Why can t non-default arguments follow default arguments

Question

Why does this piece of code throw a SyntaxError      gt  gt  gt  def fun1 a  who is you   b  True   x  y           print a b x y        File   lt stdin gt    line 1 SyntaxError  non-default argument follows default argument   While the following piece of code runs without visible errors    gt  gt  gt  def fun1 x  y  a  who is you   b  True            print a b x y

User · Answer

Required arguments  the ones without defaults   must be at the start to allow client code to only supply two   If the optional arguments were at the start  it would be confusing   fun1  who is who   3   jack     What would that do in your first example   In the last  x is  who is who   y is 3 and a    jack

User · Answer

SyntaxError  non-default argument follows default argument   If you were to allow this  the default arguments would be rendered useless because you would never be able to use their default values  since the non-default arguments  come after    In Python 3 however  you may do the following   def fun1 a  who is you   b  True      x  y       pass   which makes x and y keyword only so you can do this   fun1 x 2  y 2    This works because there is no longer any ambiguity  Note you still can t do fun1 2  2   that would set the default arguments

User · Answer

Let me clear two points here     firstly non-default argument should not follow default argument   it means you can t define  a b c  in function the order of defining parameter in function are     positional parameter or non-default parameter  i e  a b c  keyword parameter or default parameter         i e  a  b  r  j   keyword-only parameter                         i e   args  var-keyword  parameter                         i e    kwargs     def example a  b  c None  r  w    d      ae     ab     a b  are positional parameter   c none  is optional parameter   r  w   is keyword parameter    d     is list parameter     ae  is keyword-only     ab  is var-keyword parameter   now secondary thing is if i try something like this   def example a  b  c a d b     argument is not defined when default values are saved Python computes and saves default values when you define the function  c and d are not defined  does not exist  when this happens  it exists only when the function is executed     a a b  its not allowed in parameter

User · Answer

All required parameters must be placed before any default arguments  Simply because they are mandatory  whereas default arguments are not  Syntactically  it would be impossible for the interpreter to decide which values match which arguments if mixed modes were allowed  A SyntaxError is raised if the arguments are not given in the correct order   Let us take a look at keyword arguments  using your function   def fun1 a  who is you   b  True   x  y           print a b x y   Suppose its allowed to declare function as above  Then with the above declarations  we can make the following  regular  positional or keyword argument calls   func1  ok a    ok b   1     Is 1 assigned to x or   func1 1                     Is 1 assigned to a or   func1 1  2                      How you will suggest the assignment of variables in the function call  how default arguments are going to be used along with keyword arguments    gt  gt  gt  def fun1 x  y  a  who is you   b  True            print a b x y        Reference O Reilly - Core-Python                                                                Where as this function make use of the default arguments syntactically correct for above function calls  Keyword arguments calling prove useful for being able to provide for out-of-order positional arguments  but  coupled with default arguments  they can also be used to  skip over  missing arguments as well

[python] Why can't non-default arguments follow default arguments?

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