Vinod Kurup

Hospitalist/programmer in search of the meaning of life

Using Dynamic Queries in a CBV

Let’s play ‘Spot the bug’. We’re building a simple system that shows photos. Each photo has a publish_date and we should only show photos that have been published (i.e. their publish_date is in the past).

models.py
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class PhotoManager(models.Manager):
    def live(self, as_of=None):
        if as_of is None:
            as_of = timezone.now()
        return super().get_query_set().filter(publish_date__lte=as_of)

And the view to show those photos:

views.py
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class ShowPhotosView(ListView):
    queryset = Hero.objects.live()

Can you spot the bug? I sure didn’t… until the client complained that newly published photos never showed up on the site. Restarting the server fixed the problem temporarily. The newly published photos would show up, but then any photos published after the server restart again failed to display.

The problem is that the ShowPhotosView class is instantiated when the server starts. ShowPhotosView.queryset gets set to the value returned by Hero.objects.live(). That, in turn, is a QuerySet, but it’s a QuerySet with as_of set to timezone.now() WHEN THE SERVER STARTS UP. That as_of value never gets updated, so newer photos never get captured in the query.

There’s probably multiple ways to fix this, but an easy one is:

views.py
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class ShowPhotosView(ListView):
    def get_queryset(self):
        return Hero.objects.live()

Now, instead of the queryset being instantiated at server start-up, it’s instantiated only when ShowPhotosView.get_queryset() is called, which is when a request is made.

Some Emacs Posts

A few cool Emacs posts have flown across my radar, so I’m noting them here for that time in the future when I have time to play with them.

Pygments on Arch Linux

I wrote my first blog post in a little while (ok, ok… 18 months) yesterday and when I tried to generate the post, it failed. Silently failed, which is the worst kind of failure. I’m still not sure why it was silent, but I eventually was able to force it to show me an error message:

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/home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/pygments.rb-0.3.4/lib/pygments/popen.rb:354:in `rescue in get_header': Failed to get header. (MentosError)
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/pygments.rb-0.3.4/lib/pygments/popen.rb:335:in `get_header'
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/pygments.rb-0.3.4/lib/pygments/popen.rb:232:in `block in mentos'
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/timeout.rb:68:in `timeout'
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/pygments.rb-0.3.4/lib/pygments/popen.rb:206:in `mentos'
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/pygments.rb-0.3.4/lib/pygments/popen.rb:189:in `highlight'
  from /home/vinod/dev/kurup.org/plugins/pygments_code.rb:24:in `pygments'
  from /home/vinod/dev/kurup.org/plugins/pygments_code.rb:14:in `highlight'
  from /home/vinod/dev/kurup.org/plugins/backtick_code_block.rb:37:in `block in render_code_block'
  from /home/vinod/dev/kurup.org/plugins/backtick_code_block.rb:13:in `gsub'
  from /home/vinod/dev/kurup.org/plugins/backtick_code_block.rb:13:in `render_code_block'
  from /home/vinod/dev/kurup.org/plugins/octopress_filters.rb:12:in `pre_filter'
  from /home/vinod/dev/kurup.org/plugins/octopress_filters.rb:28:in `pre_render'
  from /home/vinod/dev/kurup.org/plugins/post_filters.rb:112:in `block in pre_render'
  from /home/vinod/dev/kurup.org/plugins/post_filters.rb:111:in `each'
  from /home/vinod/dev/kurup.org/plugins/post_filters.rb:111:in `pre_render'
  from /home/vinod/dev/kurup.org/plugins/post_filters.rb:166:in `do_layout'
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/jekyll-0.12.0/lib/jekyll/post.rb:195:in `render'
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/jekyll-0.12.0/lib/jekyll/site.rb:200:in `block in render'
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/jekyll-0.12.0/lib/jekyll/site.rb:199:in `each'
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/jekyll-0.12.0/lib/jekyll/site.rb:199:in `render'
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/jekyll-0.12.0/lib/jekyll/site.rb:41:in `process'
  from /home/vinod/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/jekyll-0.12.0/bin/jekyll:264:in `<top (required)>'
  from /home/vinod/.rbenv/versions/1.9.3-p286/bin/jekyll:23:in `load'
  from /home/vinod/.rbenv/versions/1.9.3-p286/bin/jekyll:23:in `<main>'

Professor Google tells me that this happens when you try to run the pygments.rb library in a Python 3 environment. (pygments.rb is a Ruby wrapper around the Python Pygments library). The fix is to run the code in a Python2 virtualenv. I guess the last time I updated my blog, Arch still had Python2 as the system default. No, I don’t want to check how long ago that was.

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$ mkvirtualenv -p `which python2` my_blog
(my_blog)$ bundle exec rake generate

So now I’m running a Ruby command in a Ruby environment (rbenv) inside a Python 2 virtualenv. Maybe it’s time to switch blog tools again…

How to Create Test Models in Django

It’s occasionally useful to be able to create a Django model class in your unit test suite. Let’s say you’re building a library which creates an abstract model which your users will want to subclass. There’s no need for your library to subclass it, but your library should still test that you can create a subclass and test out its features. If you create that model in your models.py file, then Django will think that it is a real part of your library and load it whenever you (or your users) call syncdb. That’s bad.

The solution is to create it in a tests.py file within your Django app. If it’s not in models.py, Django won’t load it during syncdb.

tests.py
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from django.db import models
from django.test import TestCase

from .models import MyAbstractModel

class MyTestModel(MyAbstractModel):
    name = models.CharField(max_length=20)

class AbstractTest(TestCase):
    def test_my_test_model(self):
        self.assertTrue(MyTestModel.objects.create(name='foo'))

A problem with this solution is that I rarely use a single tests.py file. Instead we use multiple test files collected in a tests package. If you try to create a model in tests/test_foo.py, then this approach fails because Django tries to create the model in an application named tests, but there is no such app in INSTALLED_APPS. The solution is to set app_label to the name of your app in an inner Meta class.

tests/test_foo.py
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from django.db import models
from django.test import TestCase

from ..models import MyAbstractModel

class MyTestModel(MyAbstractModel):
    name = models.CharField(max_length=20)

    class Meta:
        app_label = 'myappname'

class AbstractTest(TestCase):
    def test_my_test_model(self):
        self.assertTrue(MyTestModel.objects.create(name='foo'))

Oh, and I almost forgot… if you use South, this might not work, unless you set SOUTH_TESTS_MIGRATE to False in your settings file.

Comments and corrections welcome!

Peanut Allergy, a Personal Clinical Review

A Vignette

A 22-month-old previously healthy girl was brought to the emergency room in respiratory distress. She had been well until the day of presentation. Shortly after disembarking from an airline flight, she developed cough with one episode of emesis. En route to the emergency room, her breathing became labored and she developed hives. She was diagnosed as having an allergic reaction and treated aggressively with IV methylprednisolone, IV diphenhydramine, inhaled albuterol, and oxygen. Mechanical ventilation was not required and her symptoms improved rapidly. She was monitored in the emergency room for an extended period and discharged in stable condition with a prescription for diphenhydramine and methylprednisolone solutions. No specific allergen was identified. She was seen by a pediatric allergist where she was found to have a 6 mm response to peanut on skin prick testing, and a peanut-specific IgE level of 52 kU/L. She was diagnosed as having severe peanut allergy and avoidance was strongly recommended.

Discussion

Peanut allergy affects between 0.4 and 2.0% of children. Prevalence has been increasing in recent years, but the reason for this increase is not well understood. If reactions are identified immediately, treatment is usually effective. Despite this, about 200 people die each year from peanut reactions. Biphasic reactions, in which the initial reaction subsides, but then recurs within a few hours, are common. The second portion of a biphasic reaction is often less treatable. Because of this risk, patients should be monitored for at least 4 hours after a reaction. Patients who successfully treat their own reactions with subcutaneous epinephrine should still seek medical attention, again to monitor for biphasic reactions.

The gold standard for diagnosis is a double blind placebo controlled food challenge (DBPCFC), but this is rarely done. A diagnosis can usually be reliably made with clinical history, skin prick testing and a peanut-specific IgE blood test (psIgE). A skin prick test involves pricking the skin with a needle that is exposed to peanut extract and measuring the allergic rash that develops. The psIgE blood test measures the amount of antibody against peanut proteins. The higher it is, the more allergic the patient is. A positive skin test or psIgE is not sufficient for the diagnosis. Some type of reaction to peanuts is required (either spontaneous, or via monitored food challenge).

Peanut allergy was previously felt to be permanent, but recent evidence documents that 20% of children outgrow their allergy. More than 60% of patients with an psIgE level of less than 5 passed a food challenge. IgE levels correlate inversely with likelihood of resolution, but this correlation is by no means perfect. Some patients with undetectable psIgE levels will fail a challenge, while some patients with very high psIgE levels will pass. One expert recommends waiting until the age of 4 to consider a challenge, and to use a cutoff psIgE level of less than 5 kU/L in most patients. Recurrence of peanut allergy after resolution is rare, but is associated with the lack of regular peanut intake after resolution of the allergy.

Controversy exists regarding risk factors for developing peanut allergy. The AAP initially recommended that peanut not be introduced early in a child’s life, but rescinded that advice given the lack of evidence behind it. This leaves a void for parents as there is no recommendation to withold or introduce peanuts to toddlers. It is also not clear if maternal peanut intake during pregnancy or breast feeding increases the risk of developing peanut allergy in children. Studies have been conflicting, so more definitive studies are underway. No evidence-based advice can be given one way or the other to pregnant or breastfeeding women, but at the minimum, there is no good evidence to support avoiding peanuts.

The mainstay of treatment is education. Parents should be taught not only how to identify peanuts in foods, but to identify situations which are high-risk for exposure to peanuts. Avoidance of buffets, ice-cream parlors, and bakeries is recommended because the risk of cross-contamination in those setting is high. Religious reading of food labels is important. Patients who are allergic to peanuts are often also allergic to tree nuts, despite the dissimilarity in those food types. Highly refined peanut oil is usually NOT a risk, but cold-pressed peanut oil is not safe.

Most importantly, an epi-pen should be carried at all times and extra doses should be left at daycare facilities and schools. All caregivers should be comfortable giving an epi-pen injection and this should be provided at the first sign of a systemic allergic reaction. Waiting to give diphenhydramine or other therapies a “chance to work” can be dangerous. Follow a Food Allergy Action Plan Over 80% of people who died from allergic reactions were not given appropriate instructions on how to prevent future reactions. Most mortality occurs because of late treatment. If you take one thing away from this long post, keep an epi-pen handy and use it liberally.

Trials are underway investigating various oral-based immunotherapies, but none are strongly recommended at this point. Injection immunotherapy has not been found to be useful.

Personal Comment

So, the 22 month old above is Anika and she gave us the scare of our life a couple years ago. The scary part now is that we never saw her actually eat a peanut, so we’re especially vigilant now. Fortunately, Anika seems to understand the importance of avoiding peanuts. When she was diagnosed, I did some research but never documented my notes. She recently had another allergist appointment, so I reviewed those notes and any new data. I figured I should write up my notes so I wouldn’t have to review this again from scratch every year. I didn’t intend it to come out so ‘medical-speaky’, but there it is. Anika and Kavi are sitting next to me as I write this and they want to give some information to you, too.

Kavi says:

“Always ask at the restaurant if things have peanuts and make foods at home that do not have peanuts. Always check if there are peanuts in every food you eat.”

Anika says:

“We might have to go to the doctor if we eat peanuts. If I eat peanuts by accident, my teacher will give me a Happy-Pen (epi-pen).”

References

  1. Sampson HA. Peanut Allergy. N Engl J Med, 2002;346;1294-8. http://www.nejm.org/doi/full/10.1056/NEJMcp012667
  2. Fleischer DM, et. al. The natural progression of peanut allergy: Resolution and the possibility of recurrence. J Allergy Clin Immunol, 2003;112;183-9. http://www.jacionline.org/article/S0091-6749(03)01251-X/abstract
  3. O’B Hourihane J. Peanut Allergy. Pediatr Clin N Am, 2011;58;445-57. http://www.pediatric.theclinics.com/article/S0031-3955(11)00006-X/abstract
  4. Rinaldi M, et. al. Peanut allergy diagnoses among children residing in Olmstead County, Minnesota. J Allergy Clin Immunol, 2012:130:945-50.
  5. Burks AW. Early peanut consumption: postpone or promote? J Allergy Clin Immunol. 2009:123:417-423. http://www.jacionline.org/article/S0091-6749(08)02436-6/fulltext

RSS URL:

Here’s a medline feed URL that I monitor which should show new studies or reviews related to this topic.

Priorities and GTD

Pre-GTD task management systems recommended that you assign a priority to each task. When looking at a list of tasks, you then choose the ones with the highest priority. David Allen (author of GTD) includes ‘priority’ as one of the things that you should consider when you are deciding what to do, but only after you have considered context, time available, and energy available. Which makes sense. As Merlin Mann says, it doesn’t matter if mowing your lawn is your highest priority when you’re sitting in an airplane. Only if you’re in the right context with the right amount of time and the right amount of energy, should you consider which of the remaining available tasks is highest priority.

The risk, though, is that there are so many little ‘easy’ tasks that I could do instead of important, ‘hard’ tasks. I’m also really good at convincing myself that I don’t have the energy available to do harder tasks when I really should. So when I get back into GTD, I process all the things hiding in the back of my brain which leads to the creation of lots of simple tasks. Google this, buy that, throw away the other thing. But the two things that I really want to be doing more are writing and programming. Both of those take blocks of uninterrupted time and effort, so I end up subconsciously shying away from them. I actually feel better about it, too, because now I have all these other tasks that I can do instead. So, I feel productive, even when ignoring the more important things that I should be doing. I’m moving more efficiently, but in the wrong direction.

I’ve avoided that a little bit this time by doing a little bit of prioritization each night before I go to bed. I look through my todo list and pick about 3-5 things that I want to get done the next day. I tag those with the ‘today’ tag and then the next morning I look at that list and start working on those things before doing anything else. I’ve heard this trick from a lot of people before, so I don’t remember who I heard it from first.

So far it’s been effective for me. The problem always starts again though when I go back on service, because then I can only really focus on one thing and that is getting my work done at the hospital. We’ll see if I can somehow figure out a way to get at least one non-work thing done on each of those days.

GTD Again?

Thanks to Merlin Mann’s recent series of podcasts on GTD, I’m getting back on the GTD wagon (for about the 100th time). I read Getting Things Done in 2002 and have been trying to implement it ever since. I’ve had periods where I followed it religiously for a month at a time, but it’s never completely stuck. From what I hear, this is normal and I accept it now. This won’t be the last time that I restart GTD. Even when I’m not actively using GTD, I think in GTD terms, thinking about next actions and projects and ruthlessly placing projects on the backburner. I just have trouble sticking with the entire GTD workflow, especially the ubiquitous capture (I trust my brain too much) and the weekly reviews (they just seem too painful). I’ve used every online and offline tool in the book, too. Despite not doing so great with GTD, I’d say my life is an overwhelming success.

So why try again? Each time I have gotten back on the bandwagon, I’ve had a surge in productivity. I wrote more. I made some clear decisions on issues that I had been procrastinating on. I wouldn’t be surprised if each of my major life improvements over the past 10 years was associated with a time where I got back on the GTD wagon. I can’t prove that, because I don’t keep enough details, but I always feel better and think clearer when I’m on GTD.

But, I’ve failed so many times… what kind of hubris makes me think I’ll get it this time? “Fall down seven times, get up eight.” Parenthood has taught this to me. Of course we don’t expect our kids to learn each skill the first time they try. We expect that walking and reading and writing and bike-riding will take time, multiple efforts, and multiple failures. But eventually they will get it. Yet, with each failure, kids get upset (to varying degrees). The answer at that point is not to tell them to give up. Yet, that’s what I do to myself. So, I’m going to treat myself like a kid and let myself fail, understanding that each failure will get me closer to that one time where I will succeed.

Writing Paralysis

I think twitter and facebook stunt my writing. The more that I read tweets, the more self-conscious I get when I write. Not that my writing is all that original, but I still feel that it is something that comes from me. I worry so much about presenting ideas of others as my own without even remembering that I had seen the ideas before. In the old days, the things that I read were long form. Books, articles, maybe blog posts. They each left an impression on me, maybe good, maybe bad, but I had a sense where they were coming from. When I wrote I could reliably say where the inspiration for my idea had come from. Now with twitter, I see all these ideas and thoughts, some of which are interesting, but few of which leave a lasting impression on me. So now I have this fear that the things that I am considering my own ideas are actually the ideas of others, improperly attributed. Of course, that’s silly. No idea is original, but it’s just serious enough to stifle my writing. Because, as a writer, I am the scaredy-cat of all scaredy-cats and any criticism of my writing bites deep to the heart. I also know that this is something that I just need to get over. You cannot express ideas without inviting criticism. I also don’t just want to let criticism fall off me. I want to learn from critics. But saying all that doesn’t necessarily make it any easier. Even writing this post, I feel like a fraud. I’ve said that I’m a writer a couple times up there, but what evidence do I have to support that claim? Most of my blog posts are so simple or technical or repetitive. Nothing that would strike me as an ‘original idea’. This whole post makes it seem like I am more important or better than I really am.

What is my point with all this? I want to be able to write. I know that there are people out there who could benefit from things in my brain (maybe not a lot). I know that there are people out there who can help me (in ways that I can’t even expect). So, I want to write and I want it to be freely flowing and I don’t want to force myself to feel that what I am writing is useful or authoritative or perfect. I apologize to you, Vinod, my biggest critic. I know that you are cringing inside because you’re thinking that anyone who reads this will think that you are so full of yourself for even considering yourself a writer. But, this is my first step in letting my guard down to get this blog flowing. Because I do believe that writing is healthy and will make me a better person.

In addition to committing to blogging more frequently, I’m also going to try a twitter hiatus for a week and see what happens.

Steal My Web App Idea

Now that we have 3 kids, I’m always internally comparing one to the other. Sure that’s not healthy and will probably breed resentment in them during their teenage years, but what’s a bad parent to do?

I need an app that takes 2 kids’ birthdays and then shows me what day it was when the older kid was the age that the younger kid is now. Wow, that’s a confusing sentence. In simpler terms, if I have a 3-year-old and a 1-year-old, then the app would show me the day that the 3-year-old was 1.

Then, I want the app to take that date and go through all of my personal files (photos, notes, twitter stream, facebook stream, etc.) and show me what was happening in our life around that day.

In short, it would show a snapshot of what our life was like when the older kid was the age that the younger kid is now. I’d call it WhenIWasYourAge or something catchy like that.

Seems simple enough to build, but I don’t see myself getting to it anytime soon. Smart people say to set your ideas free and if they love you they’ll come back. (Wait, I think I’m mixing metaphors here).

How to Change the Rear Tire on a 12 Inch Bicycle

I’m not the handiest guy in the world, so I’m pretty proud that I was able to fix a flat tire on Kavi’s 12” bicycle.

DISCLAIMER: I have almost no idea what I’m doing. Follow my instructions at your peril.

I’ve fixed flats before, but it’s been a long time. Fixing it on such a tiny bike was a lot harder than I thought. The manual makes it seem so simple. “Remove the wheel, take the tube out, patch the tube and then replace the tube and tire.” Right… I got stuck at step 1. Removing the wheel isn’t straightforward. Most of the videos available online focus on adult-sized bikes which have quick-release tires. You pull a release lever back and then pull the tire off. Kids bikes have bolts instead of quick-release levers. You have to loosen the bolts, but you don’t have to take the bolts completely off. You also have to take off the plastic chain guard that prevents kids from rubbing their legs against the chain. Finally, there’s this little metal piece that attaches the wheel to the frame. I think it’s there for the pedal-brake to work. In any case, you have to take that off. Once the wheel is loosened, you have to unhook the wheel from the chain. This is also easier in big bikes which have multiple gears, making it possible to place the chain in a position to easily get it off the wheel. In a kid’s bike, it’s harder. Once I loosened the bolts on both sides of the wheel, I pushed the wheel a little forward, which loosened the chain and made it easy to release from the chain.

The next difficulty was getting the tube out of the tire. All the videos make this seem so easy. Place a tire lever in the tire to pull it over the rim. Put another tire lever in and slide it around the rim to get the rest of the tire out. Kavi’s tires are so tight that this was impossible. I could get the first tire lever in, but when I put the second tire lever in, it was completely immovable. There was no way that I was going to be able to slide it anywhere. I finally found a website where they recommended using a third tire lever. Put the first two in a few inches apart. Then put the third one in another inch away from the second one. Finally remove the middle one and move it an inch past the third one. In this way, I was able to get the tire off eventually.

I then got the tube out and found the leak. I placed a patch on it, but when I reinflated the tire, the leaking air just pushed it’s way through the patch. I tried roughing up the area with the roughing tool, but that didn’t help. Finally, I resorted to the layman’s favorite tool: duct tape. I placed the patch, then wrapped it with duct tape. It’s held together for a few months now.

Hopefully Kavi will graduate to the next size bike before my handiwork breaks down. On another note, Kavi was easily able to pick up the basics of riding a bike by following the REI guide. I took off the training wheels and the pedals, which allowed him to get the feeling of balancing the bike without having to keep his feet on the pedals. Once he got that part, it was relatively easy for him to balance with the pedals on. He also proved to be a much tougher kid than I was. Everytime he fell, he got right back on.