[python] IOError: [Errno 22] invalid mode ('r') or filename: 'c:\\Python27\test.txt'

What is wrong with the following:

test_file=open('c:\\Python27\test.txt','r')

always use 'r' to get a raw string when you want to avoid escape.

test_file=open(r'c:\Python27\test.txt','r')

\ is an escape character in Python. \t gets interpreted as a tab. If you need \ character in a string, you have to use \\.

Your code should be:
test_file=open('c:\\Python27\\test.txt','r')

\t in a string marks an escape sequence for a tab character. For a literal \, use \\.