Python Threading String Arguments

Question

I have a problem with Python threading and sending a string in the arguments.

def processLine(line) :
    print "hello";
    return;

.

dRecieved = connFile.readline();
processThread = threading.Thread(target=processLine, args=(dRecieved));
processThread.start();

Where dRecieved is the string of one line read by a connection. It calls a simple function which as of right now has only one job of printing "hello".

However I get the following error

Traceback (most recent call last):
File "C:\Python25\lib\threading.py", line 486, in __bootstrap_inner
self.run()
File "C:\Python25\lib\threading.py", line 446, in run
self.__target(*self.__args, **self.__kwargs)
TypeError: processLine() takes exactly 1 arguments (232 given)

232 is the length of the string that I am trying to pass, so I guess its breaking it up into each character and trying to pass the arguments like that. It works fine if I just call the function normally but I would really like to set it up as a separate thread.

@Maikflow Isnt that a good practice? ASI converts not semicoloned lines into semicoloned in the background afaik. — I.K., Oct 01 '19 at 12:11
@I.K. See this https://stackoverflow.com/questions/19365508/when-is-semicolon-use-in-python-considered-good-or-acceptable — Maikflow, Oct 07 '19 at 20:36

score 375 · Accepted Answer · edited Jul 30 '18 at 20:50

375

You're trying to create a tuple, but you're just parenthesizing a string :)

Add an extra ',':

dRecieved = connFile.readline()
processThread = threading.Thread(target=processLine, args=(dRecieved,))  # <- note extra ','
processThread.start()

Or use brackets to make a list:

dRecieved = connFile.readline()
processThread = threading.Thread(target=processLine, args=[dRecieved])  # <- 1 element list
processThread.start()

If you notice, from the stack trace: self.__target(*self.__args, **self.__kwargs)

The *self.__args turns your string into a list of characters, passing them to the processLine function. If you pass it a one element list, it will pass that element as the first argument - in your case, the string.

edited Jul 30 '18 at 20:50

davr

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answered Jul 11 '10 at 02:54

Stephen

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1

extra parentheses in at the end of line two of the second code block. I wanted to edit it but it is less that 6 characters – harmonickey May 09 '15 at 01:57
4

If you have arg2 with default value, do this. `threading.Thread(target=thread_function, args=(arg1,),kwargs={'arg2': arg2})` – SpiralDev Jan 24 '19 at 07:40
What about named args? – carkod Aug 23 '21 at 10:45

themefield · Answer 2 · 2019-09-21T22:17:32.557

I hope to provide more background knowledge here.

First, constructor signature of the of method threading::Thread:

class threading.Thread(group=None, target=None, name=None, args=(), kwargs={}, *, daemon=None)

args is the argument tuple for the target invocation. Defaults to ().

Second, A quirk in Python about tuple:

Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses).

On the other hand, a string is a sequence of characters, like 'abc'[1] == 'b'. So if send a string to args, even in parentheses (still a sting), each character will be treated as a single parameter.

However, Python is so integrated and is not like JavaScript where extra arguments can be tolerated. Instead, it throws an TypeError to complain.

score 0 · Answer 3 · answered Dec 29 '20 at 03:53

0

from threading import Thread
from time import sleep
def run(name):
    for x in range(10):
        print("helo "+name)
        sleep(1)
def run1():
    for x in range(10):
        print("hi")
        sleep(1)
T=Thread(target=run,args=("Ayla",))
T1=Thread(target=run1)
T.start()
sleep(0.2)
T1.start()
T.join()
T1.join()
print("Bye")

answered Dec 29 '20 at 03:53

HASEEB ALI

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The community encourages adding explanations to questions and not posting purely code answers (see [here](https://meta.stackoverflow.com/questions/300837/what-comment-should-i-add-to-code-only-answers)). – costaparas Dec 29 '20 at 04:58

Python Threading String Arguments

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