🌐 Mailman 3 March 2005 - Tutor - python.org

What does Python gain by having immutable types?
by Tony C Nov. 14, 2005

Nov. 14, 2005

A friend recently asked my this didn't modify his original string variable s1="this is a string" s1.replace("is a", "is a short") "this is a short string" print s1 "this is a string" After showing him to change it to this s1 = s1.replace("is a", "is a short") print s1 "this is a short string" He then asked Why is it that the sort operation modfies the original list variable? l1=[] l1.append(3) l1.append(44) l1.append(1) l1.append(22) l1 [3, 44, 1, 22] l1.sort() l1 [1, 3, 22, 44] I don't know, but I think it has something to do with strings being immutable, whiles lists are mutable. So, other than the non-informative answer " because that's how the language was written" are there any other reasons how Python benefits from immutable types? Does it really matter if s1 is a different object, so long as it contains the expected modified string? The list variable is the same object, and contains the sorted list, so why bother with the complexity of immutable types at all?

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How to calculate pi with this formula?
by Dick Moores April 14, 2005

April 14, 2005

Is it possible to calculate almost-pi/2 using the (24) formula on <http://mathworld.wolfram.com/PiFormulas.html> without using (23)? If it's possible, how about a hint? Recursion? Thanks, tutors. Dick Moores rdm(a)rcblue.com

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Launching a file browser
by Mike Hall April 4, 2005

April 4, 2005

I looked over the global module index and the closest thing I could find relating to my os (osx) was EasyDialogs, which has a few functions pertaining to this, "AskFileForOpen()" being one. Calling any function within EasyDialogs however yields an Apple Event error: AE.AEInteractWithUser(50000000) MacOS.Error: (-1713, 'no user interaction is allowed') I get the impression some of these "Mac" modules are more relevant to os 9 than 10(which is Unix), so maybe EasyDialogs is not the right choice here. Any suggestions are appreciated. -MH

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Re: If elif not working in comparison
by C Smith April 3, 2005

April 3, 2005

> gerardo arnaez wrote: > >> On Mon, 28 Mar 2005 09:27:11 -0500, orbitz <orbitz at drorbitz.ath.cx> >> wrote: >> >>> Floats are inherintly inprecise. So if thigns arn't working like you >>> expect don't be surprised if 0.15, 0.12, and 0.1 are closer to the same >>> number than you think. >> >> >> >> Are you telling me that I cant expect 2 digit preceision? > > > Not without using round. Have *NO* faith in floating points. This is > especially true when you are creating the decimals via division and the > like. What makes one nervous when looking at your tests with floating point numbers is that calcuations that might lead up to one of the numbers in your test might actually be a tiny bit higher or a tiny bit lower than you expect. *However*, I think the thing to keep in mind is that if you are only a billionth or less away from the boundary, it really doesn't matter which way you go in terms of dosage--you are at the border. Here is an example of two calculations that you think should get you to 2.1. One is actually just shy of it and the other just a bit past: ### >>> 3*.7 2.0999999999999996 >>> 7*.3 2.1000000000000001 ### So if you have the following test to determine if it is time to increase someone's dose of medication and the results are based on some measurement being greater than 2.1, then... ### >>> def test(x): if x>2.1: print 'increase dose' else: print 'decrease dose' >>> ### if you test the above computed values, you will get different results: ### >>> dose1, dose2 = 3*.7, 7*.3 >>> test(dose1) decrease dose >>> test(dose2) decrease dose ### OR NOT! Why do they both come out to be the same even though one number is bigger than 2.1 and the other not? Because 2.1 is not actually '2.1': ### >>> 2.1 2.1000000000000001 ### The 7*.3 is actually "equal" to 2.1: they both are 2.1000000000000001. On the one hand, we want the boundaries so we can say, "if you are past this boundary you take the next sized dose"...but if you are so close to the boundary that you can just as well say that you are right on it, does it really matter which way you go? Other than bothering the deterministic nature, are there other reasons to be concerned? As far as economics, the '2,1' case will have people staying at a lower dose longer (as would 2.2, 2.6 and 2.7 which all are a tiny bit bigger than their mathematical values, just like 2.1). Just the opposite will happen at the 2.3, 2.4, 2.8, and 2.9 since these values are represented as numbers smaller than their mathematical counterparts: ### >>> 2.9 2.8999999999999999 ### If you have Python 2.4 you might want to check out the decimal type that is now part of the language. There is a description at http://www.python.org/doc/2.4/whatsnew/node9.html which starts with: "Python has always supported floating-point (FP) numbers, based on the underlying C double type, as a data type. However, while most programming languages provide a floating-point type, many people (even programmers) are unaware that floating-point numbers don't represent certain decimal fractions accurately. The new Decimal type can represent these fractions accurately, up to a user-specified precision limit. " And it gives the following example: Once you have Decimal instances, you can perform the usual mathematical operations on them. One limitation: exponentiation requires an integer exponent: >>> import decimal >>> a = decimal.Decimal('35.72') >>> b = decimal.Decimal('1.73') >>> a+b Decimal("37.45") >>> a-b Decimal("33.99") >>> a*b Decimal("61.7956") >>> a/b Decimal("20.64739884393063583815028902") Another suggestion is to use the Round replacement that I wrote about in response to the "Float precision untrustworthy~~~" thread. With it, you don't have to worry about how large or small your numbers are when you Round them (unlike with round()). You just decide on how many "parts per x" you are interested in with your numbers. e.g. if your calculations are only precise to about 1 part per hundred then you would round them to the 2nd (or maybe 3rd) digit and compare them. With the separation of code and data as already suggested this would look something like this: ### def Round(x, n=0, sigfigs=False): if x==0: return 0 if not sigfigs: return round(x,n) else: #round to nth digit counting from first non-zero digit # e.g. if n=3 and x = 0.002356 the result is 0.00236 return round(x,n-1-int(floor(log10(abs(x))))) ... for cutoff, value in cutoffs: if Round(x,3) < Round(cutoff,3): return value .. ### IMHO, I don't think that decimal is the way to go with such calculations since calculations performed with limited precision numbers shouldn't be treated as though they had infinite precision. From basic error analysis of the calculated values above, one shouldn't probably keep more than 3 digits in a/b result of Decimal("20.64739884393063583815028902"). On the other hand, perhaps one can keep all the digits until the end and then make the precision decision--this gives the best of both worlds. /c

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wxPython / Tkinter Grid
by Liam Clarke April 1, 2005

April 1, 2005

Hi, I know a wxPython grid is totally different to a Tkinter grid, but is there a Tkinter equivalent of a wxPython grid? I'm finding wxPython to be fiddly and restrictive... Regards, Liam Clarke -- 'There is only one basic human right, and that is to do as you damn well please. And with it comes the only basic human duty, to take the consequences.

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how to setup gnu.py
by jrlen balane April 1, 2005

April 1, 2005

hi! i don't know if this is the proper forum but i'll ask anyway... how am i going to setup gnu.py(or gnuplot.py) gnuplot with python???

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Sorting more than one list
by Diego Galho Prestes April 1, 2005

April 1, 2005

Hi! I need to sort 4 lists but I need that they make the "sort together". I'll sort just one but when I change the position of the items of the 1st list I have to change the positions of the other 3 lists. Can I do this just using the sort() method of the list object? If I can't, someone know a site that have one sort method in python that its easily to implement and fast? Tks, Diego

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A Newbie Printing Question
by Richard Lyons April 1, 2005

April 1, 2005

I have little experience with programming. I have Python installed on a Windows XP system. What code do I need to use to send output from a Python script to a local printer attached to my workstation? to a network printer? Any help would be appreciated.

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Cryptography Toolkit
by Mark Thomas April 1, 2005

April 1, 2005

Does anyone have some examples on the use of A.M. Kuchling's Python Cryptography Toolkit? I've tried his examples but get "AttributeError" and "TypeError". What I'm trying to do is encrypt/decrypt a file. I'm using Python 2.3 on xp pro. Thanks -- _ ( ) Mark Thomas ASCII ribbon campaign X www.theswamp.org - against HTML email / \

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I am puzzled - help needed
by John Carmona April 1, 2005

April 1, 2005

I am not sure that it is possible to ask that question please feel free to turn me down if it is going against the forum rules. I have going through Josh Cogliati tutorial, I am stuck on one of the exercise. I need to rewrite the high_low.py program (see below) to use the last two digits of time at that moment to be the "random number". This is using the import time module. I just can't work out how to do that, I have been looking at it for the past 2,5 hours but can't break it. Has anyone done it in order for me to study it. Many thanks in advance for any help JC This is the original programme: number = 78 guess = 0 while guess != number guess = input ("Guess a number: ") if guess > number: print "Too high" elif guess < number: print "Too low" print "just right"

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