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: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.
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.
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")
