[Tutor] is v == for small ints

[Alan Gauld]

Somebody posted a question about this today and I approved
it but it hasn't shown up. We have had about 6 or 7 such
problems in the last month. Mainly they have been thank-you
messages so I didn't make an issue of it but a couple
have been genuine questions, like this was.

So if you have posted recently and it hasn't got through,
my apologies, but I have no idea what is going wrong.

As to the content of this message, the gist was that he had
tried 'is' and == with both small and large ints. When using
small ints the results were the same for both operators but
for large ints they differed. He knew the conceptual difference
between 'is' and == but didn't understand why the results
were different for large/small ints.

The answer is of course that small ints are cached internally
as an optimisation so all references to 5, for example,
refer to the same object in memory and therefore both is
(object identity) and == (value equality) are true. Large
ints result in two separate objects each with the same
value so 'is' becomes False and == remains true.

This behaviour should never be relied on since the definition
of a "small int" is potentially implementation dependant and
could even change with Python version or platform. Always
use == to test integers.

Apologies again about the missing messages.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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Re: [Tutor] interface

[Alan Gauld]

On 14/12/15 21:48, Alex Kleider wrote:

> provide a graphical interface.  There are several choices with pros and
> cons to each but an alternative more general approach might be to use a
> web based interface which I believe would make the code less platform
> dependent.

Not necessarily since the web server end still needs to live on
a particular OS platform and any OS level operations will be
just as restrictive for a web app as for a GUI. But a web app
is able to be opened up to multiple platforms as clients so
it depends on how you intend to deploy  it. If its just for
you (or for any one user) then the web doesn't help much
and adds lots of extra learning.

A GUI will involve new concepts too and a lot more code than
a CLI like you have been using. But a web app will require
whole new languages(HTML/CSS/Javascript) as well as new
concepts. A much steeper learning curve unless you already
come from a web background.

The simplest GUI framework is Tkinter, but its also fairly
limiting longer term. For putting a quick form interface
on top of an existing app its great but for a full GUI
app its less effective. It also has virtually no GUI
builder tool support.

wxPython is slightly more complex but more powerful and
looks more like the native UI on each platform. (There
is also Dabo which is wxPython based but has an IDE with
GUI builder and is very good for database oriented apps.)

pyQt/Side and PyGTK are both much more complex but also
more powerful. IDES with GUI Builders exist for both
although none of them worked very well for me.

They are all cross platform and you more or less trade
power and flexibility for simplicity. Personally I'd start
with Tkinter  - its a good general intro to the concepts of
GUI programming and standard Python so portable to other
users without extra downloads. Then decide which of the
other three to migrate too if you need more power.
Personally I find Tkinter good enough for almost all
my GUI needs, but I'm not distributing them or selling
them.

All of the above is strictly my personal opinion of
course. Others may disagree.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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Re: [Tutor] Understanding a linear runtime implementation of anagram detection

[Oscar Benjamin]

On 10 December 2015 at 12:38, Spyros Charonis  wrote:
> Dear All,
>
> I am learning about analysis of algorithms (python 2.7.6). I am reading a
> book (Problem solving with Algorithms and Data Structures) where Python is
> the language used for implementations. The author introduces algorithm
> analysis in a clear and understandable way, and uses an anagram detection
> program as a template to compare different runtime implementations
> (quadratic, log linear, linear). In the linear, and most efficient
> implementation, the code is as follows (comments added by me):

This is a good example problem for demonstrating the idea of
computational complexity but having actually wanted to write a program
that used anagram detection recently I concluded that a quadratic
performance can be acceptable or maybe even preferable for this. Here
is my own anagram function:

def anagram(s1, s2):
seen = [False] * len(s2)
for c1 in s1:
for n2, c2 in enumerate(s2):
if c1 == c2 and not seen[n2]:
seen[n2] = TrueI would do this by using a dict
break
else:
return False
else:
return all(seen)

This has O(N**2) complexity with N the string lengths. However the
real average complexity depends on the distribution of strings. The
loops in the above only run to completion if the two strings are
anagrams. Most of the time if you need to check whether two strings
are anagrams the probability that they are is small. When two strings
are not anagrams then the chances are typically high that the first
character of s1 is not in s2 at all. What that means is that the most
common runtime behaviour is to take the first character c1 from s1
then loop once through s2 verifying that none of the characters in s2
is equal to c1. So in the most common case we have a single iteration
through each character of one of the strings, whereas the "linear"
complexity algorithm you showed will always loop through both strings
fully. There are other differences such as not assuming lower-case
ascii characters etc. but these are not so important to this point.

If course in practice in Python the above code is inefficient because
it is implemented by the interpreter. The following is O(N*log(N)) but
will probably out-compete any of the examples shown:

def anagram(s1, s2):
return sorted(s1) == sorted(s2)

The sorted function in Python is very efficient so it will be hard to
beat the performance of that unless the strings really are long. This
is why anagrams are also a poor example of algorithmic performance: in
most contexts where we might practically want to check for anagrams
the strings involved are likely to be very short meaning that the
big-O type analysis is useless.


> My questions are:
...
>
> 2)
> How could I go about adapting this algorithm for multiple strings (say I
> had 20 strings and wanted to check if they are anagrams of one another).
>
> def are_anagrams(*args):
>
> """ Accepts a tuple of strings and checks if
>
>  they are anagrams of each other """
>
>
>  # Check that neither of strings are null
>
>  for i in args:
>
>  if not i:
>
>  raise TypeError, "Invalid input"
>
>  return None
>
>
>
>  # Initialize a list of counters for each string
>
>  c = ( [] for i in range(len(args) ) ???

The complexity problem here is that you want to avoid comparing all N
strings against each other. If you do then your comparing N*(N-1)/2
(quadratic) strings for anagram-ness. Peter suggested using tuples of
counts so that a dict can check them. I would use sorted string keys:

def anagrams(*words):
bykey = {}
for word in words:
key = ''.join(sorted(word))
if key not in bykey:
bykey[key] = [word]
else:
bykey[key].append(word)
return bykey.values()

--
Oscar
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Re: [Tutor] Tutor Digest, Vol 142, Issue 11

[Crusier]

Dear Alan,

Thank you very much for answering the question. If you don't mind,
please kindly let me know which library I should focus on among
beautifulsoup, selenium + PhantomJS, and dryscrape.

Have a good day

regards,
Hank

On Sun, Dec 13, 2015 at 5:11 PM,   wrote:
> Send Tutor mailing list submissions to
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> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of Tutor digest..."
>
>
> Today's Topics:
>
>1. Re: Calculation error with a simple program (Steven D'Aprano)
>2. Re: Calculation error with a simple program (Jim Gallaher)
>3. Re: Calculation error with a simple program (Todd Purple)
>4. Re: Tutor Digest, Vol 142, Issue 10 (Ken Hammer)
>5. Beautiful Soup (Crusier)
>6. Re: Beautiful Soup (Alan Gauld)
>
>
> --
>
> Message: 1
> Date: Sun, 13 Dec 2015 04:00:25 +1100
> From: Steven D'Aprano 
> To: tutor@python.org
> Subject: Re: [Tutor] Calculation error with a simple program
> Message-ID: <20151212170025.gk3...@ando.pearwood.info>
> Content-Type: text/plain; charset=us-ascii
>
> On Sat, Dec 12, 2015 at 01:03:05AM -0600, Jim Gallaher wrote:
>> Hi everyone. I'm reading through a beginners Python book and came up
>> with a super simple program. I'm not getting any errors and everything
>> runs through, but there's a logical calculation error. What the program
>> does is take an amount and calculate a couple percentages and add a
>> couple fees.
>>
>> For example, if I put in a value of 1, it will output 752.12 as the sub
>> total and 753.12 as the grand total. It's off by 1 on sub total and 2 on
>> grand total.
>
> Check your arithmetic -- with a base price of $1, the sub total is
> $752.12 and the grand total is $753.12, exactly as Python calculates.
>
>
> But I wonder whether you have made a logic mistake in your code. You
> say:
>
> dealerPrep = basePrice + 500
> destinationCharge = basePrice + 250
>
> This means that the car will cost more than TRIPLE the base price.
> Instead of $1, enter a base price of $40,000 and you will see what I
> mean: now the dealer prep is $40500 and the destination charge is
> $40250, which added to the base price gives $120750 (plus tax and
> licence). Surely that's not right, the deal charges more than the cost
> of the car for preparation? I think what you want is:
>
> dealerPrep = 500
> destinationCharge = 250
>
>
>
> --
> Steve
>
>
> --
>
> Message: 2
> Date: Sat, 12 Dec 2015 11:34:27 -0600
> From: Jim Gallaher 
> To: tutor@python.org
> Subject: Re: [Tutor] Calculation error with a simple program
> Message-ID: <8774a398-4a0f-41df-bbd2-941b49a3c...@gmail.com>
> Content-Type: text/plain;   charset=us-ascii
>
> Hi Alan,
>
> I'm 100 percent sure I'm wrong. :-) I verified it when I fixed the mistake. 
> The problem was it was adding in the basePrice and the fixed 
> rates/percentages each time. So I figured it out when Ian said something 
> about that.
>
> Thanks for everyone's help! :-)
>
> --
>
> Message: 3
> Date: Sat, 12 Dec 2015 08:13:23 -0500
> From: Todd Purple 
> To: Jim Gallaher 
> Cc: tutor@python.org
> Subject: Re: [Tutor] Calculation error with a simple program
> Message-ID: <6ec6d759-3c19-4a6d-8dd7-6efae1958...@yahoo.com>
> Content-Type: text/plain;   charset=us-ascii
>
>
>
>> On Dec 12, 2015, at 2:03 AM, Jim Gallaher  wrote:
>>
>> Hi everyone. I'm reading through a beginners Python book and came up with a 
>> super simple program. I'm not getting any errors and everything runs 
>> through, but there's a logical calculation error. What the program does is 
>> take an amount and calculate a couple percentages and add a couple fees.
>>
>> For example, if I put in a value of 1, it will output 752.12 as the sub 
>> total and 753.12 as the grand total. It's off by 1 on sub total and 2 on 
>> grand total.
>>
>> Thanks in advance! Jim Gallaher
>>
>> # Car Salesman Calculator
>>
>> # User enters the base price of the car and the program adds tax, license, 
>> dealer prep, and destination charge.
>>
>> print("Car Sales Calculator")
>> basePrice = int(input("Please enter in the price of the car: "))
>>
>> # Misc charges to be added to the total cost of the car
>> tax = basePrice * .07
>> license = basePrice * .05
>> dealerPrep = basePrice + 500
>> destinationCharge = basePrice + 250
>>
>
> I think your main problem is right here. Why do dealerPrep and 
> destinationCharge include the basePrice in their equation? This 

Re: [Tutor] is v == for small ints

[Steven D'Aprano]

On Tue, Dec 15, 2015 at 01:54:30AM +, Alan Gauld wrote:

> This behaviour should never be relied on since the definition
> of a "small int" is potentially implementation dependant and
> could even change with Python version or platform. Always
> use == to test integers.

It isn't just *potentially* implementation dependent, it certainly is 
implementation dependant.

Different versions of Python have used different rules for which small 
ints are cached, and the rules for when they will be re-used or cached. 
For example, this is with Python 2.7:


py> a = 11
py> b = 11
py> a is b
False
py> a = 12; b = 12
py> a is b
True


You CANNOT rely on this behaviour, it WILL change from version to 
version.



-- 
Steve
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Re: [Tutor] interface

[Alex Kleider]

Thank you, gentlemen (Alan, Ben, Mark,) for your advice.
The consensus seems to be in favour of tkinter
so I'll head in that direction.
Cheers,
Alex
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[Tutor] parser recommendations (was Re: Tutor Digest, Vol 142, Issue 11)

[Alan Gauld]

On 14/12/15 16:16, Crusier wrote:

Please always supply a useful subject line when replying to the digest
and also delete all irrelevant text. Some people pay by the byte and we
have all received these messages already.

> Thank you very much for answering the question. If you don't mind,
> please kindly let me know which library I should focus on among
> beautifulsoup, selenium + PhantomJS, and dryscrape.

I don't know anything about the others but Beautiful soup
is good for html, especially badly written/generated html.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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[Tutor] interface

[Alex Kleider]

So far all my python programming has been done using print for output 
and

(raw)input whenever user input is required.  I'd like to learn how to
provide a graphical interface.  There are several choices with pros and
cons to each but an alternative more general approach might be to use a
web based interface which I believe would make the code less platform
dependent.

I'd be interested in any comments regarding this idea and would be
grateful for any suggestions as to frameworks that might be helpful.
I've been looking at django but finding the learning curve to be steep
and it may not be the best solution.

Of late I've been using Python3; my platform is Ubuntu 14.4.

Thanks in advance for any guidance.

Alex
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Re: [Tutor] interface

[Mark Lawrence]

On 14/12/2015 21:48, Alex Kleider wrote:

So far all my python programming has been done using print for output and
(raw)input whenever user input is required.  I'd like to learn how to
provide a graphical interface.  There are several choices with pros and
cons to each but an alternative more general approach might be to use a
web based interface which I believe would make the code less platform
dependent.


I'd say your belief is simply wrong, many Python desktop GUIs are cross 
platform.  I'd start with tkinter as it's available with Python, there 
are loads of tutorials available for it and you'll be able to get advice 
here or on the main Python mailing list.




I'd be interested in any comments regarding this idea and would be
grateful for any suggestions as to frameworks that might be helpful.
I've been looking at django but finding the learning curve to be steep
and it may not be the best solution.


Django strikes me as pure overkill for your needs.  If you must go down 
the web route I'd start here https://wiki.python.org/moin/WebFrameworks 
at the section headed "Popular Non Full-Stack Frameworks".




Of late I've been using Python3; my platform is Ubuntu 14.4.

Thanks in advance for any guidance.

Alex


--
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence

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Re: [Tutor] interface

[Ben Finney]

Alex Kleider  writes:

> I'd be interested in any comments regarding this idea and would be
> grateful for any suggestions as to frameworks that might be helpful.

The collected wisdom of the community includes this page at the Python
wiki , and a brief
comparison of toolkits is at
.

> I've been looking at django but finding the learning curve to be steep
> and it may not be the best solution.

That seems a reasonable assessment; if you just want a UI for a
not-particularly-website program, Django may be a poor fit.

> Of late I've been using Python3; my platform is Ubuntu 14.4.

I recommend you start with the default, heavily-documented Tkinter
 and the Ttk
extension . Both
of these are part of the Python 3 system in Ubuntu.

You can find a good guide to Tk programming in Python as one stream of
the excellent .

-- 
 \“If you have the facts on your side, pound the facts. If you |
  `\ have the law on your side, pound the law. If you have neither |
_o__)   on your side, pound the table.” —anonymous |
Ben Finney

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Re: [Tutor] parser recommendations (was Re: Tutor Digest, Vol 142, Issue 11)

[bruce]

beautifulsoup, selenium + PhantomJS, and dryscrape

no knowledge of dryscape, never used it.

The other tools/apps are used to handle/parse html/websites.

Ssoup can handle xml/html as well as other input structs. Good for
being able to parse the resulting struct/dom to extract data, or to
change/modify the struct itself.

Selenium is a framework, acting as a browser env, allowing you to
'test' the site/html. It's good for certain uses regarding testing.

Phantomjs/casperjs are exxentially headless broswers, allow you to
also run/parse websites. While Soup is more for static, Phantom
because it's an actual headless browser, allows you to deal with
dynamic sites as well as static.




On Mon, Dec 14, 2015 at 2:56 PM, Alan Gauld  wrote:
> On 14/12/15 16:16, Crusier wrote:
>
> Please always supply a useful subject line when replying to the digest
> and also delete all irrelevant text. Some people pay by the byte and we
> have all received these messages already.
>
>> Thank you very much for answering the question. If you don't mind,
>> please kindly let me know which library I should focus on among
>> beautifulsoup, selenium + PhantomJS, and dryscrape.
>
> I don't know anything about the others but Beautiful soup
> is good for html, especially badly written/generated html.
>
> --
> Alan G
> Author of the Learn to Program web site
> http://www.alan-g.me.uk/
> http://www.amazon.com/author/alan_gauld
> Follow my photo-blog on Flickr at:
> http://www.flickr.com/photos/alangauldphotos
>
>
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