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Python vs. Perl
From Benjamin A. Okopnik
Answered By: Mike Orr, Jim Dennis, Rick Moen, Jimmy O'Regan
Have I mentioned, yet, that I DON'T KNOW PYTHON? Just in case I haven't, I DON'T KNOW PYTHON. Since I DON'T KNOW PYTHON, my tweaking is guided by something other than knowledge; it can be approximately imagined as a cross between voodoo, poking a long stick down a deep hole which has ominous sounds issuing from it, and using a large axe to flail wildly in random directions. The only definite part of the process is that is takes a long time.
[Sluggo] You mean you don't find it immediately more readable than Perl? Nice English words instead of obscure symbols, less packing of several operations in one statement?
[snort]
files = ["lg/" + x for x in files if x]
This is supposed to somehow be less packed than a Perl statement?
results = list() # same as files = []
for x in files:
if x:
files.append("lg/" + x)
files = results
But the list comprehension syntax is still much cleaner than the horrid:
files = map(lambda x: "lg/" + x, filter(None, files))
which is the only one-liner I can think of to implement this prior to their introduction.
The closest I can come up with in Perl at the moment is:
@files = map { "lg/" . $_ if $_} grep(/./,@files);
... which took me an unfortunately long time (much to Heather's displeasure as I'm supposed to be taking her to a social event right now). I don't use perl often enough; so, the silly @this and $that get rusty and I didn't remember Perl grep().
Is there a good Perl to Python "Rosetta stone" out there that would have helped me find Perl grep() vs. Python filter() correspondence?
Yep - Tom Christiansen's "Python to Perl Conversions" list, which I ran
across quite a while ago. Unfortunately, the original link at
mail.python.org seems to be dead - but Google still has it cached.
"Python to Perl Conversions" on http://linuxmafia.com/kb/Devtools
So... "if x" is somehow better than "if /x/"? The distinction eludes me.
As to "grep", I think you mean "map" - which I understand Python also
supports.
Erm... your judgement of perfection falls a bit short of, well,
perfection.
In the example that Jim gave, that 'grep' is completely
superfluous. However, if you wanted to use it for something like this,
you could certainly twist it to the purpose:
@files = grep s{.+}{lg/$&}, @files;
However, just because you can doesn't mean that you should. From "perldoc -f grep":
|
...............
............... |
Using 'map' for its intended purpose seems to me to be much clearer, as well as avoiding that variable repetition:
map $_="lg/$_" if /./, @files;
Oh, I wasn't saying that it wouldn't work - just that holding it up as
an example of perfection and orthodox usage wasn't appropriate. It would
work just fine, though.
That's one of those things that makes 'map' so powerful and so complex:
it can be used to modify the array being processed, or it can be used to
return the result - which means that you can use it "on the fly" to
process your data so that it can be seamlessly plugged into the rest of
the procedure.
# Voting: multiple offices, multiple candidates
my %offices = (
Janitor => [ "Arnold", "Archie", "Anna", ],
Peon => [ "Barney", "Betty", "Basil", ],
Manager => [ "Carl", "Cyril", "Cindy", ],
CEO => [ "Dave", "Dana", "Darlene", ]
);
print header, start_html, start_form,
map table(
Tr(
td( "Office: $_" ),
td( radio_group( -name => $_, -values => $offices{ $_ } ) )
)
), sort keys %offices;
print br, submit( "Done" ), end_form;
Since 'map' and 'grep' both are implicit loops that allow acess to the underlying variable, it's easy to get in trouble when using them... but that's always the case with power tools: if they can cut through an inch of steel, they can certainly take off a finger. Safety guards (a.k.a. "documentation") are there for a reason. :)
I was reading the new "Best Practices in Perl" O'Reilly book and came across the "Schwartzian Transform" example. This is known as the "DSU pattern" to Pytonistas (decorate, sort, undecorate).
Nicely descriptive, that. You might also want to take a look at
the GRT (Guttman-Rosler Transform), which is related but different (a
lot faster, for one):
#!/usr/bin/perl -w
my @words = <>; # Slurp the file contents
my @sorted = map { substr($_, 4) }
sort
map { pack( "LA*", tr/eE/eE/, $_ ) } @words;
print "@sorted";
}
I decided to write two versions of the example in Python: one that modifies the list in place and the other which returns a copy.
Returning the copy is easy and comes naturally:
from sha import sha
def sha_sorted(seq):
seq = [ (sha(x).hexdigest(), x) for x in seq ]
seq.sort()
seq = [ y for (x,y) in seq ]
return seq
(Actually I could sort by decimal or hex representations of the hash)
As you can see the first line decorates the list (by creating a list of tuples such that each tuple contains the hash, then the original item). The call to this list's .sort() method is all we need to do the right thing; since the Python .sort() methods are automagically multi-key when dealing with sequence objects like lists and tuples. Then we "undecorate."
That's easy. It's a little trickier to make it modify the object which was passed to it. In Python the seq = [ ... ] is binding a newly created list to the name seq. The object just happens to have been generated from the old object which was, co-incidentally, bound to that name prior to the new binding. We were passed an object reference, so how to me modify the object of that reference rather than creating a new object reference?
Here's the answer:
def sort_by_sha(seq):
seq[0:] = [ (sha(x).hexdigest(), x) for x in seq ]
seq.sort()
seq[0:] = [ y for (x,y) in seq ]
return None
... we assign a list of results to a slice of the list to which the name referred. It just so happens that we're replacing a slice that includes all of the items; and it doesn't matter if the new slice has more or fewer elements than the original list.
Of course that only works with mutable objects (like lists) so we can't write a function that modifies a normal Python string (which is immutable). We can write a function that returns a modified copy of a string, but we can't modify the string as a side effect of calling a function. In general we have to work to make any function modify something as a side-effect. (Of course our methods can modify their objects all they like ... but immutable members would simply be rebound to new objects with the desired transformations).
In general the Python community would use the first form, so the intended effect (changing "seq") was visible from the calling code:
seq = sha_sorted(seq)
rather than having this side effect:
seq = 'this is a test'.split()
sort_by_sha(seq)
# ...
[shrug] Peeve for you, immensely valuable for huge numbers of
programmers worldwide.
Yes, but Perl had it long before Python even existed.
Yeah, anything you steal from Perl is likely to be. :)
pairs = [(key1, value1), (key2, value2)]
d = dict(pair for pair in pairs)
I just wish they'd borrowed Perl's ternary operator (?:). That was shot down coz there were some six different syntaxes with equal support proceeding.
[nod] I'm surprised that Guido didn't repeat Larry's method - i.e., just
put his foot down and say "it shall be so." The tendency for people to
wank endlessly with trivia is well known in those circles, and even an
arbitrary decision at those times is better than none.
|
............... Following the discussion, a vote was held. While there was an overall interest in having some form of if-then-else expressions, no one format was able to draw majority support. Accordingly, the PEP was rejected due to the lack of an overwhelming majority for change. Also, a Python design principle has been to prefer the status quo whenever there are doubts about which path to take. ............... |
You can already do:
result = condition and true_value or false_value
Sure - you should be able to do that in pretty much anything that
supports logical operators.
# Perl print 3 > 2 && "true" || "false" # Bash [ 3 -gt 2 ] && echo "true" || echo "false"
etc.
[Nod]
sub am_asz
{
my $verb = shift;
my @end = qw (am asz a amy acie aj$a);
return qw (dam dasz da damy dacie dadz$a) if ($verb eq "da$c");
return qw (mam masz ma mamy macie maj$a) if ($verb eq "mie$c");
return "error" if (substr ($verb, -2) ne "a$c");
return map {substr ($verb, 0, -2).$_} @end;
};
> return qw (dam dasz da damy dacie dadz$a) if ($verb eq "da$c"); > ^^
That's not going to work - "qw" disables variable interpretation.
(I've chosen to return an array of possibilities in the event that the caller hasn't given enough information. I'm trying to find a way to sort by likelihood, but that may be a pipe dream).
What, weighted sorting? Pshaw. Pshaw, I say (perhaps because nobody
alive today pays even the slightest attention to a silly word like
"pshaw". I suspect he was bshaw's younger brother, the one that was
always picked last for the soccer team and such... But I Digress.)
#!/usr/bin/perl -w
# Created by Ben Okopnik on Thu Aug 18 01:11:46 EDT 2005
# I actually had to go look this up in a *book*. Jimmy, I'll never
# forgive you. :)
#
# Modified from algorithm shown in "Mastering Algorithms with Perl"
# (Jon Orwant, Jarkko Hietaniemi, and John Macdonald)
sub weighted {
my( $dist, $key_order, $total_weight ) = @_;
my $running_weight;
$key_order = [ sort { $dist->{$a} <=> $dist->{$b} } keys %$dist ]
unless $key_order;
unless ( $total_weight ) {
for (@$key_order) { $total_weight += $dist->{$_} }
}
# Get a random value
my $rand = rand( $total_weight );
# Use it to determine a key
while( my( $key, $weight ) = ( each %$dist ) ) {
return $key if ($running_weight += $weight) >= $rand;
}
}
%choices = ( eenie => 1, meenie => 10, mynie => 1, moe => 1 );
print weighted( \%choices ), "\n" for 1 .. 25;
Heh. That's more of an indicator of what I usually do or don't do with
Perl; I'd seen this thing before, and had actually reconstructed ~80% of
it from my memory of the underlying algorithm - but since the other 20%
wasn't instantly forthcoming, I said "HUMPH!" very loudly and went to my
bookshelf.
"MAwP" is a book of Extreme Coolness, anyway - it was a
pleasure to revisit.
Note the flexibility of the above: the key order and the total weight
can be supplied in the invocation - or they'll be computed using the
standard assumptions.
Loading the values for each verb from a *DBM hash or a MySQL DB is left to the individual student. :)
Oh, if I'd only known! I was going to make it a bunch of Smarties
colors along with the freq of occurrence, but didn't have a bag
available on the boat. :)
I'm not really clear on what you're asking here, Jimmy - what's "$c"?
What is "$a"? What are you trying to achieve by using a regex?
if ($verb =~ /(.*)ac$/)
{
return map {$1.$_} @end;
};
where the suffix stripping was done in the map statement, like I've done with substr.
Seems OK to me:
ben@Fenrir:~$ perl -wlne'sub a { map $1.$_, qw/x y z/ if /(.*)ac$/ }; print "\t$_" for a'
maniac
manix
maniy
maniz
sumac
sumx
sumy
sumz
tarmac
tarmx
tarmy
tarmz
lacy
Part of what I'm seeing, though, would be better done with a hash:
sub am_asz {
my verb = shift;
my %list = ( da => [ ("dam", "dasz", "da", "damy", "dacie", "dadz$a") ],
mie => [ ("mam", "masz", "ma", "mamy", "macie", "maj$a") ],
# Add whatever other correspondences you want
);
# I have no idea what these actually are, so I just made shit up.
my ($root, $base, $suffix) = $verb =~ /^((..).*)(..)$/;
return @{ $list{ $base } };
$suffix eq "a$c" ? map "$root$_", @end : "error";
}
I can only hope that I've hit somewhere near the target by throwing a
dart while blindfolded...
Note that your algorithm presumes a word
with a minimum of four characters, so my code reflects that restriction
as well.
/me checks
Nope. Works with a couple of random, made-up three letter verbs.
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