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',)

import os
from datetime import date
def program():
    Year = input("year of birth:" )
    Month = input("month of birth:" )
    Day = input("day of birth:" )
    Date_of_Birth = (Day + "/" + Month + "/" + Year)
    print('Your Date of Birth is ' + Date_of_Birth)
    d = date.today()
    y = d.year
    os.system("cls")
    age = y - int(Year)
    print('Your age is ' + str(age))

    if ((int(Month)==12 and int(Day) >= 22)or(int(Month)==1 and int(Day)<= 19)):
        Signo_Zodiacal = ("\n Capricorn")
    elif ((int(Month)==1 and int(Day) >= 20)or(int(Month)==2 and int(Day)<= 17)):
            zodiac_sign = ("\n aquarium")
    elif ((int(Month)==2 and int(Day) >= 18)or(int(Month)==3 and int(Day)<= 19)):
            zodiac_sign = ("\n Pices")
    elif ((int(Month)==3 and int(Day) >= 20)or(int(Month)==4 and int(Day)<= 19)):
            zodiac_sign = ("\n Aries")
    elif ((int(Month)==4 and int(Day) >= 20)or(int(Month)==5 and int(Day)<= 20)):
            zodiac_sign = ("\n Taurus")
    elif ((int(Month)==5 and int(Day) >= 21)or(int(Month)==6 and int(Day)<= 20)):
            zodiac_sign = ("\n Gemini")
    elif ((int(Month)==6 and int(Day) >= 21)or(int(Month)==7 and int(Day)<= 22)):
            zodiac_sign = ("\n Cancer")
    elif ((int(Month)==7 and int(Day) >= 23)or(int(Month)==8 and int(Day)<= 22)): 
            zodiac_sign = ("\n Leo")
    elif ((int(Month)==8 and int(Day) >= 23)or(int(Month)==9 and int(Day)<= 22)): 
            Signo_Zodiacal = ("\n Virgo")
    elif ((int(Month)==9 and int(Day) >= 23)or(int(Month)==10 and int(Day)<= 22)):
            zodiac_sign = ("\n Libra")
    elif ((int(Month)==10 and int(Day) >= 23)or(int(Month)==11 and int(Day)<= 21)): 
            zodiac_sign = ("\n Scorpio")
    elif ((int(Month)==11 and int(Day) >= 22)or(int(Month)==12 and int(Day)<= 21)):
            zodiac_sign = ("\n Sagittarius")

    print(zodiac_sign)

program()

fixed by me

The *args and **kwargs are common idioms to allow an arbitrary number of arguments to functions, as described in the section more on defining functions in the Python tutorial.

The *args will give you all positional arguments as a tuple:

def foo(*args):
    for a in args:
        print(a)        

foo(1)
# 1

foo(1, 2, 3)
# 1
# 2
# 3

The **kwargs will give you all keyword arguments as a dictionary:

def bar(**kwargs):
    for a in kwargs:
        print(a, kwargs[a])  

bar(name='one', age=27)
# name one
# age 27

Both idioms can be mixed with normal arguments to allow a set of fixed and some variable arguments:

def foo(kind, *args, bar=None, **kwargs):
    print(kind, args, bar, kwargs)

foo(123, 'a', 'b', apple='red')
# 123 ('a', 'b') None {'apple': 'red'}

It is also possible to use this the other way around:

def foo(a, b, c):
    print(a, b, c)

obj = {'b':10, 'c':'lee'}

foo(100, **obj)
# 100 10 lee

Another usage of the *l idiom is to unpack argument lists when calling a function.

def foo(bar, lee):
    print(bar, lee)

baz = [1, 2]

foo(*baz)
# 1 2

In Python 3 it is possible to use *l on the left side of an assignment (Extended Iterable Unpacking), though it gives a list instead of a tuple in this context:

first, *rest = [1, 2, 3, 4]
# first = 1
# rest = [2, 3, 4]

Also Python 3 adds a new semantic (refer PEP 3102):

def func(arg1, arg2, arg3, *, kwarg1, kwarg2):
    pass

Such function accepts only 3 positional arguments, and everything after * can only be passed as keyword arguments.

Note:

A Python dict, semantically used for keyword argument passing, is arbitrarily ordered. However, in Python 3.6+, keyword arguments are guaranteed to remember insertion order. "The order of elements in **kwargs now corresponds to the order in which keyword arguments were passed to the function." - What’s New In Python 3.6. In fact, all dicts in CPython 3.6 will remember insertion order as an implementation detail, and this becomes standard in Python 3.7.

I've done this before. The simplest solution that I ended up with was an array of the following key/values:

120:Cap, 218:Aqu, 320:Pis, 420:Ari, 521:Tau,
621:Gem, 722:Can, 823:Leo, 923:Vir, 1023:Lib
1122:Sco, 1222:Sag, 1231: Cap

Then you write the birth date in the mdd format, ie, month number (starting with 1 for January) and two digit day number (01-31). Iterate through the array, and if the date is less than or equal to an item in the array, you have your star sign.

EDIT I needed this so here's this concept as a working function

zodiacs = [(120, 'Cap'), (218, 'Aqu'), (320, 'Pis'), (420, 'Ari'), (521, 'Tau'),
           (621, 'Gem'), (722, 'Can'), (823, 'Leo'), (923, 'Vir'), (1023, 'Lib'),
           (1122, 'Sco'), (1222, 'Sag'), (1231, 'Cap')]
def get_zodiac_of_date(date):
    date_number = int("".join((str(date.date().month), '%02d' % date.date().day)))
    for z in zodiacs:
        if date_number <= z[0]:
            return z[1]

A single star * unpacks a sequence or collection into positional arguments. Suppose we have

def add(a, b):
    return a + b

values = (1, 2)

Using the * unpacking operator, we can write s = add(*values), which will be equivalent to writing s = add(1, 2).

The double star ** does the same thing for a dictionary, providing values for named arguments:

values = {'a': 1, 'b': 2}
s = add(**values) # equivalent to add(a=1, b=2)

Both operators can be used for the same function call. For example, given:

def sum(a, b, c, d):
    return a + b + c + d

values1 = (1, 2)
values2 = {'c': 10, 'd': 15}

then s = add(*values1, **values2) is equivalent to s = sum(1, 2, c=10, d=15).

See also the relevant section of the tutorial in the Python documentation.

Similarly, * and ** can be used for parameters. Using * allows a function to accept any number of positional arguments, which will be collected into a single parameter:

def add(*values):
    s = 0
    for v in values:
        s = s + v
    return s

Now when the function is called like s = add(1, 2, 3, 4, 5), values will be the tuple (1, 2, 3, 4, 5) (which, of course, produces the result 15).

Similarly, a parameter marked with ** will receive a dict:

def get_a(**values):
    return values['a']

s = get_a(a=1, b=2)      # returns 1

this allows for specifying a large number of optional parameters without having to declare them.

Again, both can be combined:

def add(*values, **options):
    s = 0
    for i in values:
        s = s + i
    if "neg" in options:
        if options["neg"]:
            s = -s
    return s

s = add(1, 2, 3, 4, 5)            # returns 15
s = add(1, 2, 3, 4, 5, neg=True)  # returns -15
s = add(1, 2, 3, 4, 5, neg=False) # returns 15

It's called unpacking operator. Here is what's written in the docs:

An asterisk * denotes iterable unpacking. Its operand must be an iterable. The iterable is expanded into a sequence of items, which are included in the new tuple, list, or set, at the site of the unpacking.

It is much like the "..." operator from Javascript ES6. (the spread operator)

https://docs.python.org/3/reference/expressions.html#expression-lists