A one-liner that converts any Python script to a one-liner. There are no literal semicolons in this script or its output (i.e. it doesn't cheat).

theultimateoneliner.py

print'exec '+(lambda L,R:lambda C:R(L,R,C))((lambda L,R,c:'type((lambda:0).__code__)(%s)'%','.join(R(L,R,getattr(c,'co_'+n))for n in'argcount:nlocals:stacksize:flags:code:consts:names:varnames:filename:name:firstlineno:lnotab:freevars:cellvars'.split(':'))),(lambda L,R,o:L(L,R,o)if type(o).__name__=='code'else'('+''.join(R(L,R,n)+','for n in o)+')'if type(o).__name__=='tuple'else repr(o)))((lambda n:compile(open(n,'r').read(),n,'exec'))(__import__('sys').argv[-1]))

theultimateoneliner.readable.py

# The outermost lambda receives two lambdas, L and R
# Because we're doing this all in one line, L and R must be passed
# as arguments to each other for all recursive calls
print 'exec ' + (lambda L, R:
    # The inner lambda receives a code object...
    lambda C:
        # ...and passes it to R
        R(L, R, C)
    )(
        # lambda L(c): converts c to a Python statement that constructs it
        (lambda L, R, c:
            # type((lambda:0).__code__) returns <type 'CodeType'> which is otherwise inaccessible
            'type((lambda:0).__code__)(%s)' % ','.join(
                R(L, R, getattr(c, 'co_' + n)) for n in
                    # CodeType requires a lot of arguments; only the last two are optional
                    ('argcount:nlocals:stacksize:flags:code:' +
                    'consts:names:varnames:filename:name:' +
                    'firstlineno:lnotab:freevars:cellvars').split(':')
            )
        ),
        # lambda R(o): repr(o), unless o is code, in which case L(o)
        (lambda L, R, o:
            L(L, R, o) if type(o).__name__ == 'code'
            else '(' + ''.join(R(L, R, n) + ',' for n in o) + ')' if type(o).__name__ == 'tuple'
            else repr(o)
        )
    # This is the code object sent to the inner lambda
    )((lambda n:
        compile(open(n, 'r').read(), n, 'exec')
    # Get program name from the last argument
    )(__import__('sys').argv[-1]))