List Comprehension Syntax

[Moosebumps]

Does anyone here find the list comprehension syntax awkward?

I like it because it is an expression rather than a series of statements,
but it is a little harder to maintain it seems.

e.g. you could do:

result = []
for element in list:
if element[0:4] == 'blah':
result.append( element.replace ( 'blah', 'moof' ) )

or just:

result = [ element.replace ( 'blah', 'moof' ) for element in list if
element[0:4] == 'blah' ]

The second looks cleaner in some cases, but it is less maintainable. It
tends to promote long lines. It all seems to run together and such. And
often you would need to add another condition or modify it, it seems better
to use the first way even though it has the ugly extra init (and which would
cause more allocs than the list comprehension? because it wouldn't know the
size off the bat?)

I am in favor of short lines like I think Guido said in the style guide. I
like each line to be so simple as to not require any thinking reading it,
e.g.

I prefer:

x = c( d, e )
y = f( g, h )
z = b( x, y )
w = a( z )

to stuff like this:

w = a( b( c( d, e ), f( g, h ) ) )

It is more maintainable, when you need to make a change, just insert a line,
rather than having to decode an expression.

Along the same lines, it seems more maintainable to split things up.

You could do:

result = [
element.replace ( 'blah', 'moof' )
for element in list
if element[0:4] == 'blah' ]

I guess, but that seems awkward to me. Looks too much like a for loop and
an if, and then the value is at the top, which reads funny to me.
(Strangely putting it on one line doesn't read as funny, but it is less
readable.) Maybe I just have to get used to it. Which do you prefer?
Comments?

MB

[Peter Hansen]

Ville Vainio wrote:

>>"Peter" == Peter Hansen <pe***@engcorp. com> writes:

Peter> But if you let emacs define the limits of your world, then
Peter> it's probably okay to do it that way. ;-)

It's just the path of least resistance - any other indentation
requires manual work, and won't reindent properly when pressing <tab>
on the line.

Not sure what you mean here. The indentation approach I'm
referring to (i.e. the "standard" way, as opposed to what I
was calling the "artistic" way) is just to hit tab in precisely
the same way you would if you were indenting an "if" block.
Anyway, the point is mostly moot because

blah = [
i + 1
for i in range(10)
if i != 3
]

indents the way you want it to, and that's the way I usually do it
anyway. I just M-x indent-region'ed your code snippet :).
Not sure which code-snippet you refer to, but the ones that I
typed were indented the same way, except perhaps that I put
the first expression the same line as the opening bracket.
Perhaps this is something that could be fixed in python-mode.el? I
mean making the indentation on the next line be previous_line + 4 in
bracket-opening situations... Or is the current behaviour better in
some generally agreed way?

Nope, definitely not in a generally agreed way. I don't think anyone
thinks this issue is on par with the tab war though, so until that
particular war is won, we're best off leaving this issue to personal
preference. (That is to say, "forever". :-)

-Peter

[Peter Hansen]

Isaac To wrote:

Except that if you write things like

a=[[[1, 2],
[3, 4],
[5, 6]],
[[7, 8],
[5, 6]],
[9]]

as

a=[[[1, 2],
[3, 4],
[5, 6]],
[[7, 8],
[5, 6]],
[9]]

and say there is no lose of legibility, then I'll seriously question
your taste.
Ugh. But nobody should write something like the first one, either.
(And I won't say how it should be written without a real example,
since otherwise we can each contrive all kinds of interesting and
meaningless ways to format something that nobody would actually
write.)
BTW, for somebody with python-mode installed and is using Emacs, it is
actually *harder* and a *bigger* waste of time to write the code as
you proposed, because by default whenever you press enter the cursor
is already at exactly the location in the former style, and you got to
count spaces in order to go to the place you proposed.

Not sure which one you think I proposed, but you definitely don't
have to count any spaces in order to do what I suggested. You
hit ENTER and TAB (once), exactly as you would with any other
block. Note that this (hitting TAB once) of course relies on
an editor which auto-indents to the same level as the preceding
line of code by default. Any editor which does not do at least
this, these days, is broken. But counting spaces would be silly.
What editor are you using that would force you to do that? vi
without a Python configuration?

-Peter

[Larry Bates]

I don't agree that the second is "less maintainable",
but do find it "wordy" for little reason. Try:

result=[x.replace('blah ','moof') for x in L if x.startswith('b lah')]

or if you prefer short lines:

result=[x.replace('blah ','moof') \
for x in L \
if x.startswith('b lah')]

Note: BAD idea to call variable 'list' as it kills the
list function for remainder of Python session (I changed
to L). For same reason never call variable 'dict', 'str',
etc. Using startswith also takes care of not having to
get the slice correct (which is easier to maintain should
you change 'blah' to something else, you don't have to
mess with the slice).

Larry Bates
Syscon, Inc.

"Moosebumps " <cr**@crap.crap > wrote in message
news:j9******** *********@newss vr25.news.prodi gy.com...

Does anyone here find the list comprehension syntax awkward?

I like it because it is an expression rather than a series of statements,
but it is a little harder to maintain it seems.

e.g. you could do:

result = []
for element in list:
if element[0:4] == 'blah':
result.append( element.replace ( 'blah', 'moof' ) )

or just:

result = [ element.replace ( 'blah', 'moof' ) for element in list if
element[0:4] == 'blah' ]

The second looks cleaner in some cases, but it is less maintainable. It
tends to promote long lines. It all seems to run together and such. And
often you would need to add another condition or modify it, it seems better to use the first way even though it has the ugly extra init (and which would cause more allocs than the list comprehension? because it wouldn't know the size off the bat?)

I am in favor of short lines like I think Guido said in the style guide. I like each line to be so simple as to not require any thinking reading it,
e.g.

I prefer:

x = c( d, e )
y = f( g, h )
z = b( x, y )
w = a( z )

to stuff like this:

w = a( b( c( d, e ), f( g, h ) ) )

It is more maintainable, when you need to make a change, just insert a line, rather than having to decode an expression.

Along the same lines, it seems more maintainable to split things up.

You could do:

result = [
element.replace ( 'blah', 'moof' )
for element in list
if element[0:4] == 'blah' ]

I guess, but that seems awkward to me. Looks too much like a for loop and
an if, and then the value is at the top, which reads funny to me.
(Strangely putting it on one line doesn't read as funny, but it is less
readable.) Maybe I just have to get used to it. Which do you prefer?
Comments?

MB

[Ville Vainio]

>>>>> "Peter" == Peter Hansen <pe***@engcorp. com> writes:

Peter> Not sure what you mean here. The indentation approach I'm
Peter> referring to (i.e. the "standard" way, as opposed to what I
Peter> was calling the "artistic" way) is just to hit tab in
Peter> precisely the same way you would if you were indenting an
Peter> "if" block.

In emacs, tab is 'indent-line', which gives the line "correct"
indentation. Occasionally one needs to dedent a line manually to close
a block. Pressing <enter> puts the cursor in the right place
immediately (indenting on ':', dedenting on 'return').

if 1:
hui();

If I press tab on line with hui(), the result is

if 1:
hui();

Anyway, the point is mostly moot because
blah = [
i + 1
for i in range(10)
if i != 3
]
indents the way you want it to, and that's the way I usually do it
anyway. I just M-x indent-region'ed your code snippet :).

Peter> Not sure which code-snippet you refer to, but the ones that
Peter> I typed were indented the same way, except perhaps that I
Peter> put the first expression the same line as the opening
Peter> bracket.

Yes, and that made the difference.
--
Ville Vainio http://tinyurl.com/2prnb

[Aahz]

In article <HI************ *******@newssvr 27.news.prodigy .com>,
Moosebumps <cr**@crap.crap > wrote:

result = [x for x in range(10) if x % 2 == 0 if x % 3 == 0]
result[0, 6] result = [ x*y for x in range(10) if x%2 == 0 for y in range(10) if y %3 == 0] result

[0, 0, 0, 0, 0, 6, 12, 18, 0, 12, 24, 36, 0, 18, 36, 54, 0, 24, 48, 72]

Just curious -- anyone care to tell me how they would format the above? (or
maybe you wouldn't write it all)

Wouldn't use either.
--
Aahz (aa**@pythoncra ft.com) <*> http://www.pythoncraft.com/

Barbara Boxer speaks for me:
http://buffaloreport.com/2004/040713....marriage.html

[David Eppstein]

In article <cd**********@p anix2.panix.com >, aa**@pythoncraf t.com (Aahz)
wrote:

In article <HI************ *******@newssvr 27.news.prodigy .com>,
Moosebumps <cr**@crap.crap > wrote:

> result = [x for x in range(10) if x % 2 == 0 if x % 3 == 0]
> result

[0, 6]

> result = [ x*y for x in range(10) if x%2 == 0 for y in range(10) if y %

3 == 0]

> result

[0, 0, 0, 0, 0, 6, 12, 18, 0, 12, 24, 36, 0, 18, 36, 54, 0, 24, 48, 72]

Just curious -- anyone care to tell me how they would format the above? (or
maybe you wouldn't write it all)

Wouldn't use either.

I would probably use instead something like

[x for x in range(0,10,6)] [0, 6] [x*y for x in range(0,10,2) for y in range(0,10,3)]

[0, 0, 0, 0, 0, 6, 12, 18, 0, 12, 24, 36, 0, 18, 36, 54, 0, 24, 48, 72]

I think two for or if clauses in a comprehension is the most I'd want to
use on a regular basis. I certainly wouldn't use two ifs in a row as in
your first example, I'd use and instead.

--
David Eppstein
Computer Science Dept., Univ. of California, Irvine
http://www.ics.uci.edu/~eppstein/