First I checked the web to see if someone had written a JSON parser using SimpleParse. I found Rob Lanphier’s JsonOrder. It at least had a grammar that I could yank as a jumping off point.

The result after about an hour of coding and benchmarking is spjson.py. At first I tried to adapt Rob’s version but I switched back to SimpleParse’s dispatch processor model. Pretty much the only thing that remains from JsonOrder is the tweaked grammar.

How does it measure up to CodeTalker’s JSON parser?

It’s a bit slower. I added a simple timeit benchmark to the spjson.py file. I used the same JSON file that Jared (the CodeTalker author) used in his benchmarks. Here are the results of running it against the latest version of CodeTalker at the time:

CodeTalker 0.0484498786926
SimpleParse 0.0623928356171

In terms of lines of code they are nearly identical. I didn’t do anything fancy to omit docstrings or comments (neither module has many of either).

$ # use 'head' to strip the 'if __name__ ...' section
$ head spjson.py -n 71 | grep -v '^\s*$' | wc -l
55
$ cat src/codetalker/codetalker/contrib/json.py | grep -v '^\s*$' | wc -l
55

The style is the biggest difference. SimpleParse uses an EBNF defined in a string to create the parser. CodeTalker uses an EBNF defined using Python code.

Both libraries let you write specialized processors for the grammar. This is one thing that I really value in SimpleParse over PLY. Your grammar and the code to act on the parse tree (or token stream) are neatly separated.

I’m still quite happy with SimpleParse. I like having the grammar in a nice contained EBNF rather than defined with Python + syntactic sugar. I’ll probably give CodeTalker a look next time a new parsing task comes up though. It’s great to see how the options for parsing in Python are expanding.

cw

I had the idea to use the hg rebase command to move all my local commits to a new branch before pushing. It worked, and here’s how I did it.

1. hg clone localrepo temp-localrepo. I always try crazy ideas in another local clone in case I trash the repo.
2. hg up to the revision before my local commits that I want to put in a branch.
3. hg branch newbranchname && hg ci -m "Branching for reason foo." This creates a new branch and head that can be used as a rebase destination.
4. Now for the rebase. I needed to rebase the first changeset in my series of local changes onto the newbranchname changeset. Something like hg rebase --source 94 --dest 105.

That’s about it. After verifying that it did what I wanted all that was left was to repeat the steps in my main local repo and push to the central server.*

cw

* Of course what I actually did was push these changes to my main local repo and then to the central repo. Woops. That left me with the changes both in default and in newbranchname.

Don’t do that. Repeat the steps after you’ve tried them in another local clone. I leave recovering from such a situation with hg strip as an exercise for the reader.

We use Mercurial for revision control so it is a logical choice for a version information source. The result of some hacking is my first Mercurial extension and commit hook.

Mercurial Version Info Plugin.

See the README file rendered at that link for the details. Click here to download version 1.0.

cw

I think this is a classic use-case for class methods. Here is my implementation.

class Vector(object):
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z
 
    @classmethod
    def from_sequence(cls, sequence):
        return cls(*sequence)
 
    @classmethod
    def from_vector(cls, vec):
        return cls(vec.x, vec.y, vec.z)
 
    def __repr__(self):
        return "Vector(%s, %s, %s)" % (self.x, self.y, self.z)
 
if __name__ == '__main__':
    print Vector(1,2,3)
    print Vector.from_sequence([4,5,6])
    print Vector.from_sequence((7,8,9))
    v = Vector(10, 11, 12)
    print Vector.from_vector(v);

Here is the output from running the script:

Vector(1, 2, 3)
Vector(4, 5, 6)
Vector(7, 8, 9)
Vector(10, 11, 12)

I like the classmethod route because it is obvious what the code is doing, it makes it easy to add new from_* methods and keeps the general __init__ method clean.

cw

I came home and was going to whip up a sammich for lunch and then get back to work.

Opened the bread to make the sammich and then had to go and bring the puppy back in the house. Eli had “accidentally” let him out. Oh well. I bring the dog in and return to the kitchen.

I got the ham and swiss laid out on the bread. Then I get called back out because Eli let the dog out again. Bring the dog and Eli in this time.

Back to making the sammich. It was taking longer than planned (something like 8 minutes had elapsed) but it was gonna be gooooood. I proceeded to drop some succulent bean sprouts onto my delicious lunch creation.

Panicked shouts from the living room.

“Curtis and Eli are peeing in the front yard!”

This just won’t do.

I go and bring the boys in and deliver a fatherly lecture on the right and wrong times and places to pee outside. I send them to their room to clean it up. Super dad.

Super hungry dad.

I get back to the kitchen to finish making the sammich.

The dog ate the sammich.

That Sucks.

cw

We tag every release in a MAJOR.MINOR.BUGFIX format. 2.32.2 for example.

Recently I wanted to get a summary of the commit log messages for each of the tagged revisions. Here is the magic command that got me the output that I wanted:

# get all log entries for releases tagged with a 2.31 MAJOR.MINOR
hg tags | grep 2\.31 | cut -c33-36 | xargs -L1 hg log -r

cw

So last night I was going over the chapter that introduces ‘let’, ‘where’, ‘case’ and guards and I wanted to try them out. I contrived a simple situation where I thought I could use them.

I made some Contestants and wanted to see if they were “valid” for some definition of the word. I chose that they had to have a real value for age (not Missing) and be 30 years old or older (I’m pretty sure the game is called “Who Wants to Be a 30-year-old Computer Nerd?”).

Here is my implementation.

-- my parameterized type
data Perhaps a = Is a
               | Missing
                 deriving (Show)
 
-- definition of a Contestant
data Contestant = Contestant {
                    name ::  String,
                    age  ::  Perhaps Int
                } deriving (Show)
 
-- some contestants
contestant1 = Contestant "Christian" (Is 31)
contestant2 = Contestant "Sarah" Missing
contestant3 = Contestant "Curtis" (Is 6)
 
-- the validation logic
contestantValid :: Contestant -> Bool
contestantValid c = 
    case age c of
        Missing      -> False
        Is true_age  -> oldenough true_age
            where
                oldenough a
                    | a >= 30 = True
                    | a < 30  = False
 
-- some code to print the results of running the validation against contestants
main = do
    print $ (contestant1, contestantValid contestant1)
    print $ (contestant2, contestantValid contestant2)
    print $ (contestant3, contestantValid contestant3)

I was pretty proud of myself. I used ‘case’, ‘where’ and guards (not to mention parameterized types - I reimplemented Maybe/Just/Nothing as Perhaps/Is/Missing).

But man, it seemed like a lot of code to do something simple.

As I was falling asleep I realized how I could simplify it. I could use pattern matching to easily handle the Missing case. It would also let me pull the age out of a Contestant with ease too.

Here is the contestantValid function updated to reflect that.

contestantValid :: Contestant -> Bool
contestantValid (Contestant _ Missing) = False
contestantValid (Contestant _ (Is age))
                                | age >= 30 = True
                                | age < 30  = False

Quite a bit simpler. I patted myself on the back. It was less fancy than the first version, but at least I had some guards in there.

Then it hit me. I didn’t need the guards.

contestantValid :: Contestant -> Bool
contestantValid (Contestant _ Missing) = False
contestantValid (Contestant _ (Is age)) = age >= 30

Well, so much for ‘where’, ‘case’ and guards. It sure is a boring function now, but I think the process of how I got there is interesting.

cw

For those of you who don’t know, PiCloud is a cloud computing platform for Python that aims to simplify the task of running code in “the cloud.”

cw

She was born this morning at 7:11am EST. She weighed in at 8lbs. 1oz. and is 19″ long. Mom and baby are doing well. I’m doing great!

cw

The Minimum Viable Tree House

A case-study in what not to do with your software project.