Python Threading String Arguments

Posted on

Question :

Python Threading String Arguments

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:Python25libthreading.py", line 486, in __bootstrap_inner
self.run()
File "C:Python25libthreading.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.

Answer #1:

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.

Answered By: Stephen

Answer #2:

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.

Answered By: themefield

Answer #3:

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 By: HASEEB ALI

Leave a Reply

Your email address will not be published.