Tao of the Machine

Contracts

Let's dissect this one, for example. Page 1: "In a weakly typed system like Python, variables don't have types." Eh? Since when is Python weakly typed? Now this issue has come up before in c.l.py, and some people like to say "weak typing" when others mean "dynamic typing", and "strong typing" for "static typing". No biggie, but sentences like this are just plain wrong, and Guido doesn't do anything to correct him. IMHO, the translation should be:

original text	translation
"In a weakly typed system like Python..."	"In a dynamically typed system like Python..."
"...variables don't have types."	"...you don't have to declare variables."

Page 3: "Doesn't the lack of type declarations on method parameters make it hard to change code, especially in public libraries? People can define a class and overload * to mean anything. They can then pass in an instance of their class to times, and use the times method in a way the method's designer never imagined, because * means something completely different."

Yes, that's the whole point. Who says you can use code only for things that its designer imagined? Signature-based polymorphism makes it *easier* to change code. A lot easier than in Java or C++ (or Pascal, for that matter) where all the declarations are in the way. You can build up a working system in no time (prototyping, something Python was originally intended for), but also change it quickly relatively painlessly.

(I am not just guessing this. Ever try refactoring a Python program, versus a Delphi program? :-)

(And of course, I can overload a method in Java, C++, etc, to mean something different and unexpected. At worst, that is just an example of bad design, and not a flaw in the language.)

Overall, the interviewer doesn't seem to know a lot about Python or dynamic languages in general. But maybe users of statically typed languages are the target audience for these articles, dunno. Still, spending a day in comp.lang.python (not python-dev! ;-) would probably teach you more about Python than this series.

Try to explain this to an American...

At the moment the euro (€) has roughly the same value as the US dollar. Hmm, I wonder what Americans would say if they had to pay $2000 or $3000 to get their license... not to mention, take weeks or months (or years, in my case :-) of drivers lessons, followed by a very strict and difficult exam, where only around 35% of the candidates actually pass. Once you have your prized license, you get rewarded by the driving experience in the Netherlands: lots and lots of traffic signs (and you are expected to see them all at once), lots of narrow streets, roundabouts, speed bumps and 30 km/h zones, and interesting rules, like, bikes coming from the right have the right of way. I am not making this up.

Also, most cars are stick-shift, making it all the more fun. And don't expect kids on bikes and mopeds (or anywhere else, for that matter) to know the traffic rules. They do whatever they want, cross red lights, etc. And if you accidentally hit them with your car, it's your fault, no matter what they did.

But if you think that is ridiculous, you don't know the price of gasoline in the Netherlands yet... ^_^

Tuesday roundup

On a totally unrelated note: everybody knows about Fredrik's decision by now. He makes some valid points. I mean, it's easy for me as somebody who doesn't develop Python to stand by the side and criticize, but what I saw recently was... worrisome. As /F says, no talk about the next Python 2.3 release; instead, abhorrent syntax changes are discussed with zeal. Especially this "thunk" (what the hell is that anyway?) stuff:

foo = bar:
    x = 1

A quirk in Kaa's embedded Python

<#
    print 'whee'
#>

<#
    print 'foo'
    print 'bar'
#>

<#
print 'foo'
    print 'bar'
#>

<#
print 'foo'
print 'bar'
#>

Spijs op je lijf

Including Kaa, I've seen three programs that use embedded code in HTML (the other two are Vellum and Spyce), and they all use a different syntax. Kaa uses <% %> and <# #>. Vellum uses <% %> as well, but allows the "=" syntax (IIRC) and weaving HTML and Python, using the word "end" as a dedent replacement. Spyce uses [[ ]] and allows "=" and weaving as well, and uses { and }. To illustrate, here's an example from the Spyce site:

<html><body>
  Hello [[print 'world!',]]
  [[ for i in range(10): { ]]
    [[=i]]
  [[ } ]]
  <br>
  Color chart:<br>
  <table border=0 cellpadding=0 cellspacing=0>
  [[ cols = ['22', '66', 'AA', 'EE' ]
     lencols3 = 5*len(cols)
     for r in cols: { ]]
       <tr>
       [[ for g in cols: { ]]
         [[ for b in cols: { ]]
           <td width=5 height=[[=lencols3]] 
           bgcolor="#[[=r+g+b]]"></td>
         [[ } ]]
       [[ } ]]
       </tr>
  [[ } ]]
  </table>
</body></html>

[[ for i in range(10): { ]]
    [[=i]]
[[ } ]]

<#
for i in range(10):
    print i,
>#

For fun and profit, here's how you would write the example above in Kaa:

<#
print "Hello world!"
for i in range(10):
    print i,
print "<br>"
print "Color chart:<br>"
print "<table border=0 cellpadding=0 cellspacing=0>"
cols = ['22', '66', 'AA', 'EE']
lencols3 = 5 * len(cols)
for r in cols:
    for g in cols:
        for b in cols:
            print "<td width=5 height=", lencols3,
            print "bgcolor=" + "#"+r+g+b, "></td>"
    print "</tr>"
print "</table>"
#>

Hello world! 0 1 2 3 4 5 6 7 8 9
Color chart:

(Note: Of course, I'm only comparing the ways of embedding Python here, not the capabilities of both systems... Spyce is a "Python Server Pages" system, Kaa is a client-based weblogging tool, two entirely different beasts.)

Try this one for size

class SizerPanel(wxPanel):

    def __init__(self, parent, direction, *args, **kwargs):
        self.id = wxNewId()
        assert direction in (wxHORIZONTAL, wxVERTICAL)
        wxPanel.__init__(self, parent, self.id, *args, **kwargs)
        self.sizer = wxBoxSizer(direction)
        self.finalize()

    def finalize(self):
        self.sizer.Fit(self)
        self.SetAutoLayout(true)
        self.SetSizer(self.sizer)
        
    def Add(self, *args, **kwargs):
        """ Shortcut to replace sizerpanel.sizer.Add(). """
        self.sizer.Add(*args, **kwargs)

p = SizerPanel(parent, wxVERTICAL) # or wxHORIZONTAL
p.Add(some_component)
p.Add(another_component)

(Note: Yes, in spite of all my Swing ramblings, I decided to code the first version of the Firedrop GUI in wxPython. If it doesn't work well on the Mac, I can always code another front-end in Swing. But that seems a lot of work, mostly in finding out how all the components work, and I wanted to get going.)