File not open for writing in Python script

Question

My script is not working, I can't figure out where the bug is, I have opened the file with the open() function each time I want to work with the file in Python, and when running it signals this error:

Traceback (most recent call last):
  File "my_example.py", line 26, in <module>
    doc.truncate()
IOError: File not open for writing

To run it I run it this way in the Terminal:

python my_example.py my_example_sample.txt

Here's the Python Script (code) :

from sys import argv
#from os.path import exists

script, filename = argv

print "The name of this program is %s" % script
print "The name of the file you want to read is %s" % filename
print "Press ENTER if you want to read the selected document."
print "Press CTRL-C to cancel."

raw_input('>')

print "%s :" % filename

doc = open(filename)

print doc.read()

#doc.close()

erase_file = raw_input("Do you want to erase the file %s Y/N? : " % filename)

if erase_file == "Y":
    doc = open(filename)
    print "Truncating the file..."
    doc.truncate()
    print "Done, truncated."
    #doc.close()
else:
    print "That's okay!"

write_file = raw_input("Do you want to write in the file %s Y/N? : " % filename) 
if write_file == "Y":
    doc = open(filename)
    print "I'm going to ask you to type in what you like to write in the file %s 
    (limited to 3 lines)" % filename
    line1 = raw_input("line 1: ")
    line2 = raw_input("line 2: ")
    line3 = raw_input("line 3: ")
    print "Perfect! writing in..."
    doc.write(line1)
    doc.write('\n')
    doc.write(line2)
    doc.write('\n')
    doc.write(line3)
    doc.write('\n')
    print "Done!"
    doc.close()
else:
    print "Ok, see you later!"
    doc.close()
# add copy and exists? features?

Any solution?

What the program does is simply reading (read()) a file (printing the file), ask the user if he wants to erase the file (truncate()) and if he wants to write in it (write()) .

Answer 1

By default, open() opens for reading. If you want to open for writing, you have to pass the second argument of open as well to specific the mode (e.g. 'w' for writing).