This program is nowhere near finished, just warning you!

I'm working on a GUI for g++ because I'm bored. I'm having strange problems with it.

from Tkinter import *
import os
import tkFileDialog

root = Tk()

tn = Label(root, text="gcc C++ Compiler GUI")
tn.grid(row=0, column=1, columnspan=3, sticky=N+S+E+W)

pathn = Label(root, text="Path to source:")
pathn.grid(row=1, column=1, sticky=N+S+E+W)

namen = Label(root, text="Name of compiled file:")
namen.grid(row=2, column=1, sticky=N+S+E+W)

path = Entry(root)
path.grid(row=1, column=2, columnspan=2, sticky=N+S+E+W)
path.delete(0, END)
path.insert(0, "")

name = Entry(root)
name.grid(row=2, column=2, columnspan=3, sticky=N+S+E+W)
name.delete(0, END)
name.insert(0, "")

quitbtn = Button(root, text="Exit", fg="red", command=exit)
quitbtn.grid(row=3, column=3, sticky=N+S+E+W)

compilebtn = Button(root, text="Compile!", fg="blue", command=quit)
compilebtn.grid(row=3, column=2, sticky=N+S+E+W)

spyro = Label(root, text="By Spyro543")
spyro.grid(row=3, column=1, sticky=N+S+E+W)

def fopen():
    cformats = [('C++ source file','*.cpp'),('Plain text file','*.txt'),('Any File','*.*')]
    filenm = tkFileDialog.askopenfile(parent=root,filetypes=cformats ,title='Select the cpp source file')
    path.insert(0, filenm)

browsebtn = Button(root, text="Browse", fg="black", command=fopen())
browsebtn.grid(row=1, column=4, sticky=N+S+E+W)

root.mainloop()

I'm having two problems: one is the definde function fopen() is running immediately instead of when I call it (when the button is pressed). Any way to fix this?
def fopen():
    cformats = [('C++ source file','*.cpp'),('Plain text file','*.txt'),('Any File','*.*')]
    filenm = tkFileDialog.askopenfile(parent=root,filetypes=cformats ,title='Select the cpp source file')
    path.insert(0, filenm)

My 2nd problem is that after fopen() runs (when it's not supposed to ) all of the widgets freeze and don't do anything. I can't activate the text boxes and none of the buttons do anything.

This is a typical bug when Tkinter programming.
You use lambda functions.
Change the line from:
browsebtn = Button(root, text="Browse", fg="black", command=fopen())

to
browsebtn = Button(root, text="Browse", fg="black", command=lambda:fopen())


Yes, that works for void functions.
The advantage with lambda is that it allows you to pass arguments
should Spyro543 decide later to do so on fopen:

browsebtn = Button(root, text="Browse", fg="black", command=(lambda: fopen(arg1,arg2,...)))

Change the line in sub fopen to read:

path.insert(0, filenm.name)

instead of:

path.insert(0, filenm)