Python for Humans

I was intrigued by the author's library "envoy", which is intended to provide a more intuitive interface to running processes from python. (https://github.com/kennethreitz/envoy)

The back story is that the older APIs that Python comes with -- os.popen and os.system -- are deprecated. Programmers are urged to use the "subprocess" module instead. Although this doesn't have the problems of the original functions, it has a rather arcane interface, in particular if you want to read the output (stdout or stderr) of a subprocess.

"envoy" seems to aim at fixing this, by providing sane defaults and being optimized for the common case. However, these defaults have drawbacks of their own.

1. envoy defaults to keeping the process output in memory, as a giant string. This can be a bad choice with regard to memory usage and performance.

2. You can run several processes in a pipe using ("cat foo | grep bla"). But otherwise as far as I can see, run() ignores regular Shell semantics, such as quotes. I imagine this can lead to unexpected results. The amount of data passed from one process to the next is capped at 10 MB -- recipe for bugs that are hard to find.

3. subprocess.call() accepts an array in the style of ["ls", "-l", "/mnt/My SD card"]. This has obvious advantages over having to deal with escaping shell characters. A good API should preserve this advantage over os.system().

4. The defaults cannot be overridden, and no preperations have been made to allow changing them. Of course this can be changed in the future. However, one of the reasons the subprocess.* API is convoluted is that it allows all kinds of flexibility, much of which is needed in many serious programs. It may be difficult to add this flexibility to envoy at a later stage. The point is that a flexible API is hard.

None of this is to discourage this initiative, which seems to me a much-needed improvement over Python's built-in API. Also, with a version number as low as 0.0.2, there is probably little need to worry about API compatibility.

  oputput = subprocess.check_output(my_command)

Great presentation on a library I've loved (and used) for awhile. However according to slide 42 I need to rewrite the regex module? I'm so busy this week though.

edit: If anyone wants to see a real-world refactor from HTTPLib/2 to requests, I did so with Pysolr here: https://github.com/mattdeboard/pysolr/commit/db63d8910dec42d...

The slides don't fit vertically on my screen, so some of the content is cut off. There's no scroll bar so initially it was difficult to figure how to see the info cut off from the bottom. Chrome's text zoom out didn't work either.

I had to highlight the text and drag downwards in order to see the content. But it was annoying having to do this for every slide with a lot of content.

Otherwise, these libraries seem really useful. Thanks for this.

Wow those libraries are indeed great, amazing compared to the standard libs. Hope they'll be included in the standard libs one day.

Audio is available here:

https://github.com/kennethreitz/python-for-humans

The portion that explains of how subprocess shuns dev/ops guys in the beginning is so true. Perl/Bash colleagues at work would basically ask me how to perform output=`command`. Once they seen subprocess, they would continue writing their script in Bash/Perl.

  from subprocess import check_output as qx

  output = qx(['command', 'arg1'])

  import subprocess
  output = subprocess.check_output('command')

  $output = `command`
  die "failed: $output" if $?

I blame GOF for making Python Standard Libs hard. The patterns described were for an OO system where functions were not first class. Python didn't need to be complicated.

If you have a look at the older libraries, most of them were written in a procedural style. Not only that, it is very amenable to testing in the REPL.

    import smtplib
    s=smtplib.SMTP("localhost")
    s.sendmail("me@my.org",tolist,msg)

note the absence of doers like "Adapters", "Handler", "Manager", "Factory"

If you have a look at the XML library, roughly when "patterns" became popular, this style of thinking infested standard library contributions. It also coincides with a time when camelCased function names crept into the python standard library.

Here's one in xml/dom/pulldom.py:

    self.documentFactory = documentFactory

Once you see this, you know you are in for some subclassing. You can no longer REPL your way to figure out how things work, and you now have to consult the manual.

Here's more pain from libraries of the same era, some of these I'd argue un-Pythonic:

    #xml/sax/xmlreader.py:    
    def setContentHandler(self, handler):

    #wsgiref/simple_server.py:
    class ServerHandler(SimpleHandler):

    #urllib2.py:
    class HTTPDigestAuthHandler(BaseHandler,
       AbstractDigestAuthHandler):

The last example is especially jarring. Abstract classes have a place in strongly typed world to declare interfaces, and help with vtable-style dispatch. In Python, where you have duck-typing and monkey patching, a class that virtually "does nothing" on its own stands out like a guy in a tux at a beach party.

Even logging is infected by the same over-patterning. logging/__init__.py:

    class StreamHandler(Handler)
    LoggerAdapter(someLogger, dict(p1=v1, p2="v2"))

"Managers" - what a pain when plain function handles would have done the job. Does this name even tell you what task the class performs?

    #multiprocessing/managers.py:
    class BaseManager(object)

If anyone remembers, Java had to do OO in a big-style with OO everywhere -- there were no alternatives.

Initially, buttons had to be subclassed just to handle click events, since functions were not first class objects. Then someone came up with a MouseListener interface, which proved too unwieldy to handle a single click. So the MouseEventAdapters came into being.

Therefore, to handle a click in a "pattern" manner involves

an anonymous class

which subclasses MouseAdapter

which implements MouseListener,

which overrides MouseClick.

Publishing how industry solves this problem of "MouseClick" over and over as a pattern [design pattern is a general reusable solution to a commonly occurring problem within a given context in software design] only gives legitimacy to an approach that has dubious wider applicability.

Heavens help the future developers who are forced to do it because it is now recognized as being industrially "good practice" and codified in a reknowned book.

It isn't!

It was a style that was forced by the constraints of a language.

This is neither pythonic nor necessary:

    panel.addMouseListener
    (
      new MouseAdapter ()
      {
        public void mouseEntered (MouseEvent e) {
          System.out.println (e.toString ());
        }
      }
    );

Embracing "foolish, unschooled" thinking, this would be rendered in Python as:

   def mouseEntered(event):
     print event
   panel.mouseEntered = mouseEntered
 

or for multiple event handlers

   panel.mouseEntered.append(mouseEntered)

This style of API again allows effective exploration on the REPL.

    > If anyone remembers, Java had to do OO in a
    > big-style with OO everywhere -- there were no
    > alternatives.

Anybody have a text version of this? I got maybe 30 slides in before I got too annoyed to continue.

This presentation brings up a tangential point that has always confused me: how error-prone is starting a subprocess, really?

I agree with the author's goals of making common tasks easier and more obvious. urllib2 is an easy target, as it was added to the standard library over a decade ago, long before REST was something people talked about. The best tools for packaging, versioning, and testing have always been a bit ambiguous in any language, including Python.

However, the author points out something that has always bothered me about Python: it is way harder to start a subprocess with an external command in Python than almost any other language. This has been true whether using sys or os or even subprocess, which is quite recent.

I always felt that this had something to do with the constant warnings in the documentation about how a pipe between the subprocess and the Python process might fill and cause the subprocess to block. Or how running the program through shell rather than exec or something might cause some sort of security issue. Are these real issues that other languages ignore in the name of user convenience, or has Python just never been able to make the right API (as the author seems to argue)?

On installing python -- the most practical way to work with it, is to have a moderately recent os-level python install and then build all the other python versions from source if required -- https://github.com/collective/buildout.python

After that use virtualenv with virtualenvwrapper.

Hangs on the first slide unless I'm doing something really wrong.

I wish that the developers of requests module stop changing it's API -- code that was working just fine with 0.6.4 suddenly began finding missing methods in version 0.8.5

seems the guy nailed down many issues i have had in the past :)

This makes a lot of good points, but some bad ones.

Esp. the "installing python" one. Just use your package manager to install all the versions you need.

And for "Packaging and Dependencies", just use pip.

The Python standard library has gotten worse over time, as it got loaded up with more and more features, obfuscating the common use cases. The irony now is that to do simple, everyday things (like http requests) you are now better off installing a third party package like "requests" than using the standard library. So much for "batteries included."

The standard library needs a reboot. Why not do it in Python 3? Nobody's using it yet anyway ;-)

I can confirm that the python subprocess api is a pain to use and also documented poorly. I recently had to use (no choice) python 2.5.x to write a script that extensively called external programs and ran into several problems. It strange that a language such as python which I find so easy to use in many cases does not already have a good as in simple, safe and well documented subprocess api.

I guess I agree things could be simpler, although the cries of "garbage!" were a bit much. I wrote a wrapper function around urllib2 about 5 years ago and haven't looked back.

It's really inexcusable that in 2012 (or 1992 even) a language that otherwise is well-suited for internet programming does not come with a first class httpclient.

What did they use to make the presentation?

The name says it all. This is much more readable compared to urllib2

yes please and thank you muchly. puts away yak shaving contraption

Why don't you just switch to Ruby?

I like the presentation, but for me it just underlines how it's smart to stay far away from Python. It's great that it's improving, but many other languages have a much better library/API situation than Python has had for years already. Will Python catch up fast enough?