[xyzzyz]

I don't buy this explanation.

Actually, this is not a design flaw, and it is not because of internals, or performance. It comes simply from the fact that functions in Python are first-class objects, and not only a piece of code.

Why in Common Lisp defaults behave the way one would expect, then? Functions are also first class, but defaults are evaluated at every call.

[sophacles]

I don't know CL, but in python function definitions can be executed multiple times... at the top level this happens at module import, so it ends up being only once. But in nested definitions, e.g.

   def foo():
      def bar():
         ...
      return bar
   foo() == foo() # false

Two different function objects are created. If, in the above examle, bar took a pram thelist=[], each call to foo would produce a bar function with a different list instance for thelist. The default values can be read as expressions passed to the function object constructor, rather than a bit of code to be evaluated each function run.

I don't know how CL works in this regard, nor do I know which is better or worse. I think the explanation linked did a terrible job conflating first class functions with execution and runtime models. Some of the answers below it explain better tho. :)

[xyzzyz]

The fact that definition can be evaluated many times is not really relevant here. What is important is how one specifies language's semantic -- for instance, Common Lisp: The Language, 2nd edition book (I don't own ANSI standard) says:

When the function represented by the lambda expression is applied to arguments, the arguments and parameters are processed in order from left to right. (...) If optional parameters are specified, then each one is processed as follows. If any unprocessed arguments remain, then the parameter variable var is bound to the next remaining arguments, just as for required parameter. If no arguments remain, however, then the initform part of the parameter specifier is evaluated, and the parameter variable is bound to the resulting value (...).

The CLTL2 specifies that the form representing the default value of optional parameter shall be evaluated every time the parameter is not provided.