Re: [Tutor] Immutable objects
"Nitin Das" wrote
class mymut(object):
def __setattr__(self,k,v):
if hasattr(self,k):
if self.__dict__.get(k) == None:
self.__dict__[k] = v
Do you need to look up the dict?
Surely you could just use
if self.k is None
self.k = v
which looks a lot simpler to me...
else:
raise TypeError("Cant Modify Attribute Value")
You could mention the class for example.
Compare it with the string and tuple messages:
'fred'[2] = 'g'
Traceback (most recent call last):
File "", line 1, in
TypeError: 'str' object does not support item assignment
(1,2,30)[1] = 5
Traceback (most recent call last):
File "", line 1, in
TypeError: 'tuple' object does not support item assignment
So if your error said
TypeError: mymut object does not support item assignment
that would be more consistent with the Python internal objects.
else:
raise TypeError("Immutable Object")
The Python immutables seem to raise an Attribute Error here
rather than a Type error. And again mentioning the class name
makes debugging easier:
'l'.x = 4
Traceback (most recent call last):
File "", line 1, in
AttributeError: 'str' object has no attribute 'x'
HTH,
--
Alan Gauld
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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[Tutor] question about a exercise.
Hello, I follow the online book " How to think like a computer scientist". But now I have a problem with a exercise. This is not homework. The exercise is : Using a text editor, create a Python script named tryme3.py . Write a function in this file called nine_lines that uses three_lines to print nine blank lines. Now add a function named clear_screen that prints out twenty-five blank lines. The last line of your program should be a call to clear_screen. So I begin with this code : def threelines(): print print print print "regel 1" threelines() print "regel 2" But now I get a terminated at line 2 message. What did I do wrong. I work on a Win7 machine with python 2.7 and as a editor SPE. Regards, Roelof ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
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Re: [Tutor] Immutable objects
On Thu, 19 Aug 2010 01:24:11 pm Nitin Das wrote: > Hello, > > Can somebody help me how to make immutable objects in python. I > have seen overriding the __new__, __setattr__ methods.. but not > comfortable with how to use them to make the object immutable. > > thanks in advance > --nitin I have found this useful: http://northernplanets.blogspot.com/2007/01/immutable-instances-in-python.html Keep in mind that the only way to make *truly* immutable objects in Python is to write them in C. Immutability of pure-Python objects can be broken if you try hard enough. Immutability is by cooperation, not enforced. -- Steven D'Aprano ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] question about a exercise.
Can you copy and paste the exact error message you receive in the traceback when you run your program. Also, copy the complete program here since it is only a handful of lines On Thu, Aug 19, 2010 at 7:31 AM, Roelof Wobben wrote: > Hello, > > I follow the online book " How to think like a computer scientist". > But now I have a problem with a exercise. > This is not homework. > > The exercise is : > > Using a text editor, create a Python script named tryme3.py . Write a > function in this file called nine_lines that uses three_lines to print > nine blank lines. Now add a function named clear_screen that prints out > twenty-five blank lines. The last line of your program should be a *call*to > clear_screen. > > So I begin with this code : > > def threelines(): > print > print > print > > print "regel 1" > threelines() > print "regel 2" > > But now I get a terminated at line 2 message. > What did I do wrong. > > I work on a Win7 machine with python 2.7 and as a editor SPE. > > Regards, > > Roelof > > > ___ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor > > -- Joel Goldstick ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Immutable objects
Nitin Das wrote:
> class mymut(object):
>
> def __setattr__(self,k,v):
> if hasattr(self,k):
> if self.__dict__.get(k) == None:
> self.__dict__[k] = v
> else:
> raise TypeError("Cant Modify Attribute Value")
> else:
> raise TypeError("Immutable Object")
>
>
> class mm(mymut):
> x = ''
> y = ''
> def __init__(self,x,y):
> self.x = x
> self.y = y
>
>
>
> p = mm(10,11)
> print p.x
> print p.y
>
>
> I have created this immutable object.Is there any other better
> implementation?
How about
>>> from collections import namedtuple
>>> Mm = namedtuple("Mm", "x y")
>>> p = Mm(10, 11)
>>> p.x
10
>>> p.y
11
>>> p.x = 42
Traceback (most recent call last):
File "", line 1, in
AttributeError: can't set attribute
>>> p.z = 42
Traceback (most recent call last):
File "", line 1, in
AttributeError: 'Mm' object has no attribute 'z'
>>>
Peter
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Re: [Tutor] question about a exercise.
Hello, I don't see a traceback only this output. regel 1 regel 2 script terminated. And the code I gave you was the whole code because Im still working on this one. I have learned to do one problem at the time so in mu opion first make the part which prints out 3 lines and then expand to 9 lines and then expand to the function clear_screen Roelof Date: Thu, 19 Aug 2010 08:37:25 -0400 Subject: Re: [Tutor] question about a exercise. From: joel.goldst...@gmail.com To: rwob...@hotmail.com CC: tutor@python.org Can you copy and paste the exact error message you receive in the traceback when you run your program. Also, copy the complete program here since it is only a handful of lines On Thu, Aug 19, 2010 at 7:31 AM, Roelof Wobben wrote: Hello, I follow the online book " How to think like a computer scientist". But now I have a problem with a exercise. This is not homework. The exercise is : Using a text editor, create a Python script named tryme3.py . Write a function in this file called nine_lines that uses three_lines to print nine blank lines. Now add a function named clear_screen that prints out twenty-five blank lines. The last line of your program should be a call to clear_screen. So I begin with this code : def threelines(): print print print print "regel 1" threelines() print "regel 2" But now I get a terminated at line 2 message. What did I do wrong. I work on a Win7 machine with python 2.7 and as a editor SPE. Regards, Roelof ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor -- Joel Goldstick ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Immutable objects
Peter Otten wrote:
> Nitin Das wrote:
>
>> class mymut(object):
>>
>> def __setattr__(self,k,v):
>> if hasattr(self,k):
>> if self.__dict__.get(k) == None:
>> self.__dict__[k] = v
>> else:
>> raise TypeError("Cant Modify Attribute Value")
>> else:
>> raise TypeError("Immutable Object")
>>
>>
>> class mm(mymut):
>> x = ''
>> y = ''
>> def __init__(self,x,y):
>> self.x = x
>> self.y = y
>>
>>
>>
>> p = mm(10,11)
>> print p.x
>> print p.y
>>
>>
>> I have created this immutable object.Is there any other better
>> implementation?
>
> How about
>
from collections import namedtuple
Mm = namedtuple("Mm", "x y")
p = Mm(10, 11)
p.x
> 10
p.y
> 11
p.x = 42
> Traceback (most recent call last):
> File "", line 1, in
> AttributeError: can't set attribute
p.z = 42
> Traceback (most recent call last):
> File "", line 1, in
> AttributeError: 'Mm' object has no attribute 'z'
By the way, you can see the class definition with
namedtuple("Mm", "x y", verbose=True)
Peter
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Re: [Tutor] question about a exercise.
"Roelof Wobben" wrote I don't see a traceback only this output. That looks about right to me, what did you expect to see that was different? regel 1 regel 2 script terminated. = I have learned to do one problem at the time so in mu opion first make the part which prints out 3 lines and then expand to 9 lines and then expand to the function clear_screen Yes thats good thinking. HTH -- Alan Gauld Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Multiple file open
Hello All,
>> Please guide with the syntax.
>
> All beginners tutorials on the web teach the syntax of python.. I am
> unsure what your questions is.
My bad with code.
>
>> below is the existing program with Formula A (Mean). Formula B will be
>> Extrapolation,
>> also I have not been able to do justice to 'except IOError:' part.The
>
> It has many errors which need fixing but..
>
>> program ends with an error.
>
> And if you get an error always make sure you provide it to this list.
> People on this list can not read minds ;-).
I was wondering ... what else i need to post. Thank you. I REALLY need
to keep that in mind.
>
>>
>> import sys,os, fileinput
>>
>>
>> file11 = raw_input('Enter PR1 File name :')
>> fp1 = open(file11,'r')
>>
>> file12 = raw_input('Enter PR3 File Name :')
>> fp2 = open(file12,'r')
>>
>> file3 = raw_input('Enter PR2 / PR4 OUTPUT File Name :')
>> fp3 = open(file3,'w')
>
> What happens when you enter a wrong filename? It will raise IOError
> the moment you use open(). Below is 1 example of what you might want
> tot do.
>
> file11 = raw_input('Enter PR1 File name :')
> try:
> fp1 = open(ffile11, 'r')
> except IOError:
> sys.exit('Could not open file: %s' % file11)
>
Adding this 'try-except'
>> while True:
>> try:
>> line1A = fp1.readline()
>
> You can read and split the line in 1 go.
> line1 = fp1.readline().split(",")
:) so much better ... adding again.
>
>> line1B = fp2.readline()
>> line1 = line1A.split(",")
>
> You can now remove the above, see earlier comment.
>
>> col1 = line1[0]
>> col2 = line1[1]
>> col3 = line1[2]
>> col4 = line1[3]
>> col5 = line1[20]
>> col6 = line1[21]
>> col7 = line1[22]
>> line2 = line1B.split(",")
>> col8 = line2[1]
>> col9 = line2[2]
>> col10 = line2[3]
>> col11 = line2[20]
>> col12 = line2[21]
>> col13 = line2[22]
>
> Above you create a list "line1 = fp1.readline().split(",")". Lists are
> mutable so you can add, remove and insert new items. You can also
> access the items in the list by index. You can even add 2 lists to
> make a new list, eg: .
>
[1,2,3,4,5] + [6,7,8,9,0]
> [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]
>
> Now try and apply this to all your col variables.
Actually I needed to do further computation, therefore I preferred to
keep the values separate.
>
>> # calculation of PR2 as per formula
>> #(A+B)/2 , ie. Mean of the two values
>> col14 = ( (float(col2)) + (float(col8)) / 2)
>
> You can write this as:
> col14 = (float(line1[1])) + (float(line2[1])) / 2)
I did this in one of my attempts. Pl dont ask me what was the error.
Did not save.
>
> And avoid creating all these col variables. But what would happen if
> col2/line1[1] has a value of "I am not a number"? Do note the comment
> below on the actual calculation.
I am keeping (cleaning) the Input data file as per the program.
>
>> col15 = ( (float(col3)) + (float(col9)) / 2)
>
>> col19 = ( (float(col7)) + (float(col13)) / 2)
>
> Your "mean" calculation is wrong (hint, division has precedence over adding).
THANKS aarrhh ... I missed another pair of braces.
>
> Greets
> Sander
>
Once the Final version is over will post it.
Thanks
Nitin
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Re: [Tutor] Immutable objects
Thanks guys,
NamedTuple implementation is preety nice solution.
--nitin
On Thu, Aug 19, 2010 at 6:28 PM, Peter Otten <__pete...@web.de> wrote:
> Peter Otten wrote:
>
> > Nitin Das wrote:
> >
> >> class mymut(object):
> >>
> >> def __setattr__(self,k,v):
> >> if hasattr(self,k):
> >> if self.__dict__.get(k) == None:
> >> self.__dict__[k] = v
> >> else:
> >> raise TypeError("Cant Modify Attribute Value")
> >> else:
> >> raise TypeError("Immutable Object")
> >>
> >>
> >> class mm(mymut):
> >> x = ''
> >> y = ''
> >> def __init__(self,x,y):
> >> self.x = x
> >> self.y = y
> >>
> >>
> >>
> >> p = mm(10,11)
> >> print p.x
> >> print p.y
> >>
> >>
> >> I have created this immutable object.Is there any other better
> >> implementation?
> >
> > How about
> >
> from collections import namedtuple
> Mm = namedtuple("Mm", "x y")
> p = Mm(10, 11)
> p.x
> > 10
> p.y
> > 11
> p.x = 42
> > Traceback (most recent call last):
> > File "", line 1, in
> > AttributeError: can't set attribute
> p.z = 42
> > Traceback (most recent call last):
> > File "", line 1, in
> > AttributeError: 'Mm' object has no attribute 'z'
>
>
> By the way, you can see the class definition with
>
> namedtuple("Mm", "x y", verbose=True)
>
> Peter
>
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[Tutor] List comprehension for dicts?
Hi,
I've been reading up on list comprehensions lately, all userful and powerful
stuff - trying to wrap my brain around it :)
As the examples all seem to relate to lists, I was wondering if there is an
elegant similar way to apply a function to all keys in a dictionary?
(without looping over it, of course)
I'm trying to convert all keys in a dict to uppercase, as in:
INPUT:
age_dict = { 'pete': 42, 'ann': 25, 'carl': 30, 'amanda': 64 }
OUTPUT:
age_dict = { 'PETE': 42, 'ANN': 25, 'CARL': 30, 'AMANDA': 64 }
I googled 'dictionary comprehension' but couldn't get any code to work with the
examples given.
http://www.siafoo.net/article/52#dictionary-comprehensions
thanks,
Pete
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Re: [Tutor] List comprehension for dicts?
Hey you can use list comprehension here
age_dict = { 'pete': 42, 'ann': 25, 'carl': 30, 'amanda': 64 }
you can create a dict from a list of tuples and you can access the dict as
a
list of tuples by accessing its items
Example:
age_dict = dict([(key.upper(), value) for key,value in age_dict.items()])
hope that helps,
Vince
On Thu, Aug 19, 2010 at 8:51 AM, Pete wrote:
> Hi,
>
> I've been reading up on list comprehensions lately, all userful and
> powerful stuff - trying to wrap my brain around it :)
>
> As the examples all seem to relate to lists, I was wondering if there is an
> elegant similar way to apply a function to all keys in a dictionary?
>
> (without looping over it, of course)
>
> I'm trying to convert all keys in a dict to uppercase, as in:
>
> INPUT:
> age_dict = { 'pete': 42, 'ann': 25, 'carl': 30, 'amanda': 64 }
>
> OUTPUT:
> age_dict = { 'PETE': 42, 'ANN': 25, 'CARL': 30, 'AMANDA': 64 }
>
> I googled 'dictionary comprehension' but couldn't get any code to work with
> the examples given.
>
> http://www.siafoo.net/article/52#dictionary-comprehensions
>
> thanks,
>
> Pete
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Re: [Tutor] List comprehension for dicts?
Pete wrote:
> Hi,
>
> I've been reading up on list comprehensions lately, all userful and
> powerful stuff - trying to wrap my brain around it :)
>
> As the examples all seem to relate to lists, I was wondering if there is
> an elegant similar way to apply a function to all keys in a dictionary?
>
> (without looping over it, of course)
>
> I'm trying to convert all keys in a dict to uppercase, as in:
>
> INPUT:
> age_dict = { 'pete': 42, 'ann': 25, 'carl': 30, 'amanda': 64 }
>
> OUTPUT:
> age_dict = { 'PETE': 42, 'ANN': 25, 'CARL': 30, 'AMANDA': 64 }
>
> I googled 'dictionary comprehension' but couldn't get any code to work
> with the examples given.
Python 2.4-2.6
>>> dict((k.upper(), v) for k, v in age_dict.iteritems())
{'PETE': 42, 'ANN': 25, 'AMANDA': 64, 'CARL': 30}
Python 2.7
>>> {k.upper(): v for k, v in age_dict.iteritems()}
{'PETE': 42, 'ANN': 25, 'AMANDA': 64, 'CARL': 30}
Python 3.x
>>> {k.upper(): v for k, v in age_dict.items()}
{'PETE': 42, 'ANN': 25, 'AMANDA': 64, 'CARL': 30}
items() instead of iteritems() works in 2.x, too, but is less efficient.
Peter
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Re: [Tutor] List comprehension for dicts?
On Thu, Aug 19, 2010 at 10:02 AM, Vince Spicer wrote:
> Hey you can use list comprehension here
>
>
> age_dict = { 'pete': 42, 'ann': 25, 'carl': 30, 'amanda': 64 }
>
> you can create a dict from a list of tuples and you can access the dict as
> a
> list of tuples by accessing its items
>
> Example:
>
> age_dict = dict([(key.upper(), value) for key,value in age_dict.items()])
>
This is a bad place to use a list comprehension. This will create a list of
values first and then create a dict from that list, so now you have a list
floating around that you didn't need.
Generator expressions, OTOH, generate the values on the fly, only as they're
needed, so there's no extra list left over once the dict is created.
Sure it will eventually be garbage collected, but "waste not, want not", as
my grandmother used to say.
-Wayne
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Re: [Tutor] List comprehension for dicts?
On Thu, Aug 19, 2010 at 8:21 PM, Pete wrote:
> Hi,
>
> I've been reading up on list comprehensions lately, all userful and
> powerful stuff - trying to wrap my brain around it :)
>
> As the examples all seem to relate to lists, I was wondering if there is an
> elegant similar way to apply a function to all keys in a dictionary?
>
> (without looping over it, of course)
>
> I'm trying to convert all keys in a dict to uppercase, as in:
>
> INPUT:
> age_dict = { 'pete': 42, 'ann': 25, 'carl': 30, 'amanda': 64 }
>
> OUTPUT:
> age_dict = { 'PETE': 42, 'ANN': 25, 'CARL': 30, 'AMANDA': 64 }
>
> I googled 'dictionary comprehension' but couldn't get any code to work with
> the examples given.
>
> http://www.siafoo.net/article/52#dictionary-comprehensions
>
> thanks,
>
There is dict comprehension too from python2.7 onwards.
>
> Pete
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--
~l0nwlf
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Re: [Tutor] List comprehension for dicts?
On Fri, 20 Aug 2010 01:40:54 am Wayne Werner wrote: > > age_dict = dict([(key.upper(), value) for key,value in > > age_dict.items()]) > > This is a bad place to use a list comprehension. This will create a > list of values first and then create a dict from that list, so now > you have a list floating around that you didn't need. How do you know that dict() doesn't need it the entire list at once? Unless you're an expert on the implementation, for all you know it looks something like this: class dict: def __new__(cls, arg): ... try: n = len(arg) except AttributeError: # probably a generator expression arg = list(arg) n = len(arg) allocate_memory_for_items(n) ... (only written in C). I'm not saying it does, or that it doesn't, but you're assuming a pattern of behaviour which might not be the case. Here's a similarly common idiom: which of these is faster? ' '.join(gen_expr) ' '.join(list_comp) [st...@sylar ~]$ python -m timeit "' '.join(str(n) for n in xrange(30))" 10 loops, best of 3: 437 msec per loop [st...@sylar ~]$ python -m timeit "' '.join([str(n) for n in xrange(30)])" 10 loops, best of 3: 401 msec per loop The list comprehension is consistently faster, because join() works more efficiently if it knows how many items it needs to pre-allocate memory for. > Generator expressions, OTOH, generate the values on the fly, only as > they're needed, so there's no extra list left over once the dict is > created. And sometimes that's a win, and sometimes it's not. Generator expressions are more computationally expensive than lists -- they're functions which remember their internal state so you can pause them and restart them at will. That doesn't happen for free -- it takes memory and time. The reason Python has generator expressions is that for large amounts of data, the saving you have by not needing to produce the entire list all at once more than makes up for the extra cost, but for small amounts of data, that's not always the case: [st...@sylar ~]$ python -m timeit "dict((k,k+1) for k in xrange(2))" 10 loops, best of 3: 5.89 usec per loop [st...@sylar ~]$ python -m timeit "dict([(k,k+1) for k in xrange(2)])" 10 loops, best of 3: 4.78 usec per loop Here, using a generator expression is a pessimation, not an optimization. > Sure it will eventually be garbage collected, but "waste not, want > not", as my grandmother used to say. Does your grandmother have a box labelled "Pieces of string, too short to use" as well? -- Steven D'Aprano ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension for dicts?
(You top-posted, so now I have to delete the older part)
Vince Spicer wrote:
Hey you can use list comprehension here
age_dict = { 'pete': 42, 'ann': 25, 'carl': 30, 'amanda': 64 }
you can create a dict from a list of tuples and you can access the dict as
a
list of tuples by accessing its items
Example:
age_dict = dict([(key.upper(), value) for key,value in age_dict.items()])
hope that helps,
Vince
The only catch to that is that two or more of the keys may map to the
same uppercase key. If one needs to detect that, one has to write an
explicit loop, rather than just converting with dict()
DaveA
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[Tutor] flow problem with a exercise
Hello, I have this exercise: Now write the function is_odd(n) that returns True when n is odd and False otherwise. Include doctests for this function as you write it. Finally, modify it so that it uses a call to is_even to determine if its argument is an odd integer. So I thought of this : def is_even(argument): remainder= argument%2 if remainder == 0 : return True else : return False def is_odd(argument): uitkomst=is_even(argument) return uitkomst even=is_odd(1) ; if even==True : print "Even getal" if even==False: print "Oneven getal" But now I get this error message : return uitkomst Syntax error : return outside function. In my opinon even calls is_odd , then uitkomst calls is_even which gives a true or false to uitkomst. So return uitkomst gives the outcome to even. But the intepreter thinks otherwise. I work on a Win7 machine with Python 2.7 Roelof ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] flow problem with a exercise
On Thu, Aug 19, 2010 at 3:01 PM, Roelof Wobben wrote: > Hello, > > I have this exercise: > > > Now write the function is_odd(n) that returns True when n is odd and > Falseotherwise. Include doctests for this function as you write it. > Finally, modify it so that it uses a call to is_even to determine if its > argument is an odd integer. > > So I thought of this : > > def is_even(argument): > remainder= argument%2 > if remainder == 0 : > return True > else : > return False > > def is_odd(argument): > uitkomst=is_even(argument) > return uitkomst > NOTE that your return statement is not indented properly. It must line up under uitkomst > > even=is_odd(1) ; > if even==True : > print "Even getal" > if even==False: > print "Oneven getal" > > > But now I get this error message : > > return uitkomst > Syntax error : return outside function. > > > In my opinon even calls is_odd , then uitkomst calls is_even which gives a > true or false to uitkomst. So return uitkomst gives the outcome to even. > But the intepreter thinks otherwise. > > I work on a Win7 machine with Python 2.7 > > Roelof > > > ___ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor > > -- Joel Goldstick ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] flow problem with a exercise
On Thu, Aug 19, 2010 at 2:01 PM, Roelof Wobben wrote: > > def is_odd(argument): > uitkomst=is_even(argument) > return uitkomst > > even=is_odd(1) ; > if even==True : > print "Even getal" > if even==False: > print "Oneven getal" > > > But now I get this error message : > > return uitkomst > Syntax error : return outside function. > Check your indention. In the email your return is at a different level than your function. Always use 4 spaces. If your editor can't convert, you need a better editor! > In my opinon even calls is_odd , then uitkomst calls is_even which gives a > true or false to uitkomst. So return uitkomst gives the outcome to even. > But the intepreter thinks otherwise. > Well, good thing opinions don't count for much when talking to a computer ;) You are correct that uitkomst contains the result of is_even, but your return is on a different indention level. Whitespace is significant here. If I were you, I would define is_odd this way: def is_odd(argument): uitkomst = not is_even(argument) return uitkomst > I work on a Win7 machine with Python 2.7 > Good job providing both the traceback and your system. They make every problem easier to debug. HTH, Wayne ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] flow problem with a exercise
On 8/19/10, Roelof Wobben wrote: > > Hello, > > > > I have this exercise: > > > > Now write the function is_odd(n) that returns True when n is odd and False > otherwise. Include doctests for this function as you write it. > Finally, modify it so that it uses a call to is_even to determine if its > argument is an odd integer. > > > > So I thought of this : > > > > def is_even(argument): > remainder= argument%2 > if remainder == 0 : > return True > else : > return False > > > def is_odd(argument): > uitkomst=is_even(argument) > return uitkomst The above line, the return statement, has to be indented; it did not appear to be in your email. Also, maybe you want to return the opposite of is_even in is_odd? Right now it looks like you are going to get something like: is_odd(3): is_even(3)=False, so is_odd(3) will echo that False. Maybe return !uitkomst instead. I could have read it wrong, though. > > > even=is_odd(1) ; > if even==True : > print "Even getal" > if even==False: > print "Oneven getal" > > > > But now I get this error message : > > > > return uitkomst > > Syntax error : return outside function. > > > > > > In my opinon even calls is_odd , then uitkomst calls is_even which gives a > true or false to uitkomst. So return uitkomst gives the outcome to even. > > But the intepreter thinks otherwise. > > > > I work on a Win7 machine with Python 2.7 > > > > Roelof > > > -- Have a great day, Alex (msg sent from GMail website) mehg...@gmail.com; http://www.facebook.com/mehgcap ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] flow problem with a exercise
Roelof Wobben wrote: def is_odd(argument): uitkomst=is_even(argument) return uitkomst You forgot to indent the return statement to match the other statement(s) in the function. DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension for dicts?
On 8/19/2010 7:51 AM Pete said... Hi, I've been reading up on list comprehensions lately, all userful and powerful stuff - trying to wrap my brain around it :) As the examples all seem to relate to lists, I was wondering if there is an elegant similar way to apply a function to all keys in a dictionary? (without looping over it, of course) Just fyi, as it sounds you may not understand. List Comprehensions loop over the list. There's no other magic going on that avoids that. result = [ ii for ii in lst if cond ] is just shorthand for result = [] for ii in lst: if cond: result.append(ii) Emile ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension for dicts?
On 2010-08-19, at 5:25 PM, Emile van Sebille wrote: > On 8/19/2010 7:51 AM Pete said... >> Hi, >> >> I've been reading up on list comprehensions lately, all userful and powerful >> stuff - trying to wrap my brain around it :) >> >> As the examples all seem to relate to lists, I was wondering if there is an >> elegant similar way to apply a function to all keys in a dictionary? >> >> (without looping over it, of course) > > Just fyi, as it sounds you may not understand. List Comprehensions loop over > the list. There's no other magic going on that avoids that. > > result = [ ii for ii in lst if cond ] > > is just shorthand for > > result = [] > for ii in lst: > if cond: >result.append(ii) > > Emile Ah, so list comprehensions are a purely syntactic construct? I had the feeling there might be a performance benefit of some kind. thanks for all the contributions on this topic, it's been greatly helpful to my understanding. -Pete ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] List comprehension for dicts?
On Fri, 20 Aug 2010 09:51:08 am Pete wrote: [...] > Ah, so list comprehensions are a purely syntactic construct? No, I don't think that's what Emile was trying to say. It's not like list comps are macros that are expanded into the explicit for-loop like that. All he is saying is that both the for-loop and the list comprehension loop over the list. There are some functional differences: for example, the list comp avoids needing to look up result.append over and over again. Because it knows it is dealing with a list, rather than some arbitrary object that conceivably might have some weird append method with strange side-effects, it can take optimizing short-cuts that aren't applicable to general for-loops. > I had the feeling there might be a performance benefit of some kind. If you write the for-loop in the most straight forward, obvious fashion, then there is a small but measurable performance cost from repeatedly looking up the append method. On the other hand, for-loops are more general -- you can call break to exit early, or continue to skip an iteration, so they're easier to optimize. There's no way to break out of a list comp early (other than raising an exception, which defeats the purpose). No matter how fast you can perform a loop, it's always faster to avoid it altogether, so this: seq = xrange(1000) result = [] for i in seq: if i >= 10: break result.append(i%2) will be much faster than this: seq = xrange(1000) result = [i%2 for i in seq if i < 10] Other than that, the speed of the loop itself is virtually the same as the speed of the list comprehension. -- Steven D'Aprano ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
