Follow the turtle up master to learn Python - function (3)

Collection function ： Parameter the user likes to pass several parameters of the feature , Definition ： Add an asterisk to the name of the parameter * Can be .

>>> def myfunc(*args):
    print(" Yes {} Parameters .".format(len(args)))
    print(" The second parameter is ：{}.".format(args[1]))

    
>>> myfunc(' Little turtle ', ' No two in yes ')
Yes 2 Parameters .
The second parameter is ： No two in yes .
>>> myfunc(1, 2, 3, 4, 5)
Yes 5 Parameters .
The second parameter is ：2.
>>> def myfunc(*args):
    print(args)

    
>>> myfunc(1, 2, 3, 4, 5)
(1, 2, 3, 4, 5)                          // The essence is tuple , Tuples have the ability to package and unpack
>>> def myfunc():
    return 1, 2, 3                      // Return multiple values

>>> myfunc()
(1, 2, 3)
>>> x, y, z = myfunc()           // Unpack
>>> x
1
>>> y
2
>>> z
3

Function parameters are passed in the same way as tuples , adopt * Packaging operation can be realized , Pack multiple parameters into a tuple

>>> def myfunc(*args):
    print(type(args))

    
>>> myfunc(1, 2, 3, 4)
<class 'tuple'>

notes ： There are other parameters after the collected parameters , This parameter can only be specified with keyword parameters .

>>> def myfunc(*args, a, b):
    print(args, a, b)

    
>>> myfunc(1, 2, 3, 4, 5)
Traceback (most recent call last):
  File "<pyshell#23>", line 1, in <module>
    myfunc(1, 2, 3, 4, 5)
TypeError: myfunc() missing 2 required keyword-only arguments: 'a' and 'b'
>>> myfunc(1, 2, 3, a=4, b=5)
(1, 2, 3) 4 5

The asterisk is actually an anonymous collection parameter

>>> def abc(a, *, b, c):
    print(a, b, c)

    
>>> abc(1, 2, 3)
Traceback (most recent call last):
  File "<pyshell#27>", line 1, in <module>
    abc(1, 2, 3)
TypeError: abc() takes 1 positional argument but 3 were given
>>> abc(1, b=2, c=3)
1 2 3

Collecting parameters can pack multiple parameters into tuples , You can also package parameters as dictionaries , It can be packaged into a dictionary by two asterisks .

>>> def myfunc(**kwargs):
    print(kwargs)

    
>>> myfunc(a=1, b=2, c=3)          // Dictionary elements are key value pairs , The keyword has values on both sides , And there is an equal sign
{'a': 1, 'b': 2, 'c': 3}

Mix it up , There are both tuples and dictionaries

>>> def myfunc(a, *b, **c):
    print(a, b, c)

    
>>> myfunc(1,2,3,4,x=5,y=6)
1 (2, 3, 4) {'x': 5, 'y': 6}

The functions that have both dictionaries and tuples are format（）

>>> help(str.format)
Help on method_descriptor:

format(...)
    S.format(*args, **kwargs) -> str
    
    Return a formatted version of S, using substitutions from args and kwargs.
    The substitutions are identified by braces ('{' and '}').

Unpack parameters ： Using asterisks on formal parameters is called parameter packing , Use... On arguments , Then unpack .

>>> args = (1, 2, 3, 4)
>>> def myfunc(a, b, c, d):
    print(a, b, c, d)

    
>>> myfunc(args)
Traceback (most recent call last):
  File "<pyshell#42>", line 1, in <module>
    myfunc(args)
TypeError: myfunc() missing 3 required positional arguments: 'b', 'c', and 'd'
>>> myfunc(*args)
1 2 3 4
>>> kwargs = {'a':1, 'b':2, 'c':3, 'd':4}
>>> myfunc(kwargs)
Traceback (most recent call last):
  File "<pyshell#48>", line 1, in <module>
    myfunc(kwargs)
TypeError: myfunc() missing 3 required positional arguments: 'b', 'c', and 'd'
>>> myfunc(**kwargs)
1 2 3 4