lisp - What does the asterisk do in Python other than multiplication and exponentiation? -
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- proper name python * operator? 7 answers
in peter norvig's lisp interpreter written in python (http://norvig.com/lispy.html), defines lisp's eval follows:
def eval(x, env=global_env): "evaluate expression in 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*) exp in x[1:]: val = eval(exp, env) return val else: # (proc exp*) exps = [eval(exp, env) exp in x] proc = exps.pop(0) return proc(*exps) isa = isinstance symbol = str this line in particular interests me:
return proc(*exps) what asterisk in of exps doing exactly?
single asterisk in before seqable object unpacks it, joran showed:
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) (note third application *: in function definition asterisk indicates variable length list of arguments, being packed 1 list, args, in in [1])
also worth noting double asterisk (**) dictionary unpacking:
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')
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