implement toString on javascript functions

[hansthen]

Describe your feature request here.

In javascript I can do the following:

exports.hello = () => { console.log('hello, world') };
console.log(exports.hello.toString())

However, I cannot call toString() from pythonmonkey. I would like to do this, so I could access the function definition from inside python. If there is another way to access the function definition as a string that would be acceptable as well.

Code example

import pythonmonkey as pm
module = pm.require("./my.js")
print(module.hello.toString())

==>

"() => { console.log('hello, world') }"

[philippedistributive]

@hansthen Please elaborate on the actual case to help us prioritise this feature request

[hansthen]

It is for the Folium library. Folium is a python library wrapping the javascript Leaflet library to generate maps. This is done by generating the html and javascript code for a Leaflet map. Users can configure many Leaflet objects by defining javascript functions (event handlers and stuff). Currently these functions are defined as strings in python code. Like this:

event_handler = JsCode("""
function (event) {
    //
}
""")

This is not really comfortable. Editor and linting support is lacking and it is difficult to write unit tests.

Several users have asked for a way to "import" the javascript functions from a *.js file into python. The javascript functions are really defined as strings, which are then passed to the templating engine. If we could parse the javascript files and retrieve the function definitions this would be a trivial task.

[hansthen]

I know this is probably not your core use case, but the PythonMonkey library is really fast and does almost everything else we need to implement this. It also has a really natural API for our purposes. I would be very grateful if you could add this. It does not seem like it would be terribly difficult to implement, but I do not know much about your code internals.

[caleb-distributive]

@hansthen
We plan on implementing being able to access JS function properties from python (such as toString), but in the meantime, to get a JSFunction as a string, you can do:

import pythonmonkey as pm
module = pm.require("./my.js")
print(pm.eval("(func) => { return func.toString(); })")(module.hello))

or, if you don't like dense one-liners:

import pythonmonkey as pm
module = pm.require("./my.js")
getFunctionString = pm.eval("(func) => { return func.toString(); }")
helloString = getFunctionString(module.hello)
print(helloString)

This just calls toString in JS land, and returns the result back to python.

[hansthen]

Thanks for the suggestion. Unfortunately, it does not seem to work in all cases. For me it returns this:

function() {
    [native code]
}

[philippedistributive]

Hi, we'll look into this further, eventually, no timeline for now besides by mid-summer

[hansthen]

Thanks for the heads up.

[caleb-distributive]

@hansthen

Thanks for the suggestion. Unfortunately, it does not seem to work in all cases. For me it returns this:

function() {
    [native code]
}

Ah, I forgot that we bind b to a when we do a.b in python if a is a JSObjectProxy and b is a function in order to get methods to work properly, since the this value of a JS method is determined by how it is accessed, while in python the self value is determined at the moment the method is created, i.e. all methods in python are bound functions (pyodide also does this, see here: https://github.com/pyodide/pyodide/blob/ee863a7f7907dfb6ee4948bde6908453c9d7ac43/src/core/jsproxy.c#L388 )

To get around this, you can do:

import pythonmonkey as pm
module = pm.require("./my.js")
unboundFunction = pm.eval("(module) => module.hello")(module)
getFunctionString = pm.eval("(func) => func.toString()")
print(getFunctionString(unboundFunction))

I made sure to actually run this code myself this time to check that it works 😉

[wesgarland]

@caleb-distributive I think our wrappers should have a better toString method if possible. '[native code]' could mention that it is an proxy and, ideally, also mentions the function name when it's available