[Sorry for the private reply earlier]
On 10 Sep 2008, at 19:43, Cliff Wells wrote:
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)
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.