope': var}

The first function works fine. The second one fails on its first line:

UnboundLocalError: local variable 'var' referenced before assignment.

I would like to read the value of var from the upper scope and place it inside a local dictionary named var.

Maybe it is possible but I would never use a variablename twice in the same file/class. I'm not sure but it might be against the pip rules. — Rik Verbeek
– Rik Verbeek, Commented Nov 20, 2014 at 18:06
Why do you want to implement JavaScript closures in Python, why not Python functions? — Daniel
– Daniel, Commented Nov 20, 2014 at 18:11
Man, that title. Is writing a proper title for a question a lost art now? — WGS
– WGS, Commented Nov 20, 2014 at 18:11
I am writing a translator and JavaScript has a bit different closures. — Piotr Dabkowski
– Piotr Dabkowski, Commented Nov 20, 2014 at 18:11
Basically inside one function you can have var be a local variable or a global variable, but you can't have it mean both things in the same function. — RemcoGerlich
– RemcoGerlich, Commented Nov 20, 2014 at 18:51

sysconfig · Accepted Answer · 2014-11-20 18:14:45Z

2

It doesn't work in f2(), because you are locally overwriting the global definition of var. If you rename var in f2(), it will work:

def f2():
   other_var = {'upper_scope': var}

Alternatively, you could define it as global, but then the change of var's value would be applied to the global variable, too, like so:

def f2():
   global var
   var = {'upper_scope': var}

That being said, it's always a better approach to pass variables as parameters to functions, unless you really need it in several places and/or need to change its value.

answered Nov 20, 2014 at 18:14

sysconfig

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9 Comments

Piotr Dabkowski Over a year ago

It has to have the same name, this is the point. And global will not work for nested functions.

user2357112 Over a year ago

@PiotrDabkowski: If you're trying to make your translator work without variable renaming, you're making things a lot harder for yourself than you need to. What happens when you need to translate a variable named $foo?

Piotr Dabkowski Over a year ago

Its all stored inside var which represents scope :). so $foo=4 will translate to var.put('$foo', Js(4))

user2357112 Over a year ago

@PiotrDabkowski: You can just number the vars according to the nesting level and have your translator use the right name for each access.

Piotr Dabkowski Over a year ago

@user2357112 I thought about it but it would be much harder to implement and the resulting code would be impossible to read. var=var is a better option than using different names for scope variable.

|

Piotr Dabkowski · Accepted Answer · 2015-03-11 23:39:53Z

1

This is probably the only way of doing that. I used this method in my JavaScript to Python converter and it does it's job.

var = {'upper_scope':None}
def first(var=var):
    var = {'upper_scope': var}
    def second(var=var):
        var = {'upper_scope': var } 
        def third(var=var):
             var = ...

Now each function 'remembers' its upper scope - its stored a as its default argument. Also note that the function automatically creates local scope when called (var = {'upper_scope': var}).

answered Mar 11, 2015 at 23:39

Piotr Dabkowski

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Comments

kammala · Accepted Answer · 2014-11-20 18:33:16Z

0

There is nonlocal statement in python3 syntax which was invented exactly for what you asked: for access to names in outer scope.

For access global varible in nest of functions you can do something like this(works in both python2 and python3):

var = {'upper_scope':None}
def first():
    global var
    var = {'upper_scope': var}
    def second():
        global var
        var = {'upper_scope': var } 
        def third():
            global var
            var = ...

In all of these functions will exist only one example of variable var — the global one. So every changes in inner scope will be available on higher level and vice versa.

edited Nov 20, 2014 at 18:33

answered Nov 20, 2014 at 18:26

kammala

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3 Comments

Piotr Dabkowski Over a year ago

I am using python 2.7 nonlocal is not implemented here :(

kammala Over a year ago

sorry, this also will not work in python3, because there is only global var exists and no others.

kammala Over a year ago

I fix my answer. It will work for closures of global variables.

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