import importlib.util
import os
import sys
import threading
import time
import types
print( sys.version_info )

PICK_API = int( os.environ[ "PICK_API" ] )
# 0: deprecated
# 1: deprecated with `module_for_loader`
# 2: new

xxx_module = \
"""
print( '=== loading module ===' )
import time
time.sleep( 3 )
x = 0
print( '=== loaded module ===' )
"""

class Loader( object ):
    def find_module( self, fullname, path ):
        if "xxx" == fullname:
            return self

    if PICK_API == 0:
        def load_module( self, fullname ):
            orig_module = sys.modules.get( fullname, None )
            module = sys.modules[ fullname ] = orig_module or types.ModuleType( fullname )
            try:
                exec( xxx_module, module.__dict__ )
            except Exception:
                if not orig_module:
                    del sys.modules[ fullname ]
                raise
            else:
                return module
    elif PICK_API == 1:
        @importlib.util.module_for_loader
        def load_module( self, module ):
            exec( xxx_module, module.__dict__ )
    elif PICK_API == 2:
        def create_module( self, fullname ):
            pass
        def exec_module( self, module ):
            exec( xxx_module, module.__dict__ )
    else:
        raise ValueError( "bad" )

sys.meta_path.insert( 0, Loader() )
def f():
    time.sleep( 1 )
    import xxx
    print( xxx.x )
t = threading.Thread( target=f )
t.daemon = True
t.start()

import xxx
print( xxx.x )
t.join()