What does the asterisk do in Python other than multiplication and exponentiation?
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在用python编写的peter norvig的lisp解释器(http://norvig.com/lispy.html)中，他将lisp的
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 | 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) isa = isinstance Symbol = str |
我特别感兴趣的是这一行：
1 | return proc(*exps) |
号
它解包参数
1 | function(1,2,3) == function(*[1,2,3]) |
它使得使用变量传递函数变得容易
1 2 | args = [1,2,3] func(*args) #much nicer than func(args[0],args[1],args[2],...) |
号
在seqable对象解包它之前输入一个星号，如joran所示：
1 2 3 4 5 6 7 | In [1]: def f(*args): return args In [2]: f(1,2,3) Out[2]: (1, 2, 3) In [3]: f(*[1,2,3,4]) Out[3]: (1, 2, 3, 4) |
(注：对
同样值得注意的是，一个双星(
1 2 3 4 5 6 7 8 9 10 | In [5]: def g(foo=None, bar=42): return foo,bar In [6]: g() Out[6]: (None, 42) In [7]: g(*[1,2]) Out[7]: (1, 2) In [8]: g(**{'foo': 'FOO', 'bar': 'BAR'}) Out[8]: ('FOO', 'BAR') |
。