tl;dr don't use metaclasses
Metaclasses in Python
11–20 of 32 posts
Re: Metaclasses in Python
#12Re: Metaclasses in Python
#13Earlier quoted context omitted.
What prototyping stuff is missing in Python?
Well in JS you could do String.prototype.fnName = function(){}; to add fnName as a method to all strings even if they are already instantiated, is that possible with Python?
No. In Python, primitives prototypes aren't mutable.
Pythonic code is meant to be modular, which leads to extreme explicitness. For example, while `string_utils.capitalize(my_string)` is a bit more longwinded than `my_string.capitalize()` in JS or even `my_string.capitalize` in Ruby, this minor bit of ongoing typing overhead will yield a beautifully explicit codebase over time.
By comparison to other dynamic languages like Javascript and Ruby, Python codebases scale effortlessly and beautifully and without the limitations of "How much freaking online documentation do I need to read to locate the imperative code that's actually running?". While that sort of necessity is the mother of beautiful documentation design you'll find in (and the ascendance of great doc writers among) the Ruby and Javascript communities, I believe its technical risks outweight those tertiary benefits. Extensive and explicit documentation is a necessity that Python does away with entirely, because the code is itself explicit. Python does this at the cost of some convenience and developer happiness. Whereas I give Ruby a 10/10 in the "day-to-day comfort" scale for us dynamic developers, and where Javascript has perhaps a 7/10, I place Python squarely at 9.
By comparison to static high-level languages like Java and ActionScript (and some dynamic languages like Perl and PHP) classical, Python is of course a breeze to write.
As a result of this explicitness, when using a Python framework, you'll rarely need consult documentation or outside advice just to find out what's happening: It's all in the code, and you just have to follow the imports and method calls down the stack.
This modularity also means Pythonists totally avoid collisions in namespacing: whereas the global namespaces of Ruby and Javascript tend to lead to headaches in large codebases, Python is built on the idea of package-as-namespace (directories) and module-as-namespace (.py files)
Re: Metaclasses in Python
#14Python is a very cool language. I wonder if using __metaclass__ in some fashion would allow for javascript-like prototyping.
What prototyping stuff is missing in Python?
In javascript you can modify an object
var foo = Object();
foo.blah = function(x,y) { ... };
foo = object()
foo.blah = lambda x,y: ...
edit: ah, as someone noted, the second code snippet should be something like foo = Foo() where
class Foo(object):
passRe: Metaclasses in Python
#15Earlier quoted context omitted.
Of course. http://codepad.org/3fWweqje
Can it be done for built-in types?
No, it can't, at least not in CPython. You can import __builtin__ and clobber str, for instance, but that will not affect the behavior of existing strings, nor will it influence the behavior of string literals.
It's something that can be done in Ruby and JavaScript, but I believe it was never allowed in Python in the name of performance (you incur overhead when every number is Fixnum with the same lookup rules as any other instance, for example), and also safety.
Re: Metaclasses in Python
#16Earlier quoted context omitted.
What prototyping stuff is missing in Python?
In javascript you can modify an object var foo = Object(); foo.blah = function(x,y) { ... }; But in python, that doesn't quite work. You can only do foo = object() foo.blah = lambda x,y: ... lambdas are a bit more limited as they are restricted to one line, and you can't have print statements, which makes complicated expressions rather ugly. edit: ah, as someone noted, the second code snippet should be something like…
That's not quite right. While you're correct that lambda statements are restricted, I have never actually seen a lambda expression used to extend an object. Instead, you use a named function, which has none of these restrictions:
foo = object()
foo.name = "Hi thar"
def hello(self):
print "Hello, I'm %s" % self.name
foo.hello = hello
Re: Metaclasses in Python
#17Python is a very cool language. I wonder if using __metaclass__ in some fashion would allow for javascript-like prototyping.
It's interesting that you bring up Javascript. I've just started working in Python full-time recently and have noticed the OOP stuff "feels" a little like working with objects in Javascript. Specifically around how the built in super works and more generally how methods are defined on classes to begin with. In Javascript you can do interesting things with call and apply on function objects to share functionality between two objects that work the same. Similarly in Python, since the methods defined in a class are also available on the class object itself, you can use them outside of an instance and simply give them a self to operate on. The class object almost acts like the prototype object on the function object you're using as a constructor. I know the two systems are different, but they do "feel" real similar. Most people I mention this to seem a bit surprised and have that "never thought of that" look on their face. Anyone else witness this?
Re: Metaclasses in Python
#18Earlier quoted context omitted.
In javascript you can modify an object var foo = Object(); foo.blah = function(x,y) { ... }; But in python, that doesn't quite work. You can only do foo = object() foo.blah = lambda x,y: ... lambdas are a bit more limited as they are restricted to one line, and you can't have print statements, which makes complicated expressions rather ugly. edit: ah, as someone noted, the second code snippet should be something like…
That's not quite right. While you're correct that lambda statements are restricted, I have never actually seen a lambda expression used to extend an object. Instead, you use a named function, which has none of these restrictions: foo = object() foo.name = "Hi thar" def hello(self): print "Hello, I'm %s" % self.name foo.hello = hello This is, in fact, one of the reasons why the self parameter is explicit.
The above doesn't really work as the builtin object class doesn't allow extra attributes. Still any pure Python class does by default:
class Foo(object):
pass
foo = Foo()
foo.name = ...
def hello(self):
...
foo.hello = helloRe: Metaclasses in Python
#19Is it me or the first metaclass example doesn't work.
I'm using Python 2.7.2+ on Debian and I can't get __metaclass__ force upper case attributes inside of a 'module'
Re: Metaclasses in Python
#20Earlier quoted context omitted.
In javascript you can modify an object var foo = Object(); foo.blah = function(x,y) { ... }; But in python, that doesn't quite work. You can only do foo = object() foo.blah = lambda x,y: ... lambdas are a bit more limited as they are restricted to one line, and you can't have print statements, which makes complicated expressions rather ugly. edit: ah, as someone noted, the second code snippet should be something like…
That's not quite right. While you're correct that lambda statements are restricted, I have never actually seen a lambda expression used to extend an object. Instead, you use a named function, which has none of these restrictions: foo = object() foo.name = "Hi thar" def hello(self): print "Hello, I'm %s" % self.name foo.hello = hello This is, in fact, one of the reasons why the self parameter is explicit.
you may not have seen it, but it does work:
n [8]: class Foo(object): ...: pass ...:
In [9]: blah = Foo()
In [10]: blah.asdf = lambda x: x
In [11]: blah.asdf(3) Out[11]: 3
In [12]: blah.asdf(5) Out[12]: 5