I am a beginner so if this question sounds stupid, please bear with me. 
I am wondering that when we write code for username/password check in python, if it is not compiled to exe ie script state, won't people will easily open the file and remove the code potion that is doing the password check?
I am assuming that the whole program is entirely written in python, no C or C++.
Even if I use a program like py2exe it can be easily decompiled back to source code. So, does that mean it is useless to do a password check?
How do professional programmers cope with that?
On a server only server administrators should have the right to change the code. Hence, to change the code you have to have administrator access, and if you do, then you can access everything anyway. :-)
The same goes for a client program. If the only security is the password check, you don't need to get around the password check, you can just read the data files directly. 
In both cases, to prevent people that has access to the files from reading those files a password check is not enough. You have to encrypt the data.
Python Password Protection