203

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.

Improve this question
3
  • 72
    Why do you have semicolons at the end of each line? Commented Sep 21, 2015 at 2:55
  • @Maikflow Isnt that a good practice? ASI converts not semicoloned lines into semicoloned in the background afaik. Commented Oct 1, 2019 at 12:11
  • @I.K. See this stackoverflow.com/questions/19365508/… Commented Oct 7, 2019 at 20:36

3 Answers 3

Reset to default
409

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.

Improve this answer
Sign up to request clarification or add additional context in comments.

3 Comments

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
If you have arg2 with default value, do this. threading.Thread(target=thread_function, args=(arg1,),kwargs={'arg2': arg2})
What about named args?
16

I hope to provide more background knowledge here.

First, let's look at the constructor signature of the 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 a TypeError to complain.

Improve this answer

Comments

1 
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")
Improve this answer

1 Comment

The community encourages adding explanations to questions and not posting purely code answers (see here).

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.