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From: Lawrence D'Oliveiro <ldo@nz.invalid>
Newsgroups: comp.lang.lisp
Subject: Re: History of lexical scope in Lisp
Date: Sat, 16 Mar 2024 07:27:57 -0000 (UTC)
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On Sat, 16 Mar 2024 00:46:21 -0600, Jeff Barnett wrote:

> First off, I believe that many if not most exception-related primitives
> expand in terms of dynamic variables.

Consider the following Python example:

    class MyException1(Exception) :
        pass
    #end MyException1

    class MyException2(Exception) :
        pass
    #end MyException2

    def func1() :
        raise MyException1
    #end func1

    def func2() :
        raise MyException2
    #end func2

    def func3() :
        class MyException1(Exception) :
            pass
        #end MyException1

        try :
            func1()
        except MyException1 :
            # will never get here
            print("caught MyException1 in func3")
        #end try
    #end func3

    def func4() :
        try :
            func2()
        except MyException2 :
            print("caught MyException2 in func4")
        #end try
    #end func4

    for f in (func1, func2, func3, func4) :
        try :
            print("* call %s" % f.__name__)
            f()
        except MyException1 :
            print("caught MyException1 at top level")
        except MyException2 :
            print("caught MyException2 at top level")
        #end try
    #end for

Here is the output it produces:

    * call func1
    caught MyException1 at top level
    * call func2
    caught MyException2 at top level
    * call func3
    caught MyException1 at top level
    * call func4
    caught MyException2 in func4

func4 shows how the search for a handler is based on dynamic execution
nesting. func3 shows how exception matching is based on lexical
scoping.