Many threads to write log file at same time in Python

print is not thread safe. Use the logging module instead (which is):

import logging
import threading
import time


FORMAT = '[%(levelname)s] (%(threadName)-10s) %(message)s'

logging.basicConfig(level=logging.DEBUG,
                    format=FORMAT)

file_handler = logging.FileHandler('results.log')
file_handler.setFormatter(logging.Formatter(FORMAT))
logging.getLogger().addHandler(file_handler)


def worker():
    logging.info('Starting')
    time.sleep(2)
    logging.info('Exiting')


t1 = threading.Thread(target=worker)
t2 = threading.Thread(target=worker)

t1.start()
t2.start()

Output (and contents of results.log):

[INFO] (Thread-1  ) Starting
[INFO] (Thread-2  ) Starting
[INFO] (Thread-1  ) Exiting
[INFO] (Thread-2  ) Exiting

Instead of using the default name (Thread-n), you can set your own name using the name keyword argument, which the %(threadName) formatting directive then will then use:

t = threading.Thread(name="My worker thread", target=worker)

---

(This example was adapted from an example from Doug Hellmann's excellent article about the threading module)

You can simply create your own locking mechanism to ensure that only one thread is ever writing to a file.

import threading
lock = threading.Lock()

def write_to_file(f, text, file_size):
    lock.acquire() # thread blocks at this line until it can obtain lock

    # in this section, only one thread can be present at a time.
    print >> f, text, file_size

    lock.release()

def filesize(asset):  
    f = open("results.txt", 'a+')  
    c = wmi.WMI(asset)  
    wql = 'SELECT FileSize,Name FROM CIM_DataFile where (Drive="D:" OR Drive="E:") and Caption like "%file%"'  
    for item in c.query(wql):  
        write_to_file(f, item.Name.split("\\")[2].strip().upper(), str(item.FileSize))

You may want to consider placing the lock around the entire for loop for item in c.query(wql): to allow each thread to do a larger chunk of work before releasing the lock.