title: Modules and Host Callbacks icon: dot

order: 8

Modules and host callbacks

A module holds Python globals. Native modules created with py_newmodule() can be imported by scripts in the current VM, just like source modules.

Create or look up a module

This fragment runs after initialization:

py_GlobalRef settings = py_newmodule("settings");
py_newint(py_emplacedict(settings, py_name("screen_width")), 1280);

bool ok = py_exec("import settings\nprint(settings.screen_width)",
                  "app.py", EXEC_MODE, NULL);
if(!ok) py_printexc();

Create a module only once per VM: recreating an existing name is an error at the native API level. py_getmodule("settings") looks up an already registered module and returns NULL if it is absent; it does not perform an import.

py_import(name) attempts to load a module and returns:

Return	Meaning
`1`	Success; the module is in `py_retval()`.
`0`	The module was not found; the caller decides how to report this.
`-1`	Loading failed with a Python exception.

Do not treat py_import() as a boolean: -1 is true in a C condition.

Where imports come from

The importer first checks registered modules, then the host's optional lazyimport callback, then bundled Python modules. For a file-based module such as game.level, it asks importfile for these paths in order:

game/level.py
game/level.pyc
game/level/__init__.py
game/level/__init__.pyc

Path separators follow the platform. Parent packages are loaded first. Supported desktop builds may then try a native dynamic module if enabled.

The default file callback opens paths relative to the process working directory. Running main scripts/app.py does not automatically add scripts/ to an import path. Start the host in the intended script directory or provide your own loader. pocketpy does not provide CPython's sys.path search mechanism.

When both source and bytecode exist, source is preferred. See bytecode deployment for the distribution format.

Load a module from memory

Replace py_callbacks()->importfile to load scripts from a game asset bundle, a virtual filesystem, or embedded strings. This complete example provides one module without reading a file:

#include "pocketpy.h"
#include <string.h>

static char* import_source(const char* path, int* data_size) {
    if(strcmp(path, "settings.py") != 0) return NULL;

    const char* source = "screen_width = 1280\n";
    size_t size = strlen(source);
    char* buffer = py_malloc(size + 1);
    memcpy(buffer, source, size + 1);
    if(data_size != NULL) *data_size = (int)size;
    return buffer;
}

int main(void) {
    py_initialize();
    py_callbacks()->importfile = import_source;

    bool ok = py_exec("import settings\nassert settings.screen_width == 1280",
                      "app.py", EXEC_MODE, NULL);
    if(!ok) py_printexc();

    py_finalize();
    return ok ? 0 : 1;
}

The loader contract matters:

Return NULL when the requested path is unavailable.
Allocate a fresh buffer with py_malloc(); the interpreter owns and frees it. Do not return a string literal, stack buffer, or shared asset pointer.
NUL-terminate source text. For bytecode, report the exact byte length.
Check whether data_size is NULL: source reloads can omit this output.
Handle every path variant that your application supports.

Callbacks belong to the current VM. Configure each VM that needs a custom loader, and configure it again after a reset.

Redirect script output

py_callbacks()->print receives NUL-terminated UTF-8 text. A single Python print() can call it more than once; do not assume each callback contains a whole line. Set flush if your output backend buffers data. py_printexc() also uses this output callback.

The getchr callback supplies input for input(). More specialized hooks, including gc_mark and displayhook, are declared in the public header.

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