Re: Planning a Python Course for Beginners

On 8/11/17 6:37 AM, Python wrote: > Marko Rauhamaa wrote: >> Python : >> >>> Marko Rauhamaa wrote: >>> I didn't disagree with any of these statements about __hash__, but only >>> your statement about id and __eq__: >>> id() is actually an ideal return value of

Re: Planning a Python Course for Beginners

Marko Rauhamaa wrote: Python : Marko Rauhamaa wrote: I didn't disagree with any of these statements about __hash__, but only your statement about id and __eq__: id() is actually an ideal return value of __hash__(). The only criterion is that the returned number

Re: Planning a Python Course for Beginners

Marko Rauhamaa wrote: Of course, some algorithms can (and, we have learned, do) prefer some bits over others, but that's inside the implementation black box. I would think every bit should carry an approximately equal weight. Ideally that would be true, but you need to consider the performance

Re: Planning a Python Course for Beginners

Steve D'Aprano wrote: On Thu, 10 Aug 2017 07:00 pm, Peter Otten wrote: /* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid excessive hash collisions for dicts and sets */ which I think agrees with my comment: using the id() itself would put too many objects in the same

Re: Planning a Python Course for Beginners

Python wrote: Marko Rauhamaa wrote: id() is actually an ideal return value of __hash__(). The only criterion is that the returned number should be different if the __eq__() is False. That is definitely true for id() nan is a clear, simple, undeniable counterexample to that claim. It's a

Re: Planning a Python Course for Beginners

Chris Angelico : > On Fri, Aug 11, 2017 at 7:17 AM, Marko Rauhamaa wrote: >> That's interesting, but suggests there's something weird (~ suboptimal) >> going on with CPython's scrambling algorithm. Also, your performance >> test might yield completely

Re: Planning a Python Course for Beginners

On Fri, Aug 11, 2017 at 7:17 AM, Marko Rauhamaa wrote: > That's interesting, but suggests there's something weird (~ suboptimal) > going on with CPython's scrambling algorithm. Also, your performance > test might yield completely different results on other Python >

Re: Planning a Python Course for Beginners

Chris Angelico : > I'm aware of this. Doesn't change the fact that the *INITIAL INDEX* is > based on exactly what I said. > > Yaknow? What you're saying is that CPython heavily prefers the low-order bits to be unique performance-wise. I don't know why that particular heuristic

Re: Planning a Python Course for Beginners

Peter Otten <__pete...@web.de>: > Marko Rauhamaa wrote: >> I see no point in CPython's rotation magic. > > Let's see: > > $ cat hashperf.py > class A(object): > __slots__ = ["_hash"] > > def __hash__(self): > return self._hash > > def no_magic(): > a = A() > a._hash =

Re: Planning a Python Course for Beginners

On Fri, Aug 11, 2017 at 6:56 AM, Marko Rauhamaa wrote: > Chris Angelico : > >> On Fri, Aug 11, 2017 at 6:03 AM, Marko Rauhamaa wrote: >>> I see no point in CPython's rotation magic. >> >> Have you ever implemented a hashtable? The most common

Re: Planning a Python Course for Beginners

Chris Angelico : > On Fri, Aug 11, 2017 at 6:03 AM, Marko Rauhamaa wrote: >> I see no point in CPython's rotation magic. > > Have you ever implemented a hashtable? The most common way to pick a > bucket for an object is to use modulo on the number of buckets.

Re: Planning a Python Course for Beginners

Marko Rauhamaa wrote: > Peter Otten <__pete...@web.de>: > >> Steve D'Aprano wrote: >>> The C code says: >>> /* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid excessive hash collisions for dicts and sets */ >>> >>> which I think agrees with my comment: using the id()

Re: Planning a Python Course for Beginners

On Fri, Aug 11, 2017 at 6:03 AM, Marko Rauhamaa wrote: > Peter Otten <__pete...@web.de>: > >> Steve D'Aprano wrote: >>> The C code says: >>> /* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid excessive hash collisions for dicts and sets */ >>> >>> which I

Re: Planning a Python Course for Beginners

Peter Otten <__pete...@web.de>: > Steve D'Aprano wrote: >> The C code says: >> >>>/* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid >>>excessive hash collisions for dicts and sets */ >> >> which I think agrees with my comment: using the id() itself would put >> too many objects

Re: Planning a Python Course for Beginners

Steve D'Aprano wrote: > The C code says: > >>/* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid >>excessive hash collisions for dicts and sets */ > > which I think agrees with my comment: using the id() itself would put too > many objects in the same bucket (i.e. too many

Re: Planning a Python Course for Beginners

Python : > Marko Rauhamaa wrote: > I didn't disagree with any of these statements about __hash__, but only > your statement about id and __eq__: > >> id() is actually an ideal return value of __hash__(). The only >> criterion is that the returned number should be different

Re: Planning a Python Course for Beginners

On Fri, Aug 11, 2017 at 2:41 AM, Steve D'Aprano wrote: > On Fri, 11 Aug 2017 12:58 am, Chris Angelico wrote: > >> On Fri, Aug 11, 2017 at 12:45 AM, Steve D'Aprano >> wrote: >> >>> The C code says: >>> /* bottom 3 or 4 bits are

Re: Planning a Python Course for Beginners

On Fri, 11 Aug 2017 12:58 am, Chris Angelico wrote: > On Fri, Aug 11, 2017 at 12:45 AM, Steve D'Aprano > wrote: > >> The C code says: >> >>> /* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid >>>excessive hash collisions for dicts and sets

Re: Planning a Python Course for Beginners

Marko Rauhamaa wrote: Python : Marko Rauhamaa wrote: Python : Marko Rauhamaa wrote: id() is actually an ideal return value of __hash__(). The only criterion is that the returned number should be different if the __eq__() is False. That is

Re: Planning a Python Course for Beginners

Python : > Marko Rauhamaa wrote: >> Python : >> >>> Marko Rauhamaa wrote: id() is actually an ideal return value of __hash__(). The only criterion is that the returned number should be different if the __eq__() is False. That is

Re: Planning a Python Course for Beginners

On Fri, Aug 11, 2017 at 12:45 AM, Steve D'Aprano wrote: > The C code says: > >> /* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid >>excessive hash collisions for dicts and sets */ > > which I think agrees with my comment: using the id()

Re: Planning a Python Course for Beginners

On Thu, 10 Aug 2017 07:00 pm, Peter Otten wrote: > Steven D'Aprano wrote: > >> On Wed, 09 Aug 2017 20:07:48 +0300, Marko Rauhamaa wrote: >> >>> Good point! A very good __hash__() implementation is: >>> >>> def __hash__(self): >>> return id(self) >>> >>> In fact, I didn't know

Re: Planning a Python Course for Beginners

Marko Rauhamaa wrote: Python : Marko Rauhamaa wrote: id() is actually an ideal return value of __hash__(). The only criterion is that the returned number should be different if the __eq__() is False. That is definitely true for id(). $ python Python 2.7.13 (default,

Re: Planning a Python Course for Beginners

Python : > Marko Rauhamaa wrote: >> id() is actually an ideal return value of __hash__(). The only criterion >> is that the returned number should be different if the __eq__() is >> False. That is definitely true for id(). > > $ python > Python 2.7.13 (default, Jan 19

Re: Planning a Python Course for Beginners

Marko Rauhamaa wrote: id() is actually an ideal return value of __hash__(). The only criterion is that the returned number should be different if the __eq__() is False. That is definitely true for id(). $ python Python 2.7.13 (default, Jan 19 2017, 14:48:08) [GCC 6.3.0 20170118] on linux2 Type

Re: Planning a Python Course for Beginners

Peter Otten <__pete...@web.de>: > Steven D'Aprano wrote: >> On Wed, 09 Aug 2017 20:07:48 +0300, Marko Rauhamaa wrote: >> >>> Good point! A very good __hash__() implementation is: >>> >>> def __hash__(self): >>> return id(self) >>> >>> In fact, I didn't know Python (kinda) did this

Re: Planning a Python Course for Beginners

Steven D'Aprano wrote: > On Wed, 09 Aug 2017 20:07:48 +0300, Marko Rauhamaa wrote: > >> Good point! A very good __hash__() implementation is: >> >> def __hash__(self): >> return id(self) >> >> In fact, I didn't know Python (kinda) did this by default already. I >> can't find that

Re: Planning a Python Course for Beginners

On Wed, 09 Aug 2017 20:07:48 +0300, Marko Rauhamaa wrote: > Good point! A very good __hash__() implementation is: > > def __hash__(self): > return id(self) > > In fact, I didn't know Python (kinda) did this by default already. I > can't find that information in the definition of

Re: Planning a Python Course for Beginners

Chris Angelico : > On Wed, Aug 9, 2017 at 11:46 PM, Marko Rauhamaa wrote: >> Really, the most obvious use case for hashed objects is their membership >> in a set. For example: >> >> invitees = set(self.bff) >> invitees |= self.classmates() >>

Re: Planning a Python Course for Beginners

On Wed, 9 Aug 2017 11:46 pm, Marko Rauhamaa wrote: > Typically, an object's equality is simply the "is" relation. "Typically"? I don't think so. Are you sure you've programmed in Python before? *wink* py> [1, 2] is [1, 2] False The most commonly used objects don't define equality as identity,

Re: Planning a Python Course for Beginners

On Wed, Aug 9, 2017 at 11:46 PM, Marko Rauhamaa wrote: > Chris Angelico : > >> On Wed, Aug 9, 2017 at 10:00 PM, Marko Rauhamaa wrote: >>> Chris Angelico : >>> Which means that its value won't change. That's what I said.

Re: Planning a Python Course for Beginners

Chris Angelico : > On Wed, Aug 9, 2017 at 10:00 PM, Marko Rauhamaa wrote: >> Chris Angelico : >> >>> Which means that its value won't change. That's what I said. Two >>> things will be equal regardless of that metadata. >> >> In

Re: Planning a Python Course for Beginners

On 8/9/2017 9:25 AM, Marko Rauhamaa wrote: > r...@zedat.fu-berlin.de (Stefan Ram): > >> Steve D'Aprano writes: >>> There's a word for frozen list: "tuple". >> Yes, but one should not forget that a tuple >> can contain mutable entries (such as lists). > Not when

Re: Planning a Python Course for Beginners

r...@zedat.fu-berlin.de (Stefan Ram): > Steve D'Aprano writes: >>There's a word for frozen list: "tuple". > > Yes, but one should not forget that a tuple > can contain mutable entries (such as lists). Not when used as keys: >>> hash(([], [])) Traceback

Re: Planning a Python Course for Beginners

On Wed, Aug 9, 2017 at 10:00 PM, Marko Rauhamaa wrote: > Chris Angelico : > >> Which means that its value won't change. That's what I said. Two >> things will be equal regardless of that metadata. > > In relational-database terms, your "value" is the primary

Re: Planning a Python Course for Beginners

On Wed, 9 Aug 2017 08:38 pm, Marko Rauhamaa wrote: > sometimes you really would like an immutable > (or rather, no-longer-mutable) list to act as a key. There's a word for frozen list: "tuple". :-) -- Steve “Cheer up,” they said, “things could be worse.” So I cheered up, and sure enough,

Re: Planning a Python Course for Beginners

On Wed, 9 Aug 2017 02:19 pm, Dennis Lee Bieber wrote: > On Tue, 8 Aug 2017 15:38:42 + (UTC), Grant Edwards > declaimed the following: > >>On 2017-08-08, Peter Heitzer wrote: [...] >>> The differences between blanks and tabs :-)

Re: Planning a Python Course for Beginners

On Wed, 9 Aug 2017 07:51 pm, Marko Rauhamaa wrote: > Dennis Lee Bieber : > >> Then there is the facet that tuples (being unmutable) can be used as >> keys into a dictionary... > > Mutable objects can be used as keys into a dictionary. Indeed. And people can also put

Re: Planning a Python Course for Beginners

On Wed, Aug 9, 2017 at 8:00 AM, Marko Rauhamaa wrote: > Yeah, when there's a will, there's a way. My Dad used to say "Where there's a will, there's relatives." -- https://mail.python.org/mailman/listinfo/python-list

Re: Planning a Python Course for Beginners

Chris Angelico : > On Wed, Aug 9, 2017 at 8:38 PM, Marko Rauhamaa wrote: >> Chris Angelico : >> >>> On Wed, Aug 9, 2017 at 7:51 PM, Marko Rauhamaa wrote: Mutable objects can be used as keys into a dictionary. >>> >>>

Re: Planning a Python Course for Beginners

On Wed, Aug 9, 2017 at 8:38 PM, Marko Rauhamaa wrote: > Chris Angelico : > >> On Wed, Aug 9, 2017 at 7:51 PM, Marko Rauhamaa wrote: >>> Mutable objects can be used as keys into a dictionary. >> >> Only when the objects' mutability does not

Re: Planning a Python Course for Beginners

On Tue, 08 Aug 2017 14:19:53 +, Stefan Ram wrote: > I am planning a Python course. > > I started by writing the course akin to courses I gave in other > languages, that means, the course starts roughly with these topics: > > - number and string literals - types of number and string

Re: Planning a Python Course for Beginners

Chris Angelico : > On Wed, Aug 9, 2017 at 7:51 PM, Marko Rauhamaa wrote: >> Mutable objects can be used as keys into a dictionary. > > Only when the objects' mutability does not affect their values. Up to equality. The objects can carry all kinds of mutable

Re: Planning a Python Course for Beginners

On Wed, Aug 9, 2017 at 7:51 PM, Marko Rauhamaa wrote: > Dennis Lee Bieber : > >> Then there is the facet that tuples (being unmutable) can be used as >> keys into a dictionary... > > Mutable objects can be used as keys into a dictionary. Only when

Re: Planning a Python Course for Beginners

Dennis Lee Bieber : > Then there is the facet that tuples (being unmutable) can be used as > keys into a dictionary... Mutable objects can be used as keys into a dictionary. Marko -- https://mail.python.org/mailman/listinfo/python-list

Re: Planning a Python Course for Beginners

Dennis Lee Bieber : > Tabs are logical entities indicating structure and should always be > used! I wrote an entire database program using only tabs. http://dilbert.com/strip/1992-09-08> Marko -- https://mail.python.org/mailman/listinfo/python-list

Re: Planning a Python Course for Beginners

On Aug 8, 2017 10:20 AM, "Stefan Ram" wrote: > > I am planning a Python course. > > I started by writing the course akin to courses I gave > in other languages, that means, the course starts roughly > with these topics: > > - number and string literals > - types

Re: Planning a Python Course for Beginners

On 2017-08-08, Peter Heitzer wrote: > Stefan Ram wrote: >> I am planning a Python course. > [different topics] >> Are there any other very simple things that >> I have missed and that should be covered very >> early in a Python

Re: Planning a Python Course for Beginners

On Tue, Aug 8, 2017 at 7:19 AM, Stefan Ram wrote: > I am planning a Python course. > > I started by writing the course akin to courses I gave > in other languages, that means, the course starts roughly > with these topics: > > - number and string literals > -

Re: Planning a Python Course for Beginners

On Wed, Aug 9, 2017 at 1:02 AM, Stefan Ram wrote: > Chris Angelico writes: >>Why a new Python course? > > It is not a course in the sense of a written text > (which I would call "course notes"). > > It is a course in the sense of an event, where I

Re: Planning a Python Course for Beginners

Stefan Ram wrote: > I am planning a Python course. [different topics] > Are there any other very simple things that > I have missed and that should be covered very > early in a Python course? The differences between blanks and tabs :-) -- Dipl.-Inform(FH) Peter

Re: Planning a Python Course for Beginners

On Wed, Aug 9, 2017 at 12:19 AM, Stefan Ram wrote: > I am planning a Python course. > Before answering any other questions, answer this one: Why a new Python course? How is it different from what already exists? The answer to that will govern just about everything