What does Python return when instantiating new classes?

I'm finding loads of quirks with Python when instantiating a new class. I'm sure it's just because I'm not used to the language, but even so, the behaviour I can see is really strange.

If I open up iPython and type the following:

class Person:
    def __init__(self, name):
        self.name = name

    def hello(self):
        print "Hello, " + self.name

Everything works exactly as I'd expect it to:

In [2]: Person
Out[2]: <class __main__.Person at 0x1c97330>

In [3]: p = Person("Jamie")

In [4]: p
Out[4]: <__main__.Person instance at 0x1c90b98>

In [5]: p.hello()
Hello, Jamie

However, if I then access a separate class inside package - nothing too fancy, I might add - and instantiate a new class, it all goes wrong. Here's the link to the code for palestrina/cache.py

In [6]: from palestrina.cache import Cache

In [7]: Cache
Out[7]: <class palestrina.cache.Cache at 0x1c97750>

In [8]: c = Cache(application = 'example', backend = 'filesystem')

In [9]: c
Out[9]: ---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

/Users/jamierumbelow/Sites/Os/palestrina/<ipython console> in <module>()

/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/IPython/Prompts.pyc in __call__(self, arg)
    550 
    551             # and now call a possibly user-defined print mechanism

--> 552             manipulated_val = self.display(arg)
    553 
    554             # user display hooks can change the variable to be stored in


/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/IPython/Prompts.pyc in _display(self, arg)
    576             return IPython.generics.result_display(arg)
    577         except TryNext:
--> 578             return self.shell.hooks.result_display(arg)
    579 
    580     # Assign the default display method:


/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/IPython/hooks.pyc in __call__(self, *args, **kw)
    139             #print "prio",prio,"cmd",cmd #dbg

    140             try:
--> 141                 ret = cmd(*args, **kw)
    142                 return ret
    143             except ipapi.TryNext, exc:

/Library/Frameworks/Python.fr开发者_运维问答amework/Versions/2.6/lib/python2.6/site-packages/IPython/hooks.pyc in result_display(self, arg)
    169 
    170     if self.rc.pprint:
--> 171         out = pformat(arg)
    172         if '\n' in out:
    173             # So that multi-line strings line up with the left column of


/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/pprint.pyc in pformat(self, object)
    109     def pformat(self, object):
    110         sio = _StringIO()
--> 111         self._format(object, sio, 0, 0, {}, 0)
    112         return sio.getvalue()
    113 

/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/pprint.pyc in _format(self, object, stream, indent, allowance, context, level)
    127             self._readable = False
    128             return
--> 129         rep = self._repr(object, context, level - 1)
    130         typ = _type(object)
    131         sepLines = _len(rep) > (self._width - 1 - indent - allowance)

/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/pprint.pyc in _repr(self, object, context, level)
    221     def _repr(self, object, context, level):
    222         repr, readable, recursive = self.format(object, context.copy(),
--> 223                                                 self._depth, level)
    224         if not readable:
    225             self._readable = False

/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/pprint.pyc in format(self, object, context, maxlevels, level)
    233         and whether the object represents a recursive construct.
    234         """
--> 235         return _safe_repr(object, context, maxlevels, level)
    236 
    237 

/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/pprint.pyc in _safe_repr(object, context, maxlevels, level)
    318         return format % _commajoin(components), readable, recursive
    319 
--> 320     rep = repr(object)
    321     return rep, (rep and not rep.startswith('<')), False
    322

TypeError: 'bool' object is not callable

I can't understand what is going on here. Can someone explain to me what might be happening?

Thanks.

Well, you removed the relevant part (the traceback) and replaced it by ... in your paste.

But it looks like you have an error in the representation of the class.

Here's a simulation of the error:

>>> class MyClass(object):
...     def __repr__(self):
...         return True()
...
>>> c = MyClass()
>>> c

Please check the traceback you removed and you'll see what is happening exactly. If you can't, please edit your question and include it, so we can explain further.

Providing the source code of that class would also help.

Not Python's fault, your package is causing the trouble.

You use some nasty tricks to save typing. Specifically, you're overriding the special __getattr__ method, which is used whenever you try to access an attribute of c. Inspecting it will look for a __repr__ method, and methods are attributes, so...

Trying to entirely replace the attributes of your class for convenience is a really bad idea. I think you'd be better off doing it with __getitem__, __setitem__ and __delitem__, so your cache access looks like c["name"] rather than c.name.

EDIT: Another tip, try not to catch all your errors unless you've got something productive to do instead. If you hadn't been catching keyerrors on get() and turning them into return False, the traceback would have shown exactly what it was trying to do. And what if I wanted to store True or False in the cache? How do I know whether False means False or "value missing"?