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From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: How to run script from interpreter?
Date: Fri, 30 May 2014 11:24:11 -0400
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Xref: csiph.com comp.lang.python:72302

On 5/30/2014 7:46 AM, Chris Angelico wrote:

> Hmm. I'm not sure that "raises SyntaxError" is any less a part of the
> language's promise than "evaluates to twice the value of x" is.

Of course it is. A real SyntaxError cannot be caught immediately.* When 
new syntax features are added, breaking the raising of a SyntaxError is 
*not* a consideration. On the other hand, we do not freely change 
AbcError to XyzError even when the latter would be more 'correct'.

* IE, you cannot type

 >>> try: ..
     except SyntaxError: print('caught')

because the SyntaxError during parsing rather than during execution.

> Also, consider:
>
>>>> for i in range(5): i*3+2
>
> 2
> 5
> 8
> 11
> 14
>
> If you do this in a script, it's perfectly legal, but won't print
> anything. So the REPL is already "chang[ing] the meaning of otherwise
> legal Python syntax",

It does not change what is calculated. It adds introspection output as a 
convenience. The minus is that one may forget that it *is* an addition 
and use the shortcut when writing a script. (I know I have, occasionally.)

 > and what's more, it's making None into a special case:

Which it is, as the 'zero' of the set of Python objects.

>>>> for i in range(5): None if i%2 else i
>
> 0
> 2
> 4
>
> Practicality beats purity. If it's more useful to the end user for
> something valid-but-illogical to have a new bit of meaning in
> interactive mode, I say go for it.

I disagree.

-- 
Terry Jan Reedy