_thread.start_new_thread: How to use kwargs

[namgang]

Baffled by kwargs in _thread.start_new_thread. The docs say The optional kwargs argument specifies a dictionary of keyword arguments. That's all it says. In spite of intense googling I can't find any example actually using this. To make things worse, I'm a mostly Java person and a bit hamhanded with Python. I may have gotten something wrong.

My function to spawn as a thread has this signature: def serverReport(self, id, cmd, val) where id must be a tuple. I've got a dict defining values for keys cmd and val. Invocation: _thread.start_new_thread(self.serverReport, self.ID, mydict). That gets me TypeError: function got multiple values for argument 'val'.

Allright, so I remove those args to get this signature: def serverReport(self, id). Same invocation. Now I get TypeError: function takes 2 positional arguments but 4 were given.

Other attempts: def serverReport(self, id, **kwargs) with or without asterisks yields TypeError: function takes 3 positional arguments but 4 were given.

MicroPython v1.25.0 on 2025-04-15; Generic ESP32 module with ESP32

[shariltumin]

I don't know the details of your script, but maybe this will help:

import _thread
import time

class test():
     def serverReport(self, id, what):
         print(f'ID: {id[0]}/{id[1]}, CMD: {what['cmd']} {what['val']}')

t = test()
# t.serverReport(('PRG', 123), {'cmd':'add', 'val':(3,4)})

# new_thread(func, (param))
_thread.start_new_thread(t.serverReport, (('PRG', 123), {'cmd':'add', 'val':(3,4)}))

time.sleep(5) # wait a bit
print('Done!')

This will give:

ID: PRG/123, CMD: add (3, 4)
Done!

[namgang]

Gosh, I'd never have guessed, but it's correct, it works! Declare one extra arg in the function to be spawned as a signal to the Pythonic machinery. When invoking, supply one single actual arg, a tuple. The first element of the tuple will be taken as the args argument found in the docs and must be a tuple. The second element should be a dict. It will be taken as the kwargs argument. (It seems the dict is not specially treated in any way. Use it as a plain old dict.)

Many thanks!

P.S. I'm just a little worried that combining everything into a single tuple will cause object allocation.

[namgang]

Follow up: It seems the second element of the tuple can be anything. I can't explain why the docs say it specifies a dictionary of keyword arguments.

[peterhinch]

The args must be presented to start_new_thread as a tuple of positional args and a dict of keyword args, but they ar processed by the user function in the normal way:

import _thread
import time

class test():
     def serverReport(self, id, num, cmd, val):  # two positional args and two kwargs
         print(f'ID: {id}, num {num}, cmd {cmd}, val {val}') 

t = test()

_thread.start_new_thread(t.serverReport, ('PRG', 123), {'cmd':'add', 'val':(3,4)})

time.sleep(5) # wait a bit
print('Done!')

prints:

ID: PRG, num 123, cmd add, val (3, 4)

[namgang]

Great clarification. I'm still getting used to Python...