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

Adam Olsen rhamph at gmail.com
Thu Sep 11 01:16:16 CEST 2008

On Wed, Sep 10, 2008 at 4:39 PM, Cliff Wells <cliff at develix.com> wrote:
> On Wed, 2008-09-10 at 15:57 -0600, Adam Olsen wrote:
>> On Wed, Sep 10, 2008 at 3:18 PM, Cliff Wells <cliff at develix.com> wrote:
>> > Further, I feel that this limitation forces programmers into using hacks
>> > and magic or overly spread-out code, which itself leads to readability
>> > concerns.  Having used Python for around a decade, I'm quite aware of
>> > the fact that you can happily write tons and tons of nice code with
>> > Python in its current state.  However, because of the direction of my
>> > work (developing a internal DSL in Python) I've suddenly become aware of
>> > this glass ceiling.  I'd bumped into it before back when I was doing a
>> > lot of GUI development, but blamed it on lambda rather than realizing
>> > that it wasn't lambda so much as what I am bringing up now.
>> Python is not intended for DSLs.  Really, don't do it.  Python is for
>> python code.
> The DSL I work on *is* Python code.  And really, this is the first time
> I've heard anyone assert anything like this.  Python is a
> general-purpose language.  It's not VBA ;-)
> DSL's are an extremely useful concept.  To summarily dispatch the whole
> of them with such an assertion is pretty much bolstering my argument:
> you've just asserted that Python is inherently limited in scope.

I agree, DSL's are useful.  They're just not something python
currently supports well.  They're the use-case you need to justify
against the substantial changes you propose.  Maybe it's worth it, or
maybe it's better to add an indent-sensitive string literal that would
then allow *arbitrary* DSL syntax?

>> If you want another language, write your own parser.  I
>> hear lisp is simple to parse, and has no annoying statements to hold
>> you back!
> Ah, except Python is the language I like in every way, *except* for this
> one particular wart.   Really, had I not entered new programming domains
> and tried to take Python with me, I'd probably never have had a
> complaint.
> Also, external parsers defeat the entire reasoning behind internal DSL's
> (taking advantage of an established VM/compiler, requiring users to
> learn a new syntax in addition to their primary programming language).

I appreciate what you're saying here, and feel much the same way about
my own pet-features, but this is a really poor argument.  Everybody
has just one wafer-thin feature they'd like to add.

>> Seriously though, there is an advantage to basing so much on
>> statements rather than expressions.  We're specialized for one
>> statement per line, which is the most common case,
> Clearly it's the most common case in existing Python code since nothing
> else is allowed.   But frankly, even Javascript doesn't follow this
> idiom anymore.   Expression-oriented languages have seen a rebirth for
> good reasons (although I admit I'm none-to-fond of many of them, for
> various reasons).

Again, "XYZ language has it" is an ineffective argument.

>>  and it allowed us
>> to have extraneous semicolons, braces, or whatever.
> Not following this.  You mean to *not* have extraneous syntax?

Consider a loop in C vs in python

for (i = 0; i < 50; i++) {

for i in range(50):

We avoid the semicolons and braces because the newline and indentation
indicate the same thing.  That's what a statement it.  Perhaps you can
retain this with your proposal, but only because a statement is still
the default, with expression as a rarely used option.

>> Readability benefits, style consistency benefits.
> I strongly disagree.  The artificial distinction between statements and
> expressions is the definition of *inconsistent*.  Why do we have two
> versions of the "if" conditional?  Why do we have "for" loops *and* list
> comprehensions?  They express the same ideas, but the limitations in one
> required growing the language another direction.   In short, we could
> have had a single, more powerful construct in place of two lesser
> constructs and simultaneously had less syntax to memorize and more
> consistency across the board.

I was referring to consistency of the programs, not the language
itself.  No silly arguments about brace position because they *are no

There's actually a problem with trying to merge a for-loop and a list
comprehension.  A generator expression is the canonical generic form,
but [genexp] would create a list containing a single genexp object.

Likewise, a for-loop would become lazy, so without some extra syntax
to evaluate it (and trigger the side effects!), your programs would
cease to do anything.

So you see, despite significant and obvious similarity between the
features, there's some important differences as well.  These are so
obvious from the contexts that you don't even think of them, so
clearly the mental load of having 3 (soon to be 5) different forms of
looping is not all that great.

Mental load is what really counts, not some abstract concept of complexity.

>> Now there are some use cases that suffer here, including the one you
>> just gave: defining a dispatch dict with the functions inline.  The
>> best I could do is define the dict first, then stick a decorator on
>> each function to register them.  That's still ugly though.  A creative
>> solution is needed, but none come to mind.
> That's because there is none.  And this is my fundamental problem: it's
> not so much that it's hard to do in Python, it's that you *cannot* do it
> in Python.  No amount of creativity, time, or experience will help, and
> this is disappointing.

I didn't mean to do it in Python.  I meant to modify the language.

> I won't pretend that any example we might toss up in here won't appear
> contrived, but there are definite cases where readability can be
> substantially enhanced with such structures.
>> An example where this has happened before is the with-statement, which
>> is spectacularly successful IMO.  Now, you may notice it could have
>> been done in a library rather than core syntax if generic anonymous
>> blocks were allowed — so what?  The library is still part of the
>> language!  It's still something that has to be learned.  And the
>> syntax would be substantially uglier using a generic mechanism, rather
>> than the specialized with-statement syntax.
> The "so what" is that it could *only* be implemented by the core devs.
> It was not possible for an average (or even above-average) Python
> programmer to write such a library, whereas it *could* have been had the
> language not prohibited it.

As important as it is to extend the language via a library, somewhere
you need to draw the line and start modifying the syntax or other
fundamental builtins.  The is a universal tenant, applying even to
lisp (which has very little syntax, rather than extensible syntax).

This is why we have a+b, rather than add(a, b).  More syntax when it's worth it.

So it's not that we don't want to allow extensibility - quite the
opposite.  It's that we want the common statement to be simpler, and
the extra syntax hasn't been justified for your use cases.

Adam Olsen, aka Rhamphoryncus

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