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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: scope of function parameters
Date: Mon, 30 May 2011 12:49:26 -0400
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On 5/30/2011 5:08 AM, Laurent Claessens wrote:
> Le 30/05/2011 11:02, Terry Reedy a =E9crit :
>> 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=20
action.

Lexers are typically based on regular grammars and implemented as very=20
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 =3D initial_state
   buffer =3D []
   for char in stream:
     state,out =3D lookup[state,char]
     if out:
       yield output(buffer)
       # convert list of chars to token expected by parser, clear buffer
     buffer +=3D char

There is no backup and no lookahead (except for the fact that output=20
excludes the current char). For python, lookup[start,'5'] =3D=3D=20
in_number,False, and lookup[in_number,'.'] =3D=3D in_float,False.

 >>> 5..__add__(6)
11.0

works because lookup[in_float,'.'] =3D=3D start,True, because buffer now =

contains a completed float ready to output and '.' signals the start of=20
a new token.

I believe we read natural language text similarly, breaking it into=20
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=20
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=20
above.

--=20
Terry Jan Reedy