Re: [Tutor] Using a list comp in place of a for loop
2009/10/3 afith13 :
> Hi tutors,
>
> I've got a loop and a comprehension that do the same thing (as far as I can
> tell):
>
> for i in range(N):
> someList.append.newObject(i)
>
> ...and...
>
> [someList.append.newObject(i) for i in range(N)]
>
> I'm tempted to write this as a list comp because it's a big list and I've
> read that list comps are faster than for loops in general. I'm hesitating
> because I'm not sure if building a big list of "None" objects and then
> throwing it away has any drawbacks.
>
> Thanks in advance,
> ___
> Tutor maillist - tu...@python.org
> To unsubscribe or change subscription options:
> http://mail.python.org/mailman/listinfo/tutor
>
If you want a list comprehension to do the same as your original code,
you need to write it as
someList = [newObject(i) for i in range(N)]
This avoids building the list of Nones, and only differs if someList
already has values in it, in which case you can use:
someList.extend(newObject(i) for i in range(N))
which will do exactly what your original code does, except using a
list comp (actually a generator, which is pretty much the same thing),
and won't generate and then throw away a list of Nones.
I prefer list comprehensions over for-loops wherever possible because
they're easier to read (IMHO). As for speed, let's take a look:
>>> t_for = Timer("""l = []
... for i in range(100):
... l.append(i)""")
>>> t_slowComp = Timer("""l = []
... [l.append(i) for i in range(100)]
... """)
>>> t_comp = Timer("""l = [i for i in range(100)""")
>>> t_addComp = Timer("""l = []
... l += [i for i in range(100)]""")
>>> t_extend = Timer("""l = []
... l.extend(i for i in range(100))""")
>>> min(t_for.repeat(3,1))
0.826678279027
>>> min(t_slowComp.repeat(3,1))
0.95501802603394026
>>> min(t_comp.repeat(3,1))
0.35897579161587601
>>> min(t_addComp.repeat(3,1))
0.37727895584498583
>>> min(t_extend.repeat(3,1))
0.59708203137552118
>>>
As you can see, list comprehensions are better than twice the speed
(in this simple example) of a regular for loop. With an existing
list, adding a comprehension is surprisingly fast (I was expecting
extend to be faster). I wasn't expecting the slow comprehension to be
as slow as it is, but as you can see, it is slower than a standard for
loop.
Hope something in here has helped you, and has answered your question(s).
--
Rich "Roadie Rich" Lovely
There are 10 types of people in the world: those who know binary,
those who do not, and those who are off by one.
P.S. In case you're wondering, I feel the minimum of Timer.repeat()
results is more applicable than an average, as I feel - and the docs
agree - that it's a sort of "best speed", ignoring as many other
drains of processing power as possible.)
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Re: [Tutor] __mul__ for different variable types?
Awesome, Rich, thanks! - Warren (war...@wantonhubris.com) On Oct 4, 2009, at 5:31 PM, Rich Lovely wrote: 2009/10/4 Warren : I'm a little confused on this one. I have a Vector class that I want to be able to multiply by either another vector or by a single float value. How would I implement this in my override of __mul__ within that class? Do you check the variable type with a stack of "if isinstance" statements or something? What is the preferred Python way of doing this? - Warren (war...@wantonhubris.com) ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor I think isinstance is probably the most pythonic way of doing this, and don't forget to add __rmul__ as well, for floats: try: from numbers import Real except NameError: Real = (int, long, float) class Vector(object): #__init__ and other methods ommitted. def __mul__(self, other): """self * other""" if isinstance(other, Vector): # code for Vector * Vector elif isinstance(other, Number): # code for Vector * number else: return NotImplemented def __rmul__(self, other): """other * self""" return self.__mul__(other) Note that I've got no type checking in __rmul__, because if only Vector * Vector has a different value if the terms are swapped, and other * self will use other.__mul__ if other is a Vector. Also note the import at the top: numbers.Real is an Abstract Base Class use for all real number types, introduced in Python 2.6, so simplifies testing types. See http://docs.python.org/library/numbers.html for info. If it's not there, the code demos another feature of isinstance most people don't notice: the type argument can be a sequence of types, so I set Real to a tuple of all the builtin real number types (that I can remember off the top of my head) if the import fails. If you want multiplication to be defined for complex numbers, change all occurances of Real to Number, and add complex to the tuple. -- Rich "Roadie Rich" Lovely There are 10 types of people in the world: those who know binary, those who do not, and those who are off by one. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] __mul__ for different variable types?
2009/10/4 Warren : > > I'm a little confused on this one. > > I have a Vector class that I want to be able to multiply by either another > vector or by a single float value. How would I implement this in my > override of __mul__ within that class? > > Do you check the variable type with a stack of "if isinstance" statements or > something? What is the preferred Python way of doing this? > > - Warren > (war...@wantonhubris.com) > > > > > ___ > Tutor maillist - tu...@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor > I think isinstance is probably the most pythonic way of doing this, and don't forget to add __rmul__ as well, for floats: try: from numbers import Real except NameError: Real = (int, long, float) class Vector(object): #__init__ and other methods ommitted. def __mul__(self, other): """self * other""" if isinstance(other, Vector): # code for Vector * Vector elif isinstance(other, Number): # code for Vector * number else: return NotImplemented def __rmul__(self, other): """other * self""" return self.__mul__(other) Note that I've got no type checking in __rmul__, because if only Vector * Vector has a different value if the terms are swapped, and other * self will use other.__mul__ if other is a Vector. Also note the import at the top: numbers.Real is an Abstract Base Class use for all real number types, introduced in Python 2.6, so simplifies testing types. See http://docs.python.org/library/numbers.html for info. If it's not there, the code demos another feature of isinstance most people don't notice: the type argument can be a sequence of types, so I set Real to a tuple of all the builtin real number types (that I can remember off the top of my head) if the import fails. If you want multiplication to be defined for complex numbers, change all occurances of Real to Number, and add complex to the tuple. -- Rich "Roadie Rich" Lovely There are 10 types of people in the world: those who know binary, those who do not, and those who are off by one. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] Using a list comp in place of a for loop
Hi tutors, I've got a loop and a comprehension that do the same thing (as far as I can tell): for i in range(N): someList.append.newObject(i) ...and... [someList.append.newObject(i) for i in range(N)] I'm tempted to write this as a list comp because it's a big list and I've read that list comps are faster than for loops in general. I'm hesitating because I'm not sure if building a big list of "None" objects and then throwing it away has any drawbacks. Thanks in advance, ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] using easy_install to download eggs
Hello, I would like to use easy_install to cache packages registered at PyPi locally. How can I do this for packages? I tried the hints from: http://peak.telecommunity.com/DevCenter/EasyInstall#installing-on-un-networked-machines It worked for some packages. But for others, the command easy_install -zxad. mercurial just creates a subdirectory and not an *.egg file. How can I 1) use easy_install to download packages from PyPi to a locally saved egg-file? 2) use easy_install to download archinves (*.tar.gz / *.zip) to download the respective software package from the link indicated on PyPi? Thanks in advance, Timmie ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] small program
On Sun, Oct 4, 2009 at 1:59 PM, Andrius wrote: > Very good. Nice to hear that from another 'untouchable'. Must to > confirm for your that I'm usually left wing fella, so, if you have > nothing what to say for my in my case - please, fuck off. Re: http://catb.org/~esr/faqs/smart-questions.html#not_losing Good luck, -Luke ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] __mul__ for different variable types?
I'm a little confused on this one. I have a Vector class that I want to be able to multiply by either another vector or by a single float value. How would I implement this in my override of __mul__ within that class? Do you check the variable type with a stack of "if isinstance" statements or something? What is the preferred Python way of doing this? - Warren (war...@wantonhubris.com) ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
