def adjust_to_correct_appdir():
     import os, sys
     try:
         appdir = sys.argv[0] #feel free to use __file__
         if not appdir:
             raise ValueError
         appdir = os.path.abspath(os.path.dirname(sys.argv[0]))
         os.chdir(appdir)
         if not appdir in sys.path:
             sys.path.insert(0,appdir)
     except:
         #placeholder for feedback, adjust to your app.
         #remember to use only python and python standart libraries
         #not any resource or module into the appdir
         #a window in Tkinter can be adequate for apps without console
         #a simple print with a timeout can be enough for console apps
         print 'Please run from an OS console.'
         import time
         time.sleep(10)
         sys.exit(1)
adjust_to_correct_appdir()

Add the code in any *.py thats directly runnable (ex. game.py , leveleditor.py ). It must be added near the begining of file, before importing any module living under the appdir, before opening any file with a filename relative to the appdir and before any pickle-unpickle operation.

Your app is started not from the appdir. As a result:

1. import cannot solve imports relative to the appdir
2. file operation with filenames relatives to appdir will fail ( like f=file('data/x.png'))
3. pickle-unpickle can fail by being unable to resolve class definitions
4. traceback error messages can be ambiguous

This can happen due to:

1. the app is started from a python interpreter console wich is not in the appdir
2. the app is started from a OS console wich is not in the appdir
3. In windows, the app is started by doubleclick in a shortcut (that not have properly set the 'start in' field)
4. the app is started from a launchbar, and the start directory has not been specified
5. others?

The second problem can only be solved if at the very beginning of the app the working directory is changed to appdir (os.chdir(appdir)).
Also, if we know the appdir then we can add to the module search paths (sys.path.insert(0,appdir)), solving the problem 1. and, (I think, confirm please) the 3.
Python doenst have functionality to directly query: what is the appdir for app 'myapp'?, but lets seek a portable solution. An old version of this recipe tells:

appdir = os.path.abspath(os.path.dirname(sys.argv[0]))

warning that sys.argv[0] its usualy the command line used to start the app, but not guaranted. By example, if the entry point for the app lies in main.py, and someone start the app from the python interpreter console:

>>>execfile(r'\appedir\main.py') #in linux-mac '/appdir/main.py' 

then sys.argv[0] would be the empty string, at least in windows A most recent recipe ,the one that Im replacing, gets appdir as

appdir = os.path.abspath(os.path.dirname(__file__))

wich in the python console case will fail with NameError: __file__ is not defined

If you cannot resolve appdir, then the better is to give feedback to the user (print, a tkinter window, any that only uses python and standart libraries) and terminate the app.
If you let execution go ahead, expect a lot of bug reports, with ambiguous interpretation.
If you really want to support starting from the python interpreter console, then you can :

... #in branch where sys.argv[0] or __file__ tells nothing try:     appdir=os.getcwd()     f=file(os.path.join(appdir,'signature'),'rb')     sig=file.read()     f.close()     if sig!='this is the myapp signature file - dont edit':        # started from the interpreter but not in the appdir.        raise     # medium confirmation that the cwd is really the appdir except:     #warn the user and quit 

Personally, I woldnt bother.

if 1 <= x <= 2:

if 1 <= x <= 2: