A brief discussion on parameter passing of functions in Python

1. Ordinary parameter passing

>>> def add(a,b): 
  return a+b 
 
>>> print add(1,2) 
3
>>> print add('abc','123') 
abc123

2. The number of parameters is optional, and the parameters are passed with default values ​​

>>> def myjoin(string,sep='_'): 
  return sep.join(string) 
 
>>> myjoin('Test') 
'T_e_s_t'
>>> myjoin('Test',';') 
'T;e;s;t' 
?
>>> def myrange(start=0,stop,step=1): 
  print stop,start,step 
   
SyntaxError: non-default argument follows default argument

The default value of parameter sep is '_' If this parameter is not given a value, the default value will be used. If it is given, the given value will be used

It should be noted that if a parameter is an optional parameter, then all the parameters following it should be optional. In addition, If the order of the optional parameters is reversed, the corresponding parameters can still be correctly assigned, but the variable name and value must be clearly specified

3. Number of variable parameters

>>> def printf(fmt,*arg): 
  print fmt%arg 
 
   
>>> printf ('%d is larger than %d',2,1) 
2 is larger than 1

The *arg in the function must be the last parameter, * means any number of parameters, *arg will put all the parameters except the previous ones into one The tuple is passed to the function and can be accessed through arg in the function

arg is a tuple, and arg can be accessed in the function through the method of accessing tuple

Another way to pass any number Parameters are passed through dictionary and can also accept multiple parameters, but each parameter needs to specify the name correspondence, such as a=1, b=2, c=3

>>> def printf(format,**keyword): 
  for k in keyword.keys(): 
    print "keyword[%s] %s %s"%(k,format,keyword[k]) 
 
     
>>> printf('is',one=1,tow=2,three=3) 
keyword[three] is 3
keyword[tow] is 2
keyword[one] is 1

These methods can be mixed together but the order must be paid attention to. The function will first accept fixed parameters, then optional parameters, then arbitrary parameters (tuple), then dictionary arbitrary parameters (dict)