Python for philosophers

[Chris Angelico]

On Mon, May 13, 2013 at 4:13 AM, llanitedave <llanitedave at veawb.coop> wrote:
> On Sunday, May 12, 2013 7:51:28 AM UTC-7, Chris Angelico wrote:
>> On Mon, May 13, 2013 at 12:17 AM, Citizen Kant <citizenkant at gmail.com> wrote:
>>
>> > Maybe It'd be good if I explain myself a bit more. What I'm trying here is
>>
>> > to grasp Python from the game's abstraction point of view, as if it were,
>>
>> > for example, chess.
>>
>>
>>
>> Maybe you're going for something a little too complicated. Let's boil
>>
>> the question down to a much MUCH simpler subset: expression
>>
>> evaluation. And let's take Python itself right out of the picture, and
>>
>> work with only the following elements:
>>
>>
>>
>> * Parentheses ( )
>>
>> * Multiplication and division * /
>>
>> * Addition and subtraction + -
>>
>> * Decimal integer constants 0123456789
>>
>>
>>
>> This will give the basics of algebraic evaluation. It's quite fun to
>>
>> build a simple little expression evaluator that just does these few
>>
>> operations, and it's surprisingly useful (embed it within a command
>>
>> interpreter, for instance). Some time ago I wrote one into a Windows
>>
>> app (written in C++), and when I pulled it out just now and made it a
>>
>> stand-alone tool, the whole thing was only ~60 lines of code. Nice and
>>
>> easy to work with.
>>
>>
>>
>> So, let's take an expression and see what it really means.
>>
>>
>>
>> 2*(3+4)+6
>>
>>
>>
>> One way to interpret this is with a stack. Here's how Python evaluates
>>
>> that expression:
>>
>>
>>
>> >>> def foo():
>>
>>       return 2*(three+4)+6
>>
>>
>>
>> >>> dis.dis(foo)
>>
>>   2           0 LOAD_CONST               1 (2)
>>
>>               3 LOAD_GLOBAL              0 (three)
>>
>>               6 LOAD_CONST               2 (4)
>>
>>               9 BINARY_ADD
>>
>>              10 BINARY_MULTIPLY
>>
>>              11 LOAD_CONST               3 (6)
>>
>>              14 BINARY_ADD
>>
>>              15 RETURN_VALUE
>>
>>
>>
>> (I had to replace one of the constants with a global, to foil the optimizer)
>>
>>
>>
>> The LOAD statements add to an internal stack, the BINARY operations
>>
>> pop two operands and push the result. This is more-or-less the same
>>
>> technique as I used in my evaluator, except that instead of compiling
>>
>> it to code, I just march straight through, left to right, and so I had
>>
>> to maintain two stacks (one of operands, one of operators).
>>
>>
>>
>> Is this what you had in mind when you wanted to grasp Python's
>>
>> internals? Because it's pretty easy to explore, thanks to the dis
>>
>> module. There's a huge amount that can be learned about the
>>
>> interpreter, its optimizers, and so on, just by disassembling
>>
>> functions. Alternatively, if you're thinking on a more abstract level,
>>
>> leave Python aside altogether and pick up a book on language design.
>>
>> Also can be awesome fun, but completely different.
>>
>>
>>
>> ChrisA
>
> No, that won't help.  You're trying to give him knowledge; he wants understanding.

I can't give him understanding. Best I can do is offer facts, which
lead to knowledge, which lead to understanding if absorbed
appropriately. Also, sources of knowledge (like dis.dis) can be VERY
powerful tools in gaining understanding.

ChrisA