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Axel Straschil
*nix forums beginner
Joined: 11 Apr 2005
Posts: 15
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 Posted: Thu Feb 03, 2005 12:59 pm Post subject:
Re: Generating modul classes with eval
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Hello!
| Quote: | After the loop has finished, the global variable TmpClass will be bound to
whatever class was created last, and the variable class_base will be bound
to that the base class of that same TmpClass. Therefore only this last class
is guaranteed to work as expected.
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Great, now it workes!
_module_name = sys.modules[__name__]
def _tag_class_factory(name, base):
class T(base):
def __init__(self, **props):
super(T, self).__init__(name=name.lower(), **props)
setattr(_module_name, name, T)
class_dic = {}
class_dic['Br'] = _Tag
class_dic['Hr'] = _Tag
class_dic['Html'] = _ContainerTag
class_dic['Table'] = _ContainerTag
class_dic['Td'] = _ContainerTag
class_dic['Tr'] = _ContainerTag
for name, base in class_dic.items():
_tag_class_factory(name, base)
print Table(contents=[Tr(contents=[Br()])])
gives: <table><tr><br/></tr></table>
Thanks,
AXEL. |
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Peter Otten
*nix forums Guru
Joined: 20 Feb 2005
Posts: 464
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| Posted: Thu Feb 03, 2005 11:18 am Post subject:
Re: Generating modul classes with eval
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Axel Straschil wrote:
| Quote: | class_dic = {}
class_dic['Br'] = _Tag
class_dic['Hr'] = _Tag
class_dic['Html'] = _ContainerTag
class_dic['Table'] = _ContainerTag
for class_name, class_base in class_dic.items():
class TmpClass(class_base):
def __init__(self, **props):
name = class_name.lower()
#super(TmpClass, self).__init__(name=name, **props)
class_base.__init__(self, name=name, **props)
setattr(_module_name, class_name, TmpClass)
|
While your workaround doesn't balk immediately, it doesn't do the right
thing either.
After the loop has finished, the global variable TmpClass will be bound to
whatever class was created last, and the variable class_base will be bound
to that the base class of that same TmpClass. Therefore only this last class
is guaranteed to work as expected.
A simplified example to demonstrate the binding problem:
| Quote: | classes = []
for text in ["alpha", "beta"]:
.... class T: |
.... def __init__(self): print text
.... classes.append(T)
....
| Quote: | classes[0] is classes[1]
False # two distinct classes, as expected
classes[0]()
beta |
<__main__.T instance at 0x402a9e2c>
<__main__.T instance at 0x402a9f8c>
And now the proof that you are actually accessing the global variable:
| Quote: | text = "gamma"
classes[0]()
gamma |
<__main__.T instance at 0x402a9f8c>
One way to fix this is to introduce a factory function:
| Quote: | def make_class(text):
.... class T: |
.... def __init__(self): print text
.... return T
....
| Quote: | classes = []
for text in ["alpha", "beta"]:
.... classes.append(make_class(text)) |
....
| Quote: | classes[0]()
alpha |
<__main__.T instance at 0x402a9e4c>
<__main__.T instance at 0x402a9f8c>
Peter |
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Axel Straschil
*nix forums beginner
Joined: 11 Apr 2005
Posts: 15
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| Posted: Thu Feb 03, 2005 9:26 am Post subject:
Re: Generating modul classes with eval
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Hello!
| Quote: | Note that we don't need eval anywhere.
|
Uuups, that looks realy cool! Thanks for that!
Im fooling around with generating html-tags. As there are only two kind
of html tags, one who can nest chields, and one who cant, i wantet to
play arround with something like:
I've got two base classes, _Tag and _ContainerTag (for tags which can
nest tags). Instead of getting an htmltag with _Tag(name='html'), I
want to have a class for each html-tag. So, I thought of creating that
classes dynamicly.
my now (nearly) working code is:
class _Tag(object):
def __init__(self, name, flags=None, **props):
[...]
class _ContainerTag(_Tag):
def __init__(self, name, contents=None, flags=None, **props):
super(_ContainerTag, self).__init__(name=name, flags=flags, **props)
self._contents = coalesce(contents, [])
_module_name = sys.modules[__name__]
class_dic = {}
class_dic['Br'] = _Tag
class_dic['Hr'] = _Tag
class_dic['Html'] = _ContainerTag
class_dic['Table'] = _ContainerTag
for class_name, class_base in class_dic.items():
class TmpClass(class_base):
def __init__(self, **props):
name = class_name.lower()
#super(TmpClass, self).__init__(name=name, **props)
class_base.__init__(self, name=name, **props)
setattr(_module_name, class_name, TmpClass)
br = Br()
print br
table = Table()
print table
br is printed OK, but for table, I get:
AttributeError: 'TmpClass' object has no attribute '_contents'
so, it seems that __init__ of _Tag is not called.
If I try to do the commented line
super(TmpClass, self).__init__(name=name, **props)
instead of
class_base.__init__(self, name=name, **props)
I get:
TypeError: super(type, obj): obj must be an instance or subtype of
type
for print table, print br ist processed OK.
Thanks for help and your perfekt examples,
AXEL. |
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Steve Holden
*nix forums Guru
Joined: 22 Feb 2005
Posts: 1255
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| Posted: Wed Feb 02, 2005 8:49 pm Post subject:
Re: Generating modul classes with eval
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Axel Straschil wrote:
| Quote: | Hello!
I was fooling around with creating classes for a module with eval,
something like:
MyModule.py:
class Base:
init(self, name):
self._name = name
for myclass in ['A', 'B', 'C']:
code="class %s(Base):\n\tinit(self, name='%s')\n\t\tsuper(%s,
self).__init(name=name)\n"%dict(myclass, myclass.lower(), myclass())
... codeop and eval stuff ...
a=A()
print a
that gives: <class '__main__.A'>, but I want MyModule.A ;-)
Can someone give me a hint how to create classes in a module with eval
and codeop so that they exist like the code was written in?
Thanks,
AXEL.
You could try just importing the module - then, when it runs, its name |
won't be "__main__" but "MyModule".
regards
Steve
--
Meet the Python developers and your c.l.py favorites March 23-25
Come to PyCon DC 2005 http://www.python.org/pycon/2005/
Steve Holden http://www.holdenweb.com/ |
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Jeremy Bowers
*nix forums Guru Wannabe
Joined: 23 Feb 2005
Posts: 129
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| Posted: Wed Feb 02, 2005 8:49 pm Post subject:
Re: Generating modul classes with eval
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On Wed, 02 Feb 2005 20:49:07 +0000, Axel Straschil wrote:
You are doing several things wrong.
| Quote: | I was fooling around with creating classes for a module with eval,
|
You shouldn't create classes with eval, because you don't need to.
"class" isn't a declaration, it is an executable statement that creates
new classes. We'll get into that momentarily...
| Quote: | something like:
MyModule.py:
class Base:
init(self, name):
self._name = name
|
Your "init" function needs to be spelled "__init__", or it will not be
automatically called.
You also did not correctly use "def" to create your function. When posting
to the newsgroup, try to use real code that you have actually executed.
| Quote: | that gives: <class '__main__.A'>, but I want MyModule.A 
|
No, it won't, since your code has syntax errors in it. Post the code you
actually ran.
That said, "__main__" indicates you ran it in the interactive shell. That
is correct, and won't change. Also, the name printing the class gives is
only very rarely important; overall you shouldn't be using that.
I'll start with giving you this:
-------
import sys
module = sys.modules[__name__]
class Base:
def __init__(self, name):
self._name = name
for myclass in ['A', 'B', 'C']:
class Tmp(Base):
myname = myclass
def __init__(self):
Base.__init__(self, self.myname)
setattr(module, myclass, Tmp)
-------
Note that we don't need eval anywhere.
But I'd suggest that this is more likely what you want:
-------
class Base:
def __init__(self, name):
self._name = name
myClasses = {}
for className in ['A', 'B', 'C']:
class Tmp(Base):
myname = className
def __init__(self):
Base.__init__(self, self.myname)
myClasses[className] = Tmp
-------
Adding things straight to modules is rarely worth it; you're better off
just collecting them somewhere.
There are too many differences to go over here between my code and yours,
so if you have questions, please ask. One of the reasons you don't want
eval is that I had to give up trying to read your class code!
Finally, while such generated classes do have their use, I'd ask what you
are planning to do with this; odds are, you don't need it.
In general, unless you are using "eval" to literally execute user supplied
input, you *almost* certainly don't need it.
A downside of my approach is that printing all three classes will say the
class name is "Tmp". Since, as I said, you really shouldn't care about
that, I don't care to try to fix it  If you can provide a compelling
reason why you need that, somebody here can help you with that. |
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Jeremy Bowers
*nix forums Guru Wannabe
Joined: 23 Feb 2005
Posts: 129
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| Posted: Wed Feb 02, 2005 8:49 pm Post subject:
Re: Generating modul classes with eval
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On Wed, 02 Feb 2005 16:20:41 -0500, Jeremy Bowers wrote:
| Quote: | That said, "__main__" indicates you ran it in the interactive shell.
|
Or ran it directly on the command line. Duh. I thought that clause really
loudly, but I guess I never actually typed it. |
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Axel Straschil
*nix forums beginner
Joined: 11 Apr 2005
Posts: 15
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| Posted: Wed Feb 02, 2005 8:49 pm Post subject:
Generating modul classes with eval
|
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|
Hello!
I was fooling around with creating classes for a module with eval,
something like:
MyModule.py:
class Base:
init(self, name):
self._name = name
for myclass in ['A', 'B', 'C']:
code="class %s(Base):\n\tinit(self, name='%s')\n\t\tsuper(%s,
self).__init(name=name)\n"%dict(myclass, myclass.lower(), myclass())
... codeop and eval stuff ...
a=A()
print a
that gives: <class '__main__.A'>, but I want MyModule.A ;-)
Can someone give me a hint how to create classes in a module with eval
and codeop so that they exist like the code was written in?
Thanks,
AXEL. |
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