sudo.py

import sys, marshal, functools, subprocess

child_script = """
import marshal, sys, types;
fn, args, kwargs = marshal.load(sys.stdin)
marshal.dump(
    types.FunctionType(fn, globals())(*args, **kwargs),
    sys.stdout)
"""

def sudo(fn):
    @functools.wraps(fn)
    def inner(*args, **kwargs):
        proc_args = [
            "sudo",
            sys.executable,
            "-c",
            child_script]
        proc = subprocess.Popen(
            proc_args,
            stdin=subprocess.PIPE,
            stdout=subprocess.PIPE)
        send_data = marshal.dumps((
            fn.func_code,
            args,
            kwargs))
        recv_data = proc.communicate(send_data)[0]
        return marshal.loads(recv_data)
    return inner

@sudo
def whoami():
    import os
    return "I am: %d" % os.getuid()

if __name__ == '__main__':
    print whoami()

As of 10/23/2021, above code won't run under py3.

But the below code would:

import sys, marshal, functools, subprocess

child_script = """
import marshal, sys, types;
fn, args, kwargs = marshal.loads(sys.stdin.buffer.read())
sys.stdout.buffer.write(
    marshal.dumps(
       types.FunctionType(fn, globals())(*args, **kwargs),
    )
)
"""

def sudo(fn):
    @functools.wraps(fn)
    def inner(*args, **kwargs):
        proc_args = [
            "sudo",
            sys.executable,
            "-c",
            child_script]
        proc = subprocess.Popen(
            proc_args,
            stdin=subprocess.PIPE,
            stdout=subprocess.PIPE)
        send_data = marshal.dumps((
            fn.__code__,
            args,
            kwargs))
        recv_data = proc.communicate(send_data)[0]
        return marshal.loads(recv_data)
    return inner

Very useful, I took @mohsenBanan code to add support to all kind of objects by using the pickle library.
The only drawback is that you should include the import or definition of what you want inside the function's definition.

#main.py
import sys, marshal, functools, subprocess, pickle

child_script = """
import pickle, sys, types, marshal;
fn, args, kwargs = marshal.loads(sys.stdin.buffer.read())
sys.stdout.buffer.write(
    pickle.dumps(
       types.FunctionType(fn, globals())(*args, **kwargs)
    )
)
"""

def sudo(fn):
    @functools.wraps(fn)
    def inner(*args, **kwargs):
        proc_args = [
            "sudo",
            sys.executable,
            "-c",
            child_script]
        proc = subprocess.Popen(
            proc_args,
            stdin=subprocess.PIPE,
            stdout=subprocess.PIPE)
        send_data = marshal.dumps((
            fn.__code__,
            args,
            kwargs))
        recv_data = proc.communicate(send_data)[0]
        out = pickle.loads(recv_data)
        return out
    return inner

@sudo
def func(x):
    from transport import Vehicle
    return (x, "Hello", Vehicle("green", 2))
if __name__ == "__main__":
    print(func(3))

#transport.py

class Vehicle:
    def __init__(self, color, wheels):
        self.color = color
        self.wheels = wheels