Why does reduce fail when run inside a timer?

Question

I have a script that watches for changes in another script, and re-runs the latter if it's changed. Gist here. It's run like this:

~/blender/blender-2.92.0-linux64/blender --python script_watcher.py -- test_group.py

It worked ok until I used the reduce() function in the watched script - snippet here (union() is a function that creates the union of two objects):

def all_union(first, *others, **kwargs):
    def _(a, b):
        return union(a, b)
    r = reduce(_, others, first)
    return r

then I get an error message like this:

NameError: name 'reduce' is not defined 

I tried replacing the timer script with one that runs only once - like this:

import bpy
import sys
import os
import traceback

prev = ""
curr = open(sys.argv[-1]).read()
if not prev==curr:
    print("%s changed" % sys.argv[-1])
    try:
        exec(curr)
    except Exception as e:
        exc_type, exc_obj, exc_tb = sys.exc_info()
        print(traceback.format_exc())

and it runs perfectly - no error.

So why is reduce() not not working under the timer? or is the issue somewhere else?