Lazy K interpreter (incomplete)

laz.py

def exprs(name):            # return lambda expression
    if name == 'S':
        return (lambda x: lambda y: lambda z: x(z)(y(z)))
    elif name == 'K':
        return (lambda x: lambda y: x)
    elif name == 'I':
        return (lambda x: x)
    elif name == 'cons':
        return (lambda s: lambda b: lambda f: f(s(b)))
    elif name == 'car':
        return (lambda p: p(lambda x: lambda y: x))
    elif name == 'cdr':
        return (lambda p: p(lambda x: lambda y: y))
    elif name == 'false':
    	return (lambda x: lambda y: y)
    elif name == 'succ':
        return (lambda n: lambda f: lambda x: f(n(f)(x)))
    elif name == 'zero':
        return (lambda f: lambda x: x)

def ev(code):               #eval
    itr = iter(code)
    f = exprs('I')
    char = ''
    while True:
        try:
            char = itr.next()
            if char in 'SKI':
                f = f(exprs(char))
            elif char == '(':
                tmp = ''; x = ''
                while x != ')':
                    tmp += x
                    x = itr.next()
                g = ev(tmp)
                f = f(g)
        except StopIteration:
            break
    return f

def char2church(char):
    n = ord(char)
    church = exprs('zero')
    succ = exprs('succ')
    for i in range(n):
        church = succ(church)

    return church

def church2char(church):
    n = 0
    n = church(lambda x: x+1)(n) # \f.\x.f(f x) -> +1(+1 0)

    return chr(n)


if __name__ == '__main__':
    import sys
    path = sys.argv[1]
    with open(path) as sf:
        code = sf.read()
    f = ev(code) 
    
    cons = exprs('cons')
    car = exprs('car') 
    cdr = exprs('cdr')
    g = exprs('false')
    
    for i in reversed(raw_input()):
        ch = char2church(i)
        g = cons(ch)(g)     # make a list
    
    def _check(f):
        try:
            return f(True)(False)
        except TypeError:
            return True
    
    out = ""
    while _check(f):
        h = car(g)
        g = cdr(g)
        out += church2char(h) 
    print out