python and macros (again) [Was: python3: 'where' keyword]

Roel Schroeven rschroev_nospam_ml at fastmail.fm
Fri Jan 14 19:33:05 CET 2005


Antoon Pardon wrote:
> Op 2005-01-14, Roel Schroeven schreef <rschroev_nospam_ml at fastmail.fm>:
> 
>>Antoon Pardon wrote:
>>
>>>IMO we have a: dogs are mamals kind of relationship in Python.
>>
>>I see what you mean, but I don't think it's true.
>>
>>
>>>Every expression can be used where a statement is expected.
>>>(And this can be worded as: every expression is a statement.)
>>
>>Not really. An expression statement is a statement that looks like an 
>>expression, but actually it's more than that: not only does it calculate 
>>the value of the expression, it also prints the value.
> 
> 
> 1) Only in an interactive environment.

True, I wanted to add that but forgot it. Doesn't change what I'm saying 
though.

> 2) That the semantics differ according to where the expression is
>    used doesn't make a difference. That an expression decides which
>    branch of an if statement is executed or what object is pass
>    as an argument in a call are also semantic difference, yet 
>    we still have an expression in both cases.

In both cases we have an expression, in both cases we have a statement, 
in both cases the expression is a part of the statement. In one case the 
expression is the only statement, in the other case the statement has 
other parts too.

> 
>>Note that it would be perfectly possible to modify the syntax into
>>
>>expression_stmt ::= "exprstmt" expression_list
>>...
> 
> If you change the syntax, of course you will change the strings
> that will be accepted. I could change the syntax to:
> 
>   if_stmt ::= "if" "ifexpr" expression ...
> 
> Have I now proved that expressions after an if are not normal
> expressions?

No, you still have a statement with one or more parts, one of which is 
an expression.

In OOP terms: I think that an expression statement 'has an' expression 
(I agree that is a very thin wrapper though), not that an expression 
statement 'is an' expression.

>>>Not every statement can be used where an expression is expected. 
>>
>>AFAIK *no* statement can be used where an expression is expected.
> 
> 
> But that was not the implication of what Guido supposedly had said.
> So that this is not the case doesn't counter what I said.

Whether statements can be used in the place of expressions is indeed not 
relevant to the discussion.

Regarding what Guido apparently said:

Op 2005-01-12, Steve Holden schreef <steve at holdenweb.com>:
 >> Given that Guido is on record as saying that expressions aren't
 >> statements because he wants those things to be separate

Antoon Pardon wrote:
 > Well, it seems that Guido is wrong then. The documentation clearly
 > states that an expression is a statement.

I don't think it says that at all.

 > More specifically, everywhere you can use a statement, you can
 > simply use an expression according to the python syntax.

  If you use an expression where a statement is expected, you really 
write an expression statement that contains the expression (and nothing 
else, but that doesn't matter).

-- 
"Codito ergo sum"
Roel Schroeven



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