Re: subclassable types
On Fri, Feb 22, 2013 at 12:00 PM, Christian Heimes wrote: > Am 22.02.2013 10:35, schrieb Wolfgang Maier: >> Also: can you use introspection to find out whether a type is valid as a >> base type? > > I don't think so. For CPython the information is stored in the type's > structure. When type->tp_flags & Py_TPFLAGS_BASETYPE is true then > subclassing is allowed. But I know of no way to retrieve that > information from Python code. There is an (undocumented and CPython-specific) way: >>> range.__flags__ & (1 << 10) 0 >>> int.__flags__ & (1 << 10) 1024 Daniel -- http://mail.python.org/mailman/listinfo/python-list
Re: len() on mutables vs. immutables
On Thu, Oct 18, 2012 at 8:42 PM, Demian Brecht wrote: >> str, bytes, bytearrays, arrays, sets, frozensets, dicts, dictviews, and >> ranges should all return len in O(1) time. That includes the possibility >> of a subtraction as indicated above. > > Awesome. Pretty much what I figured. Of course, I'll have to dig around the > source just to confirm this with my own eyes (more just curiosity than > anything), so if you know whereabouts to look, it would be most helpful :) The source is usually in Objects/*object.c (e.g., the source for list is in Objects/listobject.c, dict is in dictobject.c and so on). The implementation of __len__ is usually in a method called whatever_length (e.g., dict.__len__ is called dict_length). To be sure, you can check the PyTypeObject declaration for the type. Probably the tp_as_sequence or tp_as_mapping field contains the pointer to __len__ (sq_length or mp_length respectively). (You can also search for "lenfunc", which is the type of such functions.) -- http://mail.python.org/mailman/listinfo/python-list
Re: About a list comprehension to transform an input list
On Fri, Jun 8, 2012 at 6:10 PM, Julio Sergio wrote: > >From a sequence of numbers, I'm trying to get a list that does something to > >even > numbers but leaves untouched the odd ones, say: > > [0,1,2,3,4,...] ==> [100,1,102,3,104,...] > > I know that this can be done with an auxiliary function, as follows: > > ->>> def filter(n): > ... if (n%2 == 0): > ... return 100+n > ... return n > ... > ->>> L = range(10) > ->>> [filter(n) for n in L] > [100, 1, 102, 3, 104, 5, 106, 7, 108, 9] > > I wonder whether there can be a single list comprehension expression to get > this > result without the aid of the auxiliary function. > > Do you have any comments on this? >>> l = [0,1,2,3,4,5,6,7,8,9] >>> [n if n%2 else 100+n for n in l] [100, 1, 102, 3, 104, 5, 106, 7, 108, 9] Daniel -- http://mail.python.org/mailman/listinfo/python-list
Re: Recursion error in metaclass
On Sat, Jun 11, 2011 at 21:39, Terry Reedy wrote: > What may not be obvious from the docs is that the metaclass calculation > described in the doc section on class statements is carried out within > type.__new__ (or after a possible patch, called from within that), so that > type calls are really "a dynamic form of the class statement" even when > another another metaclass is specified or implied. "Return a new type > object." includes instances of type subclasses. I am not sure what happens > with metaclasses that are not type subclasses. There is at least one bug > report about the metaclass calculation, which is why I happen to have read > the typeobject.__new__ code. But I have not read the build-class code and > all the details of class creation. So I may have some of the details above > wrong. Just for the record, I think this is the mentioned bug: http://bugs.python.org/issue1294232 Daniel -- http://mail.python.org/mailman/listinfo/python-list
Re: function annotations in open source projects
> Do you know any open source python3 projects that use function > annotations? I would like to see some real use, so maybe I find them > useful to use in my own project. threecheck uses them for type checking: http://pypi.python.org/pypi/threecheck Daniel -- http://mail.python.org/mailman/listinfo/python-list
Re: Init a dictionary with a empty lists
On Sat, Feb 5, 2011 at 15:38, Steven D'Aprano
wrote:
> On Sat, 05 Feb 2011 14:38:29 +0100, Daniel Urban wrote:
>
>> On Sat, Feb 5, 2011 at 14:08, Lisa Fritz Barry Griffin
>> wrote:
>>> Hi there,
>>>
>>> How can I do this in a one liner:
>>>
>>> maxCountPerPhraseWordLength = {}
>>> for i in range(1,MAX_PHRASES_LENGTH+1):
>>> maxCountPerPhraseWordLength[i] = 0
>>
>> maxCountPerPhraseWordLength = {0 for i in range(1,MAX_PHRASES_LENGTH+1)}
>
> Unfortunately not. In versions before Python 2.7, it will give a
> SyntaxError. In 2.7 or better it gives a set instead of a dict:
Yeah, sorry, that should have been {i: 0 for i in
range(1,MAX_PHRASES_LENGTH+1)}
Daniel
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Re: Init a dictionary with a empty lists
On Sat, Feb 5, 2011 at 14:08, Lisa Fritz Barry Griffin
wrote:
> Hi there,
>
> How can I do this in a one liner:
>
> maxCountPerPhraseWordLength = {}
> for i in range(1,MAX_PHRASES_LENGTH+1):
> maxCountPerPhraseWordLength[i] = 0
maxCountPerPhraseWordLength = {0 for i in range(1,MAX_PHRASES_LENGTH+1)}
But I suggest using a defaultdict:
http://docs.python.org/dev/py3k/library/collections#defaultdict-objects
Daniel
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Re: can't use multiprocessing with class factory?
On Fri, Jan 28, 2011 at 20:02, Alan wrote: > Can the below example be fixed to work? > Thanks, > Alan Isaac > > import multiprocessing as mp > > class Test(object): > pass > > def class_factory(x): > class ConcreteTest(Test): > _x = x > return ConcreteTest > > def f(cls): > print cls._x > > if __name__ == '__main__': > pool = mp.Pool(2) > pool.map(f, [class_factory(i) for i in range(4)]) Only classes defined on the top level of a module are picklable (see http://docs.python.org/dev/py3k/library/pickle#what-can-be-pickled-and-unpickled ). The collections.namedtuple class factory function works around this limitation by setting the __module__ attribute of the created class, but I'm not sure if this solution can be used in this case. Daniel -- http://mail.python.org/mailman/listinfo/python-list
Re: Behaviour-based interface/protocol implementation?
On Fri, Jan 28, 2011 at 11:32, Alan Franzoni wrote: > On Thu, Jan 27, 2011 at 11:41 PM, Daniel Urban wrote: > >> Actually it can. You don't have to modify the object, just check for >> the desired methods/signature/whatever. See for example the >> implementation of collections.Hashable.__subclasshook__ in _abcoll.py >> and the abc.ABCMeta.__instancecheck__ method. > > Mmh, I don't get your examples (subclasshook checks for *classes*, not > instances, and ABCMeta is a metaclass whose instance is an ABC), but > after properly looking at the PEP (__instancecheck__ seems to be > poorly documented in python standard docs) I've seen that yes, > __instancecheck__ on an ABC can actually be used in place of > maybe_implemented_by and hooking into isinstance(). Exactly. I mentioned subclasshook because it is possible to define a similar "instancehook", which would check for instances. Daniel -- http://mail.python.org/mailman/listinfo/python-list
Re: Behaviour-based interface/protocol implementation?
On Thu, Jan 27, 2011 at 20:05, Alan Franzoni wrote: > On Thu, Jan 27, 2011 at 8:03 PM, Alan Franzoni wrote: >> Yes, __instancecheck__ could be used as an alternative hook with >> respect to maybe_implemented_by(), but there's no such logic for >> signature checking. That's a minor detail, I think. > > On the contrary, now that I double checked, it can't be used that way; > I don't want to modify the object I'm testing (it could be a third > party object) nor I want to force it to be "registered" as a certain > ABC; so __instancecheck__() is just useless here. Actually it can. You don't have to modify the object, just check for the desired methods/signature/whatever. See for example the implementation of collections.Hashable.__subclasshook__ in _abcoll.py and the abc.ABCMeta.__instancecheck__ method. Daniel -- http://mail.python.org/mailman/listinfo/python-list
Re: Behaviour-based interface/protocol implementation?
> That's just what I'd like and I suppose can't be currently done with > current ABC, PyProtocols or zope.interface implementations, right? It can. With __instancecheck__ you can override isinstance. It is possible (for example) to write a subclass of abc.ABCMeta, which extends __instancecheck__ to use an _instancehook classmethod similarly to __subclasshook__. Then in your MyInterface class you can implement _instancehook to check for methods/signatures/whatever you want. Daniel -- http://mail.python.org/mailman/listinfo/python-list
Re: collections.Set Binary Reflected Operations
On Thu, Jan 20, 2011 at 01:51, Chris Kaynor wrote: > I am implemented a custom set-like type which interacts with some > third-party software when retrieving and mutating the set, and have derived > my custom type off of collections.MutableSet, however I have noticed that > some of the magic methods supported by the built-in set do not fully > function with my custom type. From this, I looked over the > MutableSet definition in and it seems to be missing all of the reflected > operators (__rsub__, __rand__, __ror__, __rxor__, and friends) for the > operators it has defined. I can post a simple example case if desired. > I am using Python 2.6.4 (with some in-house customizations), however a quick > look over the svn repository shown on python.org makes it appear that these > are also not implemented in Python 3. I was wondering if there is some > reason for this, or if it was merely an oversight. > Chris See http://bugs.python.org/issue8743 and also http://bugs.python.org/issue2226 Daniel -- http://mail.python.org/mailman/listinfo/python-list
Re: How can a function find the function that called it?
On Fri, Dec 24, 2010 at 17:24, kj wrote: > (BTW, I don't understand why inspect doesn't provide something as > basic as the *class* that the method belongs to, whenever applicable. > I imagine there's a good reason for this coyness, but I can't figure > it out.) One function object can "belong to" (be in the namespace of) more than one class, so there is no "the class". -- http://mail.python.org/mailman/listinfo/python-list
Re: Needed: Real-world examples for Python's Cooperative Multiple Inheritance
> So far, the only situation I can find where method names necessarily > overlap is for the basics like __init__(), close(), flush(), and > save() where multiple parents need to have their own initialization > and finalization. One other possibility is subclasses of the JSONEncoder class. For example the writer of class X provides a class, XJSONEncoder, which is able to serialize X instances (by extending the default method). Similarly there is a YJSONEncoder class, which can serialize Y instances. Those classes implement the default method like this: def default(self, o): if isinstance(o, X): ... # serialize the X instance else: return super().default(o) # let the next in the MRO try to handle it If YJSONEncoder encodes Y instances similarly, one can create an encoder class, which can encode both X and Y instances: class XYJSONEncoder(XJSONEncoder, YJSONEncoder): pass It is usable this way: json.dumps([X(), Y()], cls=XYJSONEncoder) Regards, Daniel -- http://mail.python.org/mailman/listinfo/python-list
Re: decouple copy of a list
b = list(a) or b = a[:] -- http://mail.python.org/mailman/listinfo/python-list
Re: Silly newbie question - Carrot character (^)
> However, what exactly does ^ do? Bitwise XOR: http://docs.python.org/py3k/reference/expressions.html#binary-bitwise-operations -- http://mail.python.org/mailman/listinfo/python-list
Re: errors and exception
> f = open("myfile.txt")
> for line in f:
> print line
> f.close() # This is what the "with" statement guarantees; so now just do
> it yourself
Not exactly. More like:
f = open("myfile.txt")
try:
for line in f:
print line
finally:
f.close()
Daniel
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Re: how to change a string into dictionary
> a = "{'a':'1','b':'2'}"
> how to change a into a dictionary ,says, a = {'a':'1','b':'2'}
See also the ast.literal_eval function:
http://docs.python.org/py3k/library/ast.html#ast.literal_eval
Daniel
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Re: new to python - trouble calling a function from another function
> I'm building an elevator simulator for a class assignment. I recently ran > into a roadblock and don't know how to fix it. For some reason, in my > checkQueue function below, the call to self.goUp() is never executed. It is > on the last line of code I pasted in. I can put print statements before and > after the call and I have a print statement in goUp() itself. Only the > print statements before and after the call are executed. The one inside > goUp() is never executed because goUp() never seems to be executed. How can > that be? I don't get any errors when the script executes. Surely this isn't > some limitation I'm encountering? I think the self.goUp() call is executed, the goUp function gets called, returns a generator object (because goUp is a generator function), then you don't use that generator object for anything. Daniel -- http://mail.python.org/mailman/listinfo/python-list
