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Re: scope of function parameters

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On 5/30/2011 5:08 AM, Laurent Claessens wrote:
> Le 30/05/2011 11:02, Terry Reedy a écrit :
>> On 5/30/2011 3:38 AM, Laurent wrote:
>>
>>> Cool. I was thinking that "5" was the name, but
>>> >>> 5.__add__(6)
>>> File "<stdin>", line 1
>>> 5.__add__(6)
>>
>>
>> Try 5 .__add__(6)
>
> What is the rationale behind the fact to add a space between "5" and
> ".__add__" ?
> Why does it work ?

Others have given you specific answers, here is the bigger picture.

For decades, text interpreter/compilers have generally run in two phases:
1. a lexer/tokenizer that breaks the stream of characters into tokens;
2. a parser that recognizes higher-level syntax and takes appropriate 
action.

Lexers are typically based on regular grammars and implemented as very 
simple and fast deterministic finite-state automata. In outline (leaving 
out error handling and end-of-stream handling), something like:

def lexer(stream, lookup, initial_state):
   state = initial_state
   buffer = []
   for char in stream:
     state,out = lookup[state,char]
     if out:
       yield output(buffer)
       # convert list of chars to token expected by parser, clear buffer
     buffer += char

There is no backup and no lookahead (except for the fact that output 
excludes the current char). For python, lookup[start,'5'] == 
in_number,False, and lookup[in_number,'.'] == in_float,False.

 >>> 5..__add__(6)
11.0

works because lookup[in_float,'.'] == start,True, because buffer now 
contains a completed float ready to output and '.' signals the start of 
a new token.

I believe we read natural language text similarly, breaking it into 
words and punctuation. I believe the ability to read programs depends on 
being able to adjust the internal lexer a bit. Python is easier to read 
than some other algorithm languages because it tends to have at most one 
punctuation-like symbol unit between words, as is the case in the code 
above.

-- 
Terry Jan Reedy

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Thread

scope of function parameters Henry Olders <henry.olders@mcgill.ca> - 2011-05-29 04:30 -0400
  Re: scope of function parameters Mel <mwilson@the-wire.com> - 2011-05-29 07:59 -0400
    Re: scope of function parameters Terry Reedy <tjreedy@udel.edu> - 2011-05-29 16:27 -0400
  Re: scope of function parameters Peter Pearson <ppearson@nowhere.invalid> - 2011-05-29 17:21 +0000
    Re: scope of function parameters Ben Finney <ben+python@benfinney.id.au> - 2011-05-30 07:42 +1000
      Re: scope of function parameters Laurent Claessens <moky.math@gmail.com> - 2011-05-30 09:12 +0200
        Re: scope of function parameters Chris Rebert <clp2@rebertia.com> - 2011-05-30 00:27 -0700
        Re: scope of function parameters Laurent <moky.math@gmail.com> - 2011-05-30 09:38 +0200
        Re: scope of function parameters Daniel Kluev <dan.kluev@gmail.com> - 2011-05-30 19:10 +1100
        Re: scope of function parameters Terry Reedy <tjreedy@udel.edu> - 2011-05-30 05:02 -0400
          Re: scope of function parameters Laurent Claessens <moky.math@gmail.com> - 2011-05-30 11:08 +0200
            Re: scope of function parameters Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-30 09:14 +0000
            Re: scope of function parameters Ben Finney <ben+python@benfinney.id.au> - 2011-05-30 19:17 +1000
            Re: scope of function parameters Peter Otten <__peter__@web.de> - 2011-05-30 11:41 +0200
              Re: scope of function parameters Laurent Claessens <moky.math@gmail.com> - 2011-05-30 12:10 +0200
              Re: scope of function parameters Laurent Claessens <moky.math@gmail.com> - 2011-05-30 12:10 +0200
            Re: scope of function parameters Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-05-30 13:40 +0300
            Re: scope of function parameters Terry Reedy <tjreedy@udel.edu> - 2011-05-30 12:49 -0400
        Re: scope of function parameters Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-30 09:16 +0000
  Re: scope of function parameters Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 23:46 +0000
    Re: scope of function parameters Ben Finney <ben+python@benfinney.id.au> - 2011-05-30 11:31 +1000
      Re: scope of function parameters Chris Angelico <rosuav@gmail.com> - 2011-05-30 11:56 +1000
        Re: scope of function parameters Ben Finney <ben+python@benfinney.id.au> - 2011-05-30 12:08 +1000
          Re: scope of function parameters Chris Angelico <rosuav@gmail.com> - 2011-05-30 12:37 +1000
      Re: scope of function parameters Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-30 04:23 +0000
  Re: scope of function parameters rusi <rustompmody@gmail.com> - 2011-05-31 09:46 -0700
    Re: scope of function parameters rusi <rustompmody@gmail.com> - 2011-05-31 10:33 -0700

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