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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