fin = open("data.txt") # in the working folder

# open a specifically named file, data.txt, in some other folder 
fin = open("e:/python/data.txt") # PC
fin = open("/Volumes/programming/python/data.txt") # Mac

# open a file whose name is stored in a text variable
fileName = "e:/python/data.txt" # PC
fileName = "/Volumes/programming/python/data.txt" # Mac
fin = open(fileName) 

# open a file whose name is entered by the user 
fileName = input("What file do you want to use for input? ")
fin = open(fileName) 

# open a 2nd file (storing the filename in a text variable) 
fin.close() # close 1st file before opening 2nd
fileName = input("What file do you want to use for input next? ")
fin = open(fileName)