In order to support PEP-489 (multi-step module initialisation and reinitialisation), the global state of modules must become local to a module instance. Initial support for PEP-489 is in PR #1919.

Further links:

• PEP-573 about accessing module state from extension type methods
• PEP-575 about refactoring CPython's C functions
• Meta-PEP 576 about function type changes

Approach for Cython:

1. Require users to explicitly enable this feature with a compiler directive, as Cython cannot safely determine that a module supports this (there might be global state in external C libraries or C function callbacks), and there will be a visible performance impact.

2. Move all globals into a struct that gets stored in the CPython module state.

3. Use context variables (PEP 567) to pass the module state into C functions (also across coroutine dispatches etc.).

4. Copy all extension types to the heap to make them specific to a module instance and their methods aware of the module.

5. Allow users to make specific module globals shared across modules with a new declaration.

Thus, the following module:

cdef int x = 8

cdef int func():
    n = x
    return n

def get():
    print(self.x)
    return func()

def change(n):
    global x
    self.x = n

would internally be implemented as follows:

cdef int x = 8

cdef int func():
    __state = ContextVars['__cython_this_module_state_1234']
    n = __state.x
    return n

def get():
    with context(__cython_this_module_state_1234=__module_c_state__):
        print(self.x)
        return func()

def change(n):
    with context(__cython_this_module_state_1234=__module_c_state__):
        global x
        self.x = n