Is there a way to pass optional parameters to a function

Question

Is there a way in Python to pass optional parameters to a function while calling it and in the function definition have some code based on  only if the optional parameter is passed

User · Answer

You can specify a default value for the optional argument with something that would never passed to the function and check it with the is operator:

class _NO_DEFAULT:
    def __repr__(self):return "<no default>"
_NO_DEFAULT = _NO_DEFAULT()

def func(optional= _NO_DEFAULT):
    if optional is _NO_DEFAULT:
        print("the optional argument was not passed")
    else:
        print("the optional argument was:",optional)

then as long as you do not do func(_NO_DEFAULT) you can be accurately detect whether the argument was passed or not, and unlike the accepted answer you don't have to worry about side effects of ** notation:

# these two work the same as using **
func()
func(optional=1)

# the optional argument can be positional or keyword unlike using **
func(1) 

#this correctly raises an error where as it would need to be explicitly checked when using **
func(invalid_arg=7)

User · Answer

def my func mandatory arg  optional arg 100       print mandatory arg  optional arg    http   docs python org 2 tutorial controlflow html default-argument-values  I find this more readable than using   kwargs   To determine if an argument was passed at all  I use a custom utility object as the default value   MISSING   object    def func arg MISSING       if arg is MISSING

User · Answer

The Python 2 documentation  7 6  Function definitions gives you a couple of ways to detect whether a caller supplied an optional parameter   First  you can use special formal parameter syntax    If the function definition has a formal parameter preceded by a single    then Python populates that parameter with any positional parameters that aren t matched by preceding formal parameters  as a tuple   If the function definition has a formal parameter preceded by     then Python populates that parameter with any keyword parameters that aren t matched by preceding formal parameters  as a dict   The function s implementation can check the contents of these parameters for any  optional parameters  of the sort you want   For instance  here s a function opt fun which takes two positional parameters x1 and x2  and looks for another keyword parameter named  optional      gt  gt  gt  def opt fun x1  x2   positional parameters    keyword parameters           if   optional  in keyword parameters               print  optional parameter found  it is    keyword parameters  optional           else              print  no optional parameter  sorry        gt  gt  gt  opt fun 1  2  no optional parameter  sorry  gt  gt  gt  opt fun 1 2  optional  yes   optional parameter found  it is  yes  gt  gt  gt  opt fun 1 2  another  yes   no optional parameter  sorry   Second  you can supply a default parameter value of some value like None which a caller would never use  If the parameter has this default value  you know the caller did not specify the parameter  If the parameter has a non-default value  you know it came from the caller

User · Answer

If you want give some default value to a parameter assign value in     like  x  10   But important is first should compulsory argument then default value      eg     y  x  10      but    x 10  y  is wrong

User · Answer

def op a 4 b 6       add   a b     print add  i op    o p  will be  4 6  10  ii op 99   o p  will be  99 6  105  iii op 1 1   o p  will be  1 1  2  Note   If none or one parameter is passed the default passed parameter will be considered for the function

[python] Is there a way to pass optional parameters to a function?

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