[Python-ideas] Statements vs Expressions... why?

Arnaud Delobelle arnodel at googlemail.com
Wed Sep 10 21:55:00 CEST 2008


[Sorry for the private reply earlier]

On 10 Sep 2008, at 19:43, Cliff Wells wrote:

> Greetings,
>
> Something that has started to annoy me in the last couple of years is
> the fact that most Python control statements cannot be used as
> expressions.  I feel this is a pretty deep limitation and personally I
> don't feel it's well-justified.
>
> As I understand it, the reason for the distinction mostly has to do  
> with
> the premise "flat is better than nested", which I can understand,  
> but I
> don't think carries enough weight anymore.
>
> Specifically, I'd like to see things like "if" statements, "for"  
> loops,
> etc, become expressions.  This would not preclude them from being used
> as if they were statements (an expression can stand alone on a line of
> Python code), but would add a lot of expressiveness to the language as
> well as make Python more viable for creating DSLs.
>

Can you post some sample code to illustrate how statements could be  
used as expressions?

Do you propose that we write:

    y = if x == 0:
        0
    else:
        exp(x**-2)

instead of:

    y = 0 if x == 0 else exp(x**-2)

? Or, how would you write

    factors = [x for x in range(2, n) if n % x == 0]

?  Something like this maybe:

    factors = for x in range(2, n):
        if n % x == 0:
            x

Or do you suggest something else?

> Additionally, removing statements from Python would also allow the
> language to be simplified.  No need for a ternary "if" operator with
> different semantics than a normal "if" statement, "for" loops would be
> brought closer to generators in functionality, and the perceived
> limitations of lambda would disappear, amongst other things.  We'd  
> gain
> a lot of features found in languages like Lisp and Ruby as a side- 
> effect
> (i.e. anonymous code blocks).
>
> Overall it seems this design decision is specifically geared toward
> forcing programmers into an imperative style in order to enforce  
> program
> readability.  In Python 1.5, this made a bit of sense, but as Python  
> has
> "matured" (or in my view, gotten over-complicated) this makes much  
> less
> sense.  Many parts of Python's extensive syntax are explicit  
> workarounds
> to this design decision.  So on the one hand we have the perceived
> danger that programmers will write nested code and on the other we  
> have
> an ever-expanding syntax.  I'd take the former any day.
>

So do you think readability is not as important now as it was?

> I've not delved into the internals of the Python interpreter to check,
> but I suspect that converting most statements to expressions would not
> be very difficult (changing the grammar definition and generated
> bytecode a small amount in most cases).
>
> Any thoughts on this?  I'm sure it's been brought up before, but I
> haven't found any definitive discussions on why this rather arbitrary
> design decision continues to hold in the face of a general migration
> away from imperative languages (especially when it seems it could be
> changed without much backwards-compatibility issues).

I think to call this feature of Python an arbitrary design decision is  
a misjudgement.  To me it is central to the identity of Python.

-- 
Arnaud



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