Python: 頑固的 variable length argument

這邊卡了很久，做個筆記...

可變長度的參數傳遞

def p(fmt, *arg):
    print fmt % arg

# 傳多個參數
s1 = "hello"
s2 = "world"
p("%s %s", s1, s2)

# 把 list 的 element 當作參數
l = ["Allen", "Billy", "Elmer"]
p("%s %s %s", *tuple(l))

# 混合模式
l = ["Allen", "Billy", "Elmer"]
s = "hello"
p("%s, %s %s %s", *((s, ) + tuple(l)))

Output:

hello world
Allen Billy Elmer
hello, Allen Billy Elmer

Wrapper function 的寫法

def func(name, tel, city=None):
    print "name=%s" % name
    print "tel=%d" % tel
    print "city=%s" % city

def wrapper(*args, **kwargs):
    print args
    print kw
    func(*args, **kwargs)
    
wrapper("benson", 3345, city="Taipei")

Output:

('benson', 3345)
{'city': 'Taipei'}
name=benson
tel=3345
city=Taipei

由此可見，arg 是 tuple，kwargs 是 dict 型態
這邊沒寫好可能會出現 error

only named arguments may follow *expression

*args 與 args 的差別

def func(x, y, z):
    print x+y+z

def wrapper1(*args): # with star
    func(*args)

def wrapper2(args): # without star
    func(*args)

wrapper1(1, 2, 3)
wrapper2([1, 2, 3])

Output:

6
6

參考資料
Python, tuple arguments playing nicely with others - Stack Overflow