Packaging and unpacking of * and * * in Python

The * and * * In Python allow the function to support any number of arguments, which have different purposes in function definition and invocation.

I. Packing parameters* Function: In the function definition, collect all positional parameters into a new tuple and assign the tuple to the variable args

def F (*args):    print(args)    >>> F () ()>>> F (1 ) (1,)>>> F (1, 2, 3, 4) (1, 2, 3, 4)

* * Role: In the function definition, the Collection keyword argument is passed to a dictionary and the dictionary is assigned to the variable Kwargs

def F (* *Kwargs)    :print(Kwargs    )>>> f () {}>> > f (a=1, b=2) {'a'b': 2}

two. Unpacking Parameters* Function: * Ability to unpack tuples or lists into different parameters in a functional call

def func (A, B, C, D):     Print (A, B, C, D)     >>> args = (1, 2, 3, 4)>>> func (*args)1 2 3 4>>> args = [1, 2, 3, 4] >>> func (*args)1 2 3 4

* * Function: In a function call, * * will unpack a dictionary as a key/value, making it a separate keyword parameter

def func (A, B, C, D):     Print (A, B, C, D)     >>> Kwargs = {"a""b""C " " " D ": 4}>>> func (* *Kwargs)1 2 3 4

three. Note1. When the function is defined, * means packing, inside the function body, * still means unpacking (print (*args) is actually called the print function)

>>>defFoo (*args, * *Kwargs):Print(args)#Unpacked Parameters    Print(*args)#Unpacking Parameters>>> v = (1, 2, 4)>>> d = {'a': 1,'b': 12}>>>foo (V, D) ((1, 2, 4), {'a': 1,'b': 12})(1, 2, 4) {'a': 1,'b': 12}

2. Packaging and unpacking can not be separated from the function of the existence

On the surface, there is no function, in fact there is a function call of format

>>> C = {"name"'Zhang'"Age" : 2}>>> * *csyntaxerror:invalid syntax"name:{name}, Age:{age}  ". Format (* *c)'name:zhang, age:2'

Definition of the Format function in the reference source

But this dictionary unpacking cannot be output with the print function

Print (* *c) Traceback (most recent):   " <pyshell#40> "  in <module>    print(**"Age"is   for this function

Because this is the way the dictionary is solved:

**c = name='Zhang', age=2

The print function supports only *args and does not support **kwargs

3. Application in DDT

@ddt. Data (*all_casedatas), data is a function that invokes a function when the parameter *all_casedatas automatically switches the argument list [{}, {}, {} ...] Unpack {},{},{} ..., in the DEF data (*values) function, * automatically packages each positional parameter into a new tuple ({}, {}, {} ...), then @ddt.data can get each piece of data as a test case

@ddt. Data (*All_casedatas)deftest_my_request (Self, case_data):GlobalGlobal_varifLen (global_var)! = 0 andcase_data["Request_data"] is  notNone: forKey, ValueinchGlobal_var.items ():ifcase_data["Request_data"].find (key)! =-1: case_data["Request_data"] = case_data["Request_data"].replace (key, value)

Four. Practice

Please write out the results of the following code

defF (str1, *args, * *Kwargs):Print(str1, args, Kwargs) L= [1, 2, 3]t= [4, 5, 6]d= {"a": 7,"b": 8,"C": 9}f (1, 2) F (1, 2, 3,"python") F ("python", A=1, b=2, c=3)Print("================") F ("python", L, D) F ("python", *t) F ("python", *l, * *d) F ("python", q="winning", **d) Operation Result:1 (2,) {}1 (2, 3,'python') {}python () {'a': 1,'b': 2,'C': 3}================python ([1, 2, 3], {'a': 7,'b': 8,'C'8 {}) {}python (4, 5, 6) {}python (1, 2, 3) {'a': 7,'b': 8,'C'8 {}python () {'a': 7,'b': 8,'Q':'winning','C': 9}

It is important to note that f ("Python", *t) is to unpack the list T first into 4, 5, 6, and then in def f (str1, *args, **kwargs): RePack 4, 5, 6 into a new tuple (4, 5, 6) and pass it to the variable args

Packaging and unpacking of * and * * in Python