Contributing to the “core” C Code#

This file is intended as guidelines to help contributors trying to modify the C source files in src/core.

What the files do#

The primary purpose of core is to implement type translations between Python and JavaScript. Here is a breakdown of the purposes of the files.

  • main – responsible for configuring and initializing the Python interpreter, initializing the other source files, and creating the _pyodide_core module which is used to expose Python objects to pyodide_py. main.c also tries to generate fatal initialization error messages to help with debugging when there is a mistake in the initialization code.

  • keyboard_interrupt – This sets up the keyboard interrupts system for using Pyodide with a webworker.

Backend utilities#

  • hiwire – A helper framework. It is impossible for wasm to directly hold owning references to JavaScript objects. The primary purpose of hiwire is to act as a surrogate owner for JavaScript references by holding the references in a JavaScript Map. hiwire also defines a wide variety of EM_JS helper functions to do JavaScript operations on the held objects. The primary type that hiwire exports is JsRef. References are created with Hiwire.new_value (only can be done from JavaScript) and must be destroyed from C with hiwire_decref or hiwire_CLEAR, or from JavaScript with Hiwire.decref.

  • error_handling – defines macros useful for error propagation and for adapting JavaScript functions to the CPython calling convention. See more in the Error Handling Macros section.

Type conversion from JavaScript to Python#

  • js2python – Translates basic types from JavaScript to Python, leaves more complicated stuff to jsproxy.

  • jsproxy – Defines Python classes to proxy complex JavaScript types into Python. A complex file responsible for many of the core behaviors of Pyodide.

Type conversion from Python to JavaScript#

  • python2js – Translates types from Python to JavaScript, implicitly converting basic types and creating pyproxies for others. It also implements explicit conversion from Python to JavaScript (the toJs method).

  • python2js_buffer – Attempts to convert Python objects that implement the Python Buffer Protocol. This includes bytes objects, memoryviews, array.array and a wide variety of types exposed by extension modules like numpy. If the data is a 1d array in a contiguous block it can be sliced directly out of the wasm heap to produce a JavaScript TypedArray, but JavaScript does not have native support for pointers, so higher dimensional arrays are more complicated.

  • pyproxy – Defines a JavaScript Proxy object that passes calls through to a Python object. Another important core file, PyProxy.apply is the primary entrypoint into Python code. pyproxy.c is much simpler than jsproxy.c though.

CPython APIs#

Conventions for indicating errors#

The two main ways to indicate errors:

  1. If the function returns a pointer, (most often PyObject*, char*, or const char*) then to indicate an error set an exception and return NULL.

  2. If the function returns int or float and a correct output must be nonnegative, to indicate an error set an exception and return -1.

Certain functions have “successful errors” like PyIter_Next (successful error is StopIteration) and PyDict_GetItemWithError (successful error is KeyError). These functions will return NULL without setting an exception to indicate the “successful error” occurred. Check what happened with PyErr_Occurred. Also, functions that return int for which -1 is a valid return value will return -1 with no error set to indicate that the result is -1 and -1 with an error set if an error did occur. The simplest way to handle this is to always check PyErr_Occurred.

Lastly, the argument parsing functions PyArg_ParseTuple, PyArg_Parse, etc are edge cases. These return true on success and return false and set an error on failure.

Python APIs to avoid:#

  • PyDict_GetItem, PyDict_GetItemString, and _PyDict_GetItemId These APIs do not do correct error reporting and there is talk in the Python community of deprecating them going forward. Instead, use PyDict_GetItemWithError and _PyDict_GetItemIdWithError (there is no PyDict_GetItemStringWithError API because use of GetXString APIs is also discouraged).

  • PyObject_HasAttrString, PyObject_GetAttrString, PyDict_GetItemString, PyDict_SetItemString, PyMapping_HasKeyString etc, etc. These APIs cause wasteful repeated string conversion. If the string you are using is a constant, e.g., PyDict_GetItemString(dict, "identifier"), then make an id with Py_Identifier(identifier) and then use _PyDict_GetItemId(&PyId_identifier). If the string is not constant, convert it to a Python object with PyUnicode_FromString() and then use e.g., PyDict_GetItem.

  • PyModule_AddObject. This steals a reference on success but not on failure and requires unique cleanup code. Instead, use PyObject_SetAttr.

Error Handling Macros#

The file error_handling.h defines several macros to help make error handling as simple and uniform as possible.

Error Propagation Macros#

In a language with exception handling as a feature, error propagation requires no explicit code, it is only if you want to prevent an error from propagating that you use a try/catch block. On the other hand, in C all error propagation must be done explicitly.

We define macros to help make error propagation look as simple and uniform as possible. They can only be used in a function with a finally: label which should handle resource cleanup for both the success branch and all the failing branches (see structure of functions section below). When compiled with DEBUG_F, these commands will write a message to console.error reporting the line, function, and file where the error occurred.

  • FAIL() – unconditionally goto finally;.

  • FAIL_IF_NULL(ptr)goto finally; if ptr == NULL. This should be used with any function that returns a pointer and follows the standard Python calling convention.

  • FAIL_IF_MINUS_ONE(num)goto finally; if num == -1. This should be used with any function that returns a number and follows the standard Python calling convention.

  • FAIL_IF_NONZERO(num)goto finally; if num != 0. Can be used with functions that return any nonzero error code on failure.

  • FAIL_IF_ERR_OCCURRED()goto finally; if the Python error indicator is set (in other words if PyErr_Occurred()).

  • FAIL_IF_ERR_MATCHES(python_err_type)goto finally; if PyErr_ExceptionMatches(python_err_type), for example FAIL_IF_ERR_MATCHES(PyExc_AttributeError);

JavaScript to CPython calling convention adaptors#

If we call a JavaScript function from C and that JavaScript function throws an error, it is impossible to catch it in C. We define two EM_JS adaptors to convert from the JavaScript calling convention to the CPython calling convention. The point of this is to ensure that errors that occur in EM_JS functions can be handled in C code using the FAIL_*`` macros. When compiled with DEBUG_F, when a JavaScript error is thrown a message will also be written to console.error`. The wrappers do roughly the following:

try {
  // body of function here
} catch (e) {
  // wrap e in a Python exception and set the Python error indicator
  // return error code

There are two variants: EM_JS_NUM returns -1 as the error code, EM_JS_REF returns NULL == 0 as the error code. A couple of simple examples: Use EM_JS_REF when return value is a JsRef:

EM_JS_REF(JsRef, hiwire_call, (JsRef idfunc, JsRef idargs), {
  let jsfunc = Hiwire.get_value(idfunc);
  let jsargs = Hiwire.get_value(idargs);
  return Hiwire.new_value(jsfunc(... jsargs));

Use EM_JS_REF when return value is a PyObject:

EM_JS_REF(PyObject*, __js2python, (JsRef id), {
  // body here

If the function returns void, use EM_JS_NUM with return type errcode. errcode is a typedef for int. EM_JS_NUM will automatically return -1 if an error occurs and 0 if not:

EM_JS_NUM(errcode, hiwire_set_member_int, (JsRef idobj, int idx, JsRef idval), {
  Hiwire.get_value(idobj)[idx] = Hiwire.get_value(idval);

If the function returns int or bool use EM_JS_NUM:

EM_JS_NUM(int, hiwire_get_length, (JsRef idobj), {
  return Hiwire.get_value(idobj).length;

These wrappers enable the following sort of code:

except JsException:
  print("Caught an exception thrown in JavaScript!")

Structure of functions#

In C it takes special care to correctly and cleanly handle both reference counting and exception propagation. In Python (or other higher level languages), all references are released in an implicit finally block at the end of the function. Implicitly, it is as if you wrote:

def f():
  try: # implicit
    a = do_something()
    b = do_something_else()
    c = a + b
    return some_func(c)
    # implicit, free references both on successful exit and on exception

Freeing all references at the end of the function allows us to separate reference counting boilerplate from the “actual logic” of the function definition. When a function does correct error propagation, there will be many different execution paths, roughly linearly many in the length of the function. For example, the above pseudocode could exit in five different ways: do_something could raise an exception, do_something_else could raise an exception, a + b could raise an exception, some_func could raise an exception, or the function could return successfully. (Even a Python function like def f(a,b,c,d): return (a + b) * c - d has four execution paths.) The point of the try/finally block is that we know the resources are freed correctly without checking once for each execution path.

To do this, we divide any function that produces more than a couple of owned PyObject*s or JsRefs into several “segments”. The more owned references there are in a function and the longer it is, the more important it becomes to follow this style carefully. By being as consistent as possible, we reduce the burden on people reading the code to double-check that you are not leaking memory or errors. In short functions it is fine to do something ad hoc.

  1. The guard block. The first block of a function does sanity checks on the inputs and argument parsing, but only to the extent possible without creating any owned references. If you check more complicated invariants on the inputs in a way that requires creating owned references, this logic belongs in the body block.

Here’s an example of a METH_VARARGS function:

JsImport_CreateModule(PyObject* self, PyObject* args)
  // Guard
  PyObject* name;
  PyObject* jsproxy;
  // PyArg_UnpackTuple uses an unusual calling convention:
  // It returns `false` on failure...
  if (!PyArg_UnpackTuple(args, "create_module", 2, 2, &spec, &jsproxy)) {
    return NULL;
  if (!JsProxy_Check(jsproxy)) {
    PyErr_SetString(PyExc_TypeError, "package is not an instance of jsproxy");
    return NULL;
  1. Forward declaration of owned references. This starts by declaring a success flag bool success = false. This will be used in the finally block to decide whether the finally block was entered after a successful execution or after an error. Then declare every reference counted variable that we will create during execution of the function. Finally, declare the variable that we are planning to return. Typically, this will be called result, but in this case the function is named CreateModule so we name the return variable module.

  bool success = false;
  // Note: these are all the objects that we will own. If a function returns
  // a borrow, we XINCREF the result so that we can CLEAR it in the finally block.
  // Reference counting is hard, so it's good to be as explicit and consistent
  // as possible!
  PyObject* sys_modules = NULL;
  PyObject* importlib_machinery = NULL;
  PyObject* ModuleSpec = NULL;
  PyObject* spec = NULL;
  PyObject* __dir__ = NULL;
  PyObject* module_dict = NULL;
  // result
  PyObject* module = NULL;
  1. The body of the function. The vast majority of API calls can return error codes. You MUST check every fallible API for an error. Also, as you are writing the code, you should look up every Python API you use that returns a reference to determine whether it returns a borrowed reference or a new one. If it returns a borrowed reference, immediately Py_XINCREF() the result to convert it into an owned reference (before FAIL_IF_NULL, to be consistent with the case where you use custom error handling).

  name = PyUnicode_FromString(name_utf8);
  sys_modules = PyImport_GetModuleDict(); // returns borrow
  module = PyDict_GetItemWithError(sys_modules, name); // returns borrow
  if(module && !JsImport_Check(module)){
      "Cannot mount with name '%s': there is an existing module by this name that was not mounted with 'pyodide.mountPackage'."
      , name
// ... [SNIP]
  1. The finally block. Here we will clear all the variables we declared at the top in exactly the same order. Do not clear the arguments! They are borrowed. According to the standard Python function calling convention, they are the responsibility of the calling code.

  success = true;
  return result;

One case where you do need to Py_CLEAR a variable in the body of a function is if that variable is allocated in a loop:

  // refcounted variable declarations
  PyObject* pyentry = NULL;
  // ... other stuff
  Py_ssize_t n = PySequence_Length(pylist);
  for (Py_ssize_t i = 0; i < n; i++) {
    pyentry = PySequence_GetItem(pydir, i);
    Py_CLEAR(pyentry); // important to use Py_CLEAR and not Py_decref.

  success = true
  // have to clear pyentry at end too in case do_something failed in the loop body


Any nonstatic C function called some_name defined not using EM_JS will be exposed as pyodide._module._some_name, and this can be used in tests to good effect. If the arguments / return value are not just numbers and booleans, it may take some effort to set up the function call.

If you want to test an EM_JS function, consider moving the body of the function to an API defined on Module. You should still wrap the function with EM_JS_REF or EM_JS_NUM in order to get a function with the CPython calling convention.