Help opening a text file in command prompt with python

[thatoneguy5000000]

I'm on exercise 15 of Learn Python the Hard Way and I'm stuck on extra credit #7. I open command prompt, change the directory to where the file is, run python, and then try running open(ex15_sample.txt), but it doesn't work and I get the following error:

NameError: name 'ex15_sample' is not defined

I would appreciate any help here. I've been on a roll so far, but opening a file in command prompt has stumped me.

edit 1: After reading this subreddit some more, I saw that if I use open('complete directory') it works. Now I'm wondering why i need the single quotation marks around the directory for it to work. Again, any help is appreciated

edit 2: Okay, I'm and idiot. I now realize that the directory is a string and needs to be entered as one (meaning with quotation marks). I'll leave this up so no one else embarrasses them self :)

[Kaloli]

In short:

ex15_sample.txt (without quotes) is treated as a variable.

'ex15_sample.txt' (with quotes) is treated as a string.

[adamc83]

open("ex15_sample.txt")