Question

Does * have a special meaning in Python as it does in C? I saw a function like this in the Python Cookbook:

def get(self, *a, **kw) 

Would you please explain it to me or point out where I can find an answer (Google interprets the * as wild card character and thus I cannot find a satisfactory answer).

Answer 1

See Function Definitions in the Language Reference.

If the form *identifier is present, it is initialized to a tuple receiving any excess positional parameters, defaulting to the empty tuple. If the form **identifier is present, it is initialized to a new dictionary receiving any excess keyword arguments, defaulting to a new empty dictionary.

Also, see Function Calls.

Assuming that one knows what positional and keyword arguments are, here are some examples:

Example 1:

# Excess keyword argument (python 3) example: def foo(a, b, c, **args):     print("a = %s" % (a,))     print("b = %s" % (b,))     print("c = %s" % (c,))     print(args)      foo(a="testa", d="excess", c="testc", b="testb", k="another_excess") 

As you can see in the above example, we only have parameters a, b, c in the signature of the foo function. Since d and k are not present, they are put into the args dictionary. The output of the program is:

a = testa b = testb c = testc {'k': 'another_excess', 'd': 'excess'} 

Example 2:

# Excess positional argument (python 3) example: def foo(a, b, c, *args):     print("a = %s" % (a,))     print("b = %s" % (b,))     print("c = %s" % (c,))     print(args)          foo("testa", "testb", "testc", "excess", "another_excess") 

Here, since we’re testing positional arguments, the excess ones have to be on the end, and *args packs them into a tuple, so the output of this program is:

a = testa b = testb c = testc ('excess', 'another_excess') 

You can also unpack a dictionary or a tuple into arguments of a function:

def foo(a,b,c,**args):     print("a=%s" % (a,))     print("b=%s" % (b,))     print("c=%s" % (c,))     print("args=%s" % (args,))  argdict = dict(a="testa", b="testb", c="testc", excessarg="string") foo(**argdict) 

Prints:

a=testa b=testb c=testc args={'excessarg': 'string'} 

And

def foo(a,b,c,*args):     print("a=%s" % (a,))     print("b=%s" % (b,))     print("c=%s" % (c,))     print("args=%s" % (args,))  argtuple = ("testa","testb","testc","excess") foo(*argtuple) 

Prints:

a=testa b=testb c=testc args=('excess',)