eval() of lis.py, a tiny lisp simulator in python by Peter Norvig

gistfile1.py

def eval(x, env=global_env):
    "Evaluate an expression in an environment."
    if isa(x, Symbol):             # variable reference
        return env.find(x)[x]
    elif not isa(x, list):         # constant literal
        return x                
    elif x[0] == 'quote':          # (quote exp)
        (_, exp) = x 
        return exp 
    elif x[0] == 'if':             # (if test conseq alt)
        (_, test, conseq, alt) = x 
        return eval((conseq if eval(test, env) else alt), env)
    elif x[0] == 'set!':           # (set! var exp)
        (_, var, exp) = x 
        env.find(var)[var] = eval(exp, env)
    elif x[0] == 'define':         # (define var exp)
        (_, var, exp) = x 
        env[var] = eval(exp, env)
    elif x[0] == 'lambda':         # (lambda (var*) exp)
        (_, vars, exp) = x 
        return lambda *args: eval(exp, Env(vars, args, env))
    elif x[0] == 'begin':          # (begin exp*)
        for exp in x[1:]:
            val = eval(exp, env)
        return val 
    else:                          # (proc exp*)
        exps = [eval(exp, env) for exp in x]
        proc = exps.pop(0)
        return proc(*exps)