questions about scope/threading

[Dennis Lee Bieber]

eugene kim fed this fish to the penguins on Wednesday 16 October 2002 
11:22 pm:

> The scheduling of threads and thread switching is tightly controlled
> by a global interpreter lock that allows only a single thread of
> execution to be running in the interpreter at once.
> : what does this mean? i thought threads are supposed to be running in
> parrellel.
>
        Unless you have a multiple processor machine, nothing ever runs in 
parallel.

        Instead, the system rapidly switches between different programs, 
letting them have a certain amount of time before they are suspended 
and another program runs.
 
> Thread switching can only occur between the execution of individual
> bytecodes in the interpreter.
> : what is 'individual bytecodes' ? is one python statement supposed to
> : have
> many bytecodes?(just like c & assembly)
>
        C (unless you are talking about M$ .NET version) and assembly generate 
direct native hardware instructions. Python's interpreter first 
"compiles" to byte-codes for a virtual machine (same thing Java, UCSD 
Pascal, and Alcor Pascal) do. THEN a virtual machine emulator runs 
using the byte-codes as instructions.

        The Python interpreter/virtual machine swaps among threads at some 
defined number of byte-code instructions.

>  Threads share global variables.
> The trick with Python is that ALL objects are truly global
> : ?? i 'm really confused, what is refered as global? i thought 'var1'
> : is
> only global here on below example.
> <file start>
> var1 = 1
> 
> class Class1:
>    self.var2 = 2
>    var3 = 3
> 
> class Class2(threading.Thread)
>    self.var4 = 4
>    var5 = 5
>    def method1(self):
>        var6 = 6
> <file end>
>
        Well, Class1 and Class2 are also global symbols (though where your 
Classx are getting "self" from I don't know... You don't have any 
initialization function with self as the first argument).

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