Trying to learn about metaclasses

[Steven W. Orr]

I have been doing a lot of reading. I'm starting to get it. I think it's
really cool as well as dangerous, but I plan on being respectful of the
construct. I found a web page that I found quite readable.

http://cleverdevil.org/computing/78/

So, I tried to run the example code (below), and I get AttributeError. If
someone can get me over this hump, I'd be grateful. It complains that Person
has no _fields attribute, but the print hello I have in EnforcerMeta doesn't
happen either. Am I doing something wrong?

I'm running python 2.6.2

Here's the error:

585 > ./meta1.py
Traceback (most recent call last):
File "./meta1.py", line 38, in <module>
swo.name = 'swo'
File "./meta1.py", line 27, in __setattr__
if key in self._fields:
AttributeError: 'Person' object has no attribute '_fields'


And here's the code:

#! /usr/bin/python
# http://cleverdevil.org/computing/78/
class Field(object):
def __init__(self, ftype):
self.ftype = ftype

def is_valid(self, value):
return isinstance(value, self.ftype)

class EnforcerMeta(type):
def __init(cls, name, bases, ns):
# Store the field definitions on the class as a dictionary
# mapping the field name to the Field instance.
print 'Hello'
cls._fields = {}

# loop through the namespace looking for Field instances.
for key, value in ns.items():
if isinstance(value, Field):
cls._fields[key] = value

class Enforcer(object):
# attach the metaclass
__metaclass__ = EnforcerMeta

def __setattr__(self, key, value):
if key in self._fields:
if not self._fields[key].is_valid(value):
raise TypeError('Invalid type for field.')
super(Enforcer, self).__setattr__(key, value)

class Person(Enforcer):
name = Field(str)
age = Field(int)

if __name__ == '__main__':
swo = Person()
swo.name = 'swo'
print 'swo:', swo


--
Time flies like the wind. Fruit flies like a banana. Stranger things have .0.
happened but none stranger than this. Does your driver's license say Organ ..0
Donor?Black holes are where God divided by zero. Listen to me! We are all- 000
individuals! What if this weren't a hypothetical question?
steveo at syslang.net

 

[jfine]

Hi

I gave a tutorial at this year's EuroPython that covered metaclasses.
You can get the tutorial materials from a link on:
http://ep2011.europython.eu/conference/talks/objects-and-classes-in-python-and-javascript


Jonathan

 

[Karim]

Very good.

Karim

 

[bruno.desthuilliers]

I have been doing a lot of reading. I'm starting to get it. I think it's
really cool as well as dangerous,

Dangerous ??? Why so ? Is there anything "dangerous" in a constructor
or an initialiser ??? A metaclass is just a class, and a class is just
an object.

 

[Steven D'Aprano]

Dangerous ??? Why so ? Is there anything "dangerous" in a constructor
or an initialiser ??? A metaclass is just a class, and a class is just
an object.

Obviously not dangerous in the sense of "Arghh, I just lost a finger!!!"

I think it is fair to talk about language features' power on a continuum,
where something like 1+1 has very little power (it just adds two numbers),
classes have more power, metaclasses more again, and exec even more still.
Power can be used for good things, but it can also be used for bad. I
consider metaclasses to be about equal in power to decorators, but more
obscure and less straightforward. Consider a decorator example:

@decorate
def func(x):
return x+1


func(42)
=> returns "the result is 43"

Surprise! What the function does is barely related to the code you can see.
That's surprising and therefore dangerous. Metaclasses give you the same
ability to surprise the reader, hence, dangerous. That's not necessarily a
bad thing: an oxy-acetylene torch is dangerous, and you shouldn't use one
for lighting a cigarette, but if you need to cut through iron bars in a
hurry, it gets the job done.