On 2009-06-12 09:05, Paul LaFollette wrote:
Kind people,
Using Python 3.0 on a Gatesware machine (XP).
I am building a class in which I want to constrain the types that can
be stored in various instance variables. For instance, I want to be
certain that self.loc contains an int. This is straightforward (as
long as I maintain the discipline of changing loc through a method
rather than just twiddling it directly.
You may want to consider using Enthought's Traits package (disclosure: I work
for Enthought).
http://pypi.python.org/pypi/Traits/
Type-checking attributes is not my favorite feature of Traits (I try to avoid
using this feature if I can avoid it), but it is designed for precisely your use
cases.
In [2]: %cpaste
Pasting code; enter '--' alone on the line to stop.
:from enthought.traits.api import Float, HasTraits, Instance, Int, This
:
:class FancyClass(HasTraits):
: """ An example traited class.
: """
: x = Float(10.0)
: y = Int(-20)
:
:class Node(HasTraits):
: """ A node in a single-linked list.
: """
:
: # The location of this Node.
: loc = Int(0)
:
: # The next Node. This() is a special trait that accepts instances of
: # this class, or None.
: next = This()
:
: # The data attached to the Node. It will accept instances of FancyClass
: # or None.
: data = Instance(FancyClass)
:
:
:--
In [3]: n = Node()
In [4]: n.next
In [5]: n.data
In [6]: n.loc
Out[6]: 0
In [7]: fc = FancyClass(x=15.0)
In [8]: fc.x
Out[8]: 15.0
In [9]: fc.y
Out[9]: -20
In [10]: fc.y = 'a string'
---------------------------------------------------------------------------
TraitError Traceback (most recent call last)
/Users/rkern/<ipython console> in <module>()
/Users/rkern/svn/et/Traits/enthought/traits/trait_handlers.pyc in error(self,
object, name, value)
173 """
174 raise TraitError( object, name, self.full_info( object, name,
value ),
--> 175 value )
176
177 def arg_error ( self, method, arg_num, object, name, value ):
TraitError: The 'y' trait of a FancyClass instance must be an integer, but a
value of 'a string' <type 'str'> was specified.
In [11]: n.data = fc
In [12]: n.data = None
In [13]: n.data = n
---------------------------------------------------------------------------
TraitError Traceback (most recent call last)
/Users/rkern/<ipython console> in <module>()
/Users/rkern/svn/et/Traits/enthought/traits/trait_handlers.pyc in error(self,
object, name, value)
173 """
174 raise TraitError( object, name, self.full_info( object, name,
value ),
--> 175 value )
176
177 def arg_error ( self, method, arg_num, object, name, value ):
TraitError: The 'data' trait of a Node instance must be a FancyClass or None,
but a value of <__main__.Node object at 0x17999c0> <class '__main__.Node'> was
specified.
In [14]: n.next = Node(loc=1)
In [15]: n.next
Out[15]: <__main__.Node at 0x1892e70>
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
--
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