Favorite non-python language trick?

[NickC]

with colour do begin
red := 0; blue := 255; green := 0;
end;

instead of:

colour.red := 0; colour.blue := 255; colour.green := 0;

c = colour
c.red = 0; c.blue = 255; c.green = 0
del c # Not strictly needed, but limits the scope of c

When everything's a reference, the Pascal 'with' syntax doesn't gain
you anything over a single-letter variable name. As I recall, it's
handy in Pascal because record assignment has value semantics rather
than reference semantics.

Cheers,
Nick.

 

[Benji York]

http://www.logilab.org/projects/python-logic/

This is something pretty new to me, so I can't comment on how well
it would meet your expectations, but I see now that the site does mention
OZ/Mozart as comparables.

I've used both, the logilab stuff is cool, but no where near the
maturity (or speed) of OZ/Mozart. OTOH, I can actually get things
done with the logilab code. But that might say more about me than
Mozart.

 

[todddeluca]

If I had to choose one feature, I would like to see better support for
nested lexical scopes. However, I imagine this is no easy "trick" to
add to the language.

 

[Terry Reedy]

Sadly, its not a solution that I'm after, but a particular toolkit that
can be used for solving that type of problem.

Presuming this is an anwer to my comment about generators... I was pointing
you more to the method used than the particular solutions. Whether your
constraint problems are similar enough, I have no idea.

Terry J. Reedy

 

[Terry Hancock]

If I had to choose one feature, I would like to see better support for
nested lexical scopes. However, I imagine this is no easy "trick" to
add to the language.

Doesn't that already happen in versions 2.2+?

What exactly do you mean by "better" support for nested lexical
scopes? Can you give an example of something that doesn't work,
but should?

I'm just curious. The whole question of lexical scoping seemed
a bit esoteric to me at the time --- I think I must instinctively
avoid situations where it causes trouble. The most important
exception would have to be the behavior of lambda and locally-defined
functions --- I still expect them to know the variables in the defining
function's namespace. But I think that lexical scoping fixed this
so that they do, IIRC (I don't use them often, so I'm not so sure).

 

[ncf]

Eh, just figured it'd be worth noting...map, filter, and reduce should
be possible with the extended list syntaxes. Well, filter I know is,
but hte others /should/ be possible.

filter(lambda: <<condition>>, <<seq>>)
[some_var for some_var in <<seq>> if <<condition>>]


Honestly, though, I hope they don't drop the map functions and such.
That's one of the stronger functions of the language (IMO).

-Wes

 

[ncf]

Hmm...I think it's time I do better justice to what I previously wrote.


http://www.artima.com/weblogs/viewpost.jsp?thread=98196

The above article was linked by Python's PEP...

 

[Erik Max Francis]

Eh, just figured it'd be worth noting...map, filter, and reduce should
be possible with the extended list syntaxes. Well, filter I know is,
but hte others /should/ be possible.

filter(lambda: <<condition>>, <<seq>>)
[some_var for some_var in <<seq>> if <<condition>>]

reduce isn't.

 

[ncf]

Sorry, I realized that shortly after my post =X

 

[Pekka Karjalainen]

I'm curious -- what is everyone's favorite trick from a non-python
language? And -- why isn't it in Python?

Being able to use my native language without a hitch when naming my
variables &c. Java allows that because it supports almost any Unicode
character in its identifiers. With Python I occasionally have to stop and
think: "I can't name it that. What's better?"

Not everyone always writes their programs in English. Once you get used
to Java's naming possibilities, it's plain annoying to go back, even if
it's just a couple of letters that're lacking in my case.

 

[Devan L]

With the exception of reduce(lambda x,y:x*y, sequence), reduce can be
replaced with sum, and Guido wants to add a product function.

 

[Christos TZOTZIOY Georgiou]

I think some sort of inline or deferred local statement would be useful
also. It would serve as a limited lambda (after it's removed), eval
alternative, and as a inlined function in some situations as well I think.

Something like:

name = defer <expression>

then used as:

result = name()

The expression name() will never have arguments as it's meant to
reference it's variables as locals and probably will be replaced
directly with names's byte code contents at compile time.

Defer could be shortened to def I suppose, but I think defer would be
clearer. Anyway, it's only a wish list item for now.

This is similar:

http://groups-beta.google.com/group/comp.lang.python/msg/6fc884147852d23d

 

[Rocco Moretti]

I'm curious -- what is everyone's favorite trick from a non-python
language? And -- why isn't it in Python?

I'm not aware of a language that allows it, but recently I've found
myself wanting the ability to transparently replace objects. For
example, if you have a transparent wrapper class around a certain
object, and then determine that you no longer need to wrap the object,
you can say the magic incantation, and the wrapper instance is replaced
by what it is wrapping everywhere in the program. Or you have a complex
math object, and you realize you can reduce it to a simple integer, you
can substitue the integer for the math object, everywhere.

I mainly look for it in the "object replaces self" form, but I guess you
could also have it for arbitrary objects, e.g. to wrap a logging object
around a function, even if you don't have access to all references of
that function.

Why isn't it in Python? It's completely counter to the conventional
object semantics.

 

[Shai]

I'm curious -- what is everyone's favorite trick from a non-python
language? And -- why isn't it in Python?

1. Lisp's "dynamically scoped" variables (Perl has them, and calls them
"local", but as far as I've seen their use their is discouraged). These
are global variables which are given time-local bindings. That is,
structuring the syntax after what's used for globals,

x=10
def foo():
# No need to define x as it is only read -- same as globals
print x

def bar():
dynamic x
x = 11
foo()

def baz():
bar() # prints 11
foo() # prints 10; the binding in bar is undone when bar exits

This feature makes using "globals" sensible, providing a way to avoid
many important uses (and some say, misuses) of objects if you are so
inclined. It allows you to do some things better than objects do,
because it does to library parameters, what exceptions do to return
codes: instead of passing them in all the way from outside until a
piece of code which actually uses them, they are only mentioned where
you set them and where you really need to access them.

It would not be too hard to implement a version of this (inefficiently)
in the existing language, if frame objects could carry a modifiable
dictionary.

I suppose it is not in Python because (most) Pythoners are not looking
(hard enough) for alternatives to OOP.

2. Prolog's ability to add operators to the language. Though this
facility is quite clanky in Prolog (because there is no elegant way to
specify precedence), the idea is appealing to me. It would allow a
better implementation of my (awkward, granted) recipe for adding logic
programming constructs to Python. It is not in the language because it
might fragmentize it, and because it is very hard to make
recursive-descent parsers like CPython's programmable this way.

3. Lisp's Macros, of course, which have been mentioned already in this
thread. Even Boo-like macros, which are nowhere as strong as Lisp's,
would be very useful. Not in the language, besides its being hard in
any non-lisp-like language, for the reasons mentioned for adding
operators.

On the other hand, there's no end to the features I wish I could copy
from Python to other languages...

 

[Scott David Daniels]

Joseph Garvin wrote:

I'm not aware of a language that allows it, but recently I've found
myself wanting the ability to transparently replace objects....
I mainly look for it in the "object replaces self" form, but I guess you
could also have it for arbitrary objects, e.g. to wrap a logging object
around a function, even if you don't have access to all references of
that function.

Why isn't it in Python? It's completely counter to the conventional
object semantics.

Actually this is the old (and terrifying) Smalltalk message 'becomes:'.
There is a concrete reason it is not in python: objects are represented
as pointers to their data structures, do not have identical sizes, and
therefore cannot be copied into each others data space. Smalltalk
implementations often have a level of indirection that allows it to
simply tweak an indirection table to implement this method.

The reason I find it terrifying is that I can be passed an object,
place it in a dictionary (for example) based on its value, and then
it can magically be changed into something else which does not fit
in that spot in the dictionary.

--Scott David Daniels
(e-mail address removed)

 

[Mike Meyer]

1. Lisp's "dynamically scoped" variables (Perl has them, and calls them
"local", but as far as I've seen their use their is discouraged). These
are global variables which are given time-local bindings. That is,
structuring the syntax after what's used for globals,

Perl started life with nothing but dynamically scoped variables. They
added lexical scoping after they realized what a crock dynamic scoping
was.

x=10
def foo():
# No need to define x as it is only read -- same as globals
print x

def bar():
dynamic x
x = 11
foo()

def baz():
bar() # prints 11
foo() # prints 10; the binding in bar is undone when bar exits

This feature makes using "globals" sensible, providing a way to avoid
many important uses (and some say, misuses) of objects if you are so
inclined. It allows you to do some things better than objects do,
because it does to library parameters, what exceptions do to return
codes: instead of passing them in all the way from outside until a
piece of code which actually uses them, they are only mentioned where
you set them and where you really need to access them.

It would not be too hard to implement a version of this (inefficiently)
in the existing language, if frame objects could carry a modifiable
dictionary.

I suppose it is not in Python because (most) Pythoners are not looking
(hard enough) for alternatives to OOP.

Last time I checked, dynamic binding variables were frowned on in LISP
systems as well. Scheme doesn't have them. Common LISP requires
special forms to use them.

The problem with the given use case is that it lets every routine in
the call chain substitute it's own variable for the library parameter
you want to use, with no local indication that this is going
on. This makes bugs in dynamically scoped variables a PITA to find.

Given that it's a feature I don't want programmers using, I'd only be
willing to see it added to the language if you can show that it has no
overhead so long as you don't use it. I'm not sure that can be done.

<mike

 

[Chris Rebert (cybercobra)]

My personal favorite would be ruby's iterators and blocks.
Instead of writing a bunch of repetitive list comprehensions or
defining a bunch of utility functions, you just use the iterators
supported by container objects.
For instance,

[f(x) for x in y]

could be written in Ruby as

y.collect |x| do
#body of f
end

You don't have to use a lambda or define f() externally.
Best of all, Ruby's containers come with many iterators for common
cases builtin.

 

[Mike Meyer]

[Lots of quoted text left in...]

I started thinking about this, and realized that there was a way to do
what you wanted, with no execution time overhead, and without
providing ways to radically change the program behavior behind the
scenes.

Perl started life with nothing but dynamically scoped variables. They
added lexical scoping after they realized what a crock dynamic scoping
was.

That's a bit harsher than I intended. I mean that having nothing but
dynamically scoped variables - like early Perl and LISP - is a
crock. I'll add that dynamic scoping as the default is a crock. But
you aren't asking for those.

Here's the problem with that. Consider this script:

import foo
x = 10
def bar():
print x

foo.foogle(bar)

If foo.foogle includes "dynamic x" and then invokes bar, bar could
print anything. This makes the behavior of bar unpredictable by
examining the sourc, with no hint that that is going on.

Given that it's a feature I don't want programmers using, I'd only be
willing to see it added to the language if you can show that it has no
overhead so long as you don't use it. I'm not sure that can be done.

Here's a proposal for dynamically bound variables that you should be
able to implement without affecting the runtime behavior of code that
doesn't use it.

Instead of dynamic meaning "all references to the named variable(s)
will be dynamic until this function exits", have it mean "the named
variable(s) will be dynamic in this function." Whether it should only
check local variables in the calling routines, check local + global,
or check for all free variables, is an open question.

I.e. - your example would be written:

x = 10
def foo():
dynamic x
print x

def bar():
x = 11
foo()

def baz():
bar() # prints 11
foo() # Possibly an error?

For my example above, bar would *always* print 10. Nothing that
foo.foogle did would change that. However, you could write:

import foo
def bar():
dynamic x
print x

foo.foogle(bar)

In this case, bar will print whatever foo.foogle sets x to - and it's
noted in the source to bar. This means that functions that don't
declare a dynamic variable can be compiled to the same code they are
compiled to now.

Would this version provide the functionality you wanted?

<mike

 

[=?ISO-8859-1?Q?BJ=F6rn_Lindqvist?=]

I like C++ templates so that you can ensure that a list only contain
items of one type. I also like the JMP instruction in x86 assembler,
you could do some nasty tricks with that.

 

[Terry Reedy]

With the exception of reduce(lambda x,y:x*y, sequence), reduce can be
replaced with sum, and Guido wants to add a product function.

The update function is not at all limited to sums and products, but can be
any callable with the appropriate signature. The new any() and all()
functions are examples that use other updates.

Terry J. Reedy