[Tutor] Tutor Digest, Vol 26, Issue 56
nywbon001 at mail.uct.ac.za
nywbon001 at mail.uct.ac.za
Tue Apr 18 10:55:11 CEST 2006
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> Today's Topics:
>
> 1. Changing lists in place (Paul D. Kraus)
> 2. Re: Changing lists in place (Paul D. Eden)
> 3. Re: Changing lists in place (Bob Gailer)
> 4. Re: Meaning of %g ? (Terry Carroll)
> 5. Re: Changing lists in place (Kent Johnson)
> 6. Re: Tutor Digest, Vol 26, Issue 55 (Carroll, Barry)
> 7. Re: Changing lists in place (Paul D. Eden)
> 8. Re: functions in Python (Alan Gauld)
> 9. Raw Bits! (Control characters ahoy!) (doug shawhan)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 17 Apr 2006 14:02:37 -0400
> From: "Paul D. Kraus" <paul.kraus at gmail.com>
> Subject: [Tutor] Changing lists in place
> To: tutor at python.org
> Message-ID:
> <4e2aea430604171102y136f3d83x725d38aad66376a6 at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> I have a list that I want to change in a for loop. It changes in the loop
> but the changes are not persistant outside of the loop.
> I thought lists were mutuable objects so I am a bit confused.
>
> Sample Code....
> #!/usr/bin/env python
> """ Testing lists """
>
> mylist = [ 'One ', ' two', ' three ' ]
> print mylist
> for element in mylist:
> element = element.strip()
> print "<>" + element + "<>"
> print mylist
>
>
> Sample Output....
> ['One ', ' two', ' three ']
> <>One<>
> <>two<>
> <>three<>
> ['One ', ' two', ' three ']
> -------------- next part --------------
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> ------------------------------
>
> Message: 2
> Date: Mon, 17 Apr 2006 11:11:44 -0700
> From: "Paul D. Eden" <paul at benchline.org>
> Subject: Re: [Tutor] Changing lists in place
> To: pkraus at pelsupply.com
> Cc: tutor at python.org
> Message-ID: <4443DA60.2060509 at benchline.org>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Lists are mutable, you are right.
>
> But the code you gave does not change the list. It changes the variable
> element which is separate from the list myList.
>
> If you want to change the list try something like this:
>
> mylist = [ 'One ', ' two', ' three ' ]
> print mylist
> newlist = []
> for element in mylist:
> element = element.strip()
> newlist.append(element)
> print "<>" + element + "<>"
> print newlist
>
> OR
>
> mylist = [ 'One ', ' two', ' three ' ]
> print mylist
> mylist = [element.strip() for element in mylist]
> for element in mylist:
> print "<>" + element + "<>"
> print mylist
>
> Paul
>
> Paul D. Kraus wrote:
> > I have a list that I want to change in a for loop. It changes in the
> > loop but the changes are not persistant outside of the loop.
> > I thought lists were mutuable objects so I am a bit confused.
> >
> > Sample Code....
> > #!/usr/bin/env python
> > """ Testing lists """
> >
> > mylist = [ 'One ', ' two', ' three ' ]
> > print mylist
> > for element in mylist:
> > element = element.strip()
> > print "<>" + element + "<>"
> > print mylist
> >
> >
> > Sample Output....
> > ['One ', ' two', ' three ']
> > <>One<>
> > <>two<>
> > <>three<>
> > ['One ', ' two', ' three ']
> >
> >
> > ------------------------------------------------------------------------
> >
> > _______________________________________________
> > Tutor maillist - Tutor at python.org
> > http://mail.python.org/mailman/listinfo/tutor
>
>
> ------------------------------
>
> Message: 3
> Date: Mon, 17 Apr 2006 11:17:18 -0700
> From: Bob Gailer <bgailer at alum.rpi.edu>
> Subject: Re: [Tutor] Changing lists in place
> To: pkraus at pelsupply.com
> Cc: tutor at python.org
> Message-ID: <4443DBAE.40003 at alum.rpi.edu>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Paul D. Kraus wrote:
> > I have a list that I want to change in a for loop. It changes in the
> > loop but the changes are not persistant outside of the loop.
> > I thought lists were mutuable objects
> They are.
> > so I am a bit confused.
> >
> > Sample Code....
> > #!/usr/bin/env python
> > """ Testing lists """
> >
> > mylist = [ 'One ', ' two', ' three ' ]
> > print mylist
> > for element in mylist:
> element is a variable to which the successive items in mylist are
> assigned. element has no "magic" connection to the list.
> > element = element.strip()
> In order to change a list item you must do something like:
> mylist[itemIndex] = element.strip()
>
> So you need both the item's value and its position in the list. That's
> what enumerate is for:
>
> for itemIndex, element in enumerate(mylist):
> mylist[itemIndex] = element.strip()
> > [snip]
>
>
> ------------------------------
>
> Message: 4
> Date: Mon, 17 Apr 2006 11:35:13 -0700 (PDT)
> From: Terry Carroll <carroll at tjc.com>
> Subject: Re: [Tutor] Meaning of %g ?
> To: tutor at python.org
> Message-ID: <Pine.LNX.4.44.0604171126330.7018-100000 at violet.rahul.net>
> Content-Type: TEXT/PLAIN; charset=US-ASCII
>
> On Mon, 17 Apr 2006, Alan Gauld wrote:
>
> > >>>> a = 123456.78
> > >>>> print "%g\n%e\n%f" % (a,a,a)
> > > 123457
> > > 1.234568e+005
> > > 123456.780000
> >
> > >Float number loses digits and becomes integer in the output ?
> >
> > Yep, I am rapidly coming to the comnclusion that %g is broken
> > in Python. I must do some tests with gcc to see what it does
> > with %g, it may be the wierd behaviour is coming from there.
> > I've never used %g with gcc...
>
> FWIW, (Cygwin) Perl gives a similar result:
>
> > perl -e 'my $a = 123456.78;printf ("%g\n%e\n%f", $a, $a, $a);'
> 123457
> 1.234568e+05
> 123456.780000
>
> The only difference being the two-digit vs. three-digit exponent on %e
> (Python's 1.234568e+005 vs. Perl's 1.234568e+05).
>
> Hmm.. Upon further review, Cygwin Perl gives 1.234568e+05; while
> Activestate Perl gives 1.234568e+005. (For reasons that elude me,
> Activestate Perl won't do the command-line version, but I don't care
> enough to track it down.)
>
>
>
> ------------------------------
>
> Message: 5
> Date: Mon, 17 Apr 2006 15:01:59 -0400
> From: Kent Johnson <kent37 at tds.net>
> Subject: Re: [Tutor] Changing lists in place
> Cc: tutor at python.org
> Message-ID: <4443E627.4030103 at tds.net>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Paul D. Eden wrote:
> > Lists are mutable, you are right.
> >
> > But the code you gave does not change the list. It changes the variable
> > element which is separate from the list myList.
> >
> > If you want to change the list try something like this:
> >
> > mylist = [ 'One ', ' two', ' three ' ]
> > print mylist
> > newlist = []
> > for element in mylist:
> > element = element.strip()
> > newlist.append(element)
> > print "<>" + element + "<>"
> > print newlist
> >
> > OR
> >
> > mylist = [ 'One ', ' two', ' three ' ]
> > print mylist
> > mylist = [element.strip() for element in mylist]
> > for element in mylist:
> > print "<>" + element + "<>"
> > print mylist
>
> Neither of these changes the original list either. They both create new
> lists with the desired contents. The second example binds the new list
> to the old name, but it is still a new list. In many cases this is fine,
> but the distinction is important. For example if you are writing a
> function that modifies a list passed to it, these solutions won't work.
> Bob's solution using enumerate() is the simplest way to modify a list in
> place.
>
> Kent
>
>
>
> ------------------------------
>
> Message: 6
> Date: Mon, 17 Apr 2006 11:48:08 -0700
> From: "Carroll, Barry" <Barry.Carroll at psc.com>
> Subject: Re: [Tutor] Tutor Digest, Vol 26, Issue 55
> To: <tutor at python.org>
> Message-ID:
> <2BBAEE949D384D40A2B851287ADB6A432C3658 at eugsrv400.psc.pscnet.com>
> Content-Type: text/plain; charset="US-ASCII"
>
> Payal:
>
> > -----Original Message-----
> > Date: Mon, 17 Apr 2006 13:24:31 -0400
> > From: Payal Rathod <payal-python at scriptkitchen.com>
> > Subject: Re: [Tutor] functions in Python
> > To: Steve Nelson <sanelson at gmail.com>
> > Cc: "Python\[Tutor\]" <tutor at python.org>
> > Message-ID: <20060417172431.GA26463 at tranquility.scriptkitchen.com>
> > Content-Type: text/plain; charset=us-ascii
> >
> > On Mon, Apr 17, 2006 at 05:42:05PM +0100, Steve Nelson wrote:
> > > When you define a function, you are writing a block of code which
> you
> > > can ask to perform a task. The task may be simple, and not require
> > > any additional information, or it may be more complex and need
> > > information.
> >
> > What is the difference between,
> >
> > >>> def f(x):
> > ... return x
> > ...
> > >>> f(4)
> > 4
> >
> > >>> def f(x):
> > ... print x
> > ...
> > >>> f(4)
> > 4
> >
> > Both give same results. So, why return statement is needed?
> > With warm regards,
> > -Payal
>
> Let's use your two functions named and see what we can do with them. In
> order to use both function definitions in the same interactive session,
> I will name the first 'f1' and the second 'f2'.
>
> >>>>>>>>>>
> >>> def f1(x):
> ... return x
> ...
> >>> def f2(x):
> ... print x
> ...
> >>>>>>>>>>
>
> Okay so far?
>
> Now, let's use each of these functions in a simple computation. To
> start, we'll just add two integers (called literals in programming
> jargon),
>
> >>>>>>>>>>
> >>> 4 + 4
> 8
> >>>>>>>>>>
>
> The '+' operator takes two integers, adds them together and gives back
> (returns) the result. (Actually, the '+' operator can do more than
> this, but this is enough for this example.)
>
> Now, we'll do the same calculation using f1 instead of the literals.
>
> >>>>>>>>>>
> >>> f1(4) + f1(4)
> 8
> >>>>>>>>>>
>
> This worked as expected: f1 sent back (returned) the value we gave it
> (the argument 4) and the '+' operator did the addition.
> Questions?
>
> Now, let's do the same calcuation once more, using f2 this time.
>
> >>>>>>>>>>
> >>> f2(4) + f2(4)
> 4
> 4
> Traceback (most recent call last):
> File "<input>", line 1, in ?
> TypeError: unsupported operand type(s) for +: 'NoneType' and 'NoneType'
> >>>>>>>>>>
>
> This time the operation failed. The f2 function did what we wrote it to
> do: it printed the argument to the Standard Output device (also called
> STDOUT), the interactive shell window in this case. But, as the error
> message indicates, it also RETURNED a value: in this case None, which
> has the type NoneType. The '+' operator then tried to add the two None
> values together and failed.
>
> The examples above show that, although the output of f1 and f2 look the
> same, it is actually different. The 'f2' function displays its argument
> for humans to read. The 'f1' function makes the argument available for
> other code to use. That is the purpose of the 'return' statement: to
> make the results of the function available to other code.
>
> It is important to remember that, in Python, EVERY function returns a
> value. If a 'return' statement is included in the function, the value
> the result of the expression associated with the statement (in your
> example, 4). If no 'return' statement is included, the function returns
> None.
>
> I hope this makes sense to you. If not, keep asking questions.
>
> Best Regards,
>
> Barry
> barry.carroll at psc.com
> 541-302-1107
> ________________________
> We who cut mere stones must always be envisioning cathedrals.
>
> -Quarry worker's creed
>
>
>
>
> ------------------------------
>
> Message: 7
> Date: Mon, 17 Apr 2006 12:04:57 -0700
> From: "Paul D. Eden" <paul at benchline.org>
> Subject: Re: [Tutor] Changing lists in place
> To: Kent Johnson <kent37 at tds.net>
> Cc: tutor at python.org
> Message-ID: <4443E6D9.5050101 at benchline.org>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Very true.
>
> Paul
>
> Kent Johnson wrote:
> > Paul D. Eden wrote:
> >
> >>Lists are mutable, you are right.
> >>
> >>But the code you gave does not change the list. It changes the variable
> >>element which is separate from the list myList.
> >>
> >>If you want to change the list try something like this:
> >>
> >>mylist = [ 'One ', ' two', ' three ' ]
> >>print mylist
> >>newlist = []
> >>for element in mylist:
> >> element = element.strip()
> >> newlist.append(element)
> >> print "<>" + element + "<>"
> >>print newlist
> >>
> >>OR
> >>
> >>mylist = [ 'One ', ' two', ' three ' ]
> >>print mylist
> >>mylist = [element.strip() for element in mylist]
> >>for element in mylist:
> >> print "<>" + element + "<>"
> >>print mylist
> >
> >
> > Neither of these changes the original list either. They both create new
> > lists with the desired contents. The second example binds the new list
> > to the old name, but it is still a new list. In many cases this is fine,
> > but the distinction is important. For example if you are writing a
> > function that modifies a list passed to it, these solutions won't work.
> > Bob's solution using enumerate() is the simplest way to modify a list in
> > place.
> >
> > Kent
> >
> > _______________________________________________
> > Tutor maillist - Tutor at python.org
> > http://mail.python.org/mailman/listinfo/tutor
>
>
> ------------------------------
>
> Message: 8
> Date: Mon, 17 Apr 2006 21:05:02 +0100
> From: "Alan Gauld" <alan.gauld at freenet.co.uk>
> Subject: Re: [Tutor] functions in Python
> To: "Payal Rathod" <payal-python at scriptkitchen.com>, "Steve Nelson"
> <sanelson at gmail.com>
> Cc: "Python\[Tutor\]" <tutor at python.org>
> Message-ID: <01bd01c6625a$36ae0230$0a01a8c0 at xp>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> reply-type=original
>
> Wow! I checked the list at lunchtime and there were only a few things
> here, then I check again now and lots of stuff from my tutor! Fortunately
> most of it has been answered already - thanks folks - but I feel honour
> bound to contribute something...
>
> > What is the difference between,
> >
> >>>> def f(x):
> > ... return x
>
> >>>> def f(x):
> > ... print x
> > ...
> >>>> f(4)
> > 4
>
> > Both give same results. So, why return statement is needed?
>
> As has been stated the print displays on the output
> The return sends the value of the function back to the caller.
> If the caller is the interactive prompt(as here) the prompt
> prints the value so the effect is the same.
>
> If you need to save the result for later use then you must
> store it in a variable. You cannot do that if the function
> just prints the result - the value gets lost when the function
> ends.
>
> Try this:
>
> >>> def f(x): return x
> ...
> >>> def g(x): print x
> ...
> >>> fVal = f(4)
> >>> gVal = g(4)
> 4
>
> Now notice that the two functoons behave differently
> f() doesn't display a value but g does because of the
> print statement. So far g seems to win. But...
>
> >>> print fVal
> 4
> >>> print gVal
> None
> >>>
>
> Now f() wins because we have stored its value in fVal
> and can use it over and over again as we wish.
>
> The reason gVal has stored None is because any function that
> does not return an explicit value is treated as retiurning the
> special value None. Since g() only printed the parameter 4
> but did not return it gVal stored a None value.
>
> If we return the value of f() we can display it any time we
> want by using print:
>
> >>> print f(4)
> 4
> >>>
>
> So we do not lose anything by using return instead of print
> but we gain a lot by being able to store the returned value.
>
> HTH,
>
> Alan G
> Author of the learn to program web tutor
> http://www.freenetpages.co.uk/hp/alan.gauld
>
>
>
>
> ------------------------------
>
> Message: 9
> Date: Mon, 17 Apr 2006 16:40:45 -0500
> From: "doug shawhan" <doug.shawhan at gmail.com>
> Subject: [Tutor] Raw Bits! (Control characters ahoy!)
> To: tutor at python.org
> Message-ID:
> <5e1ceb8a0604171440r6b34dd57j4ab20a9061a831b3 at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> I am in the middle of a project that requires me to send and retrieve
> information from a machine connected to a serial port. My problems are
> these:
>
> 1. I cannot send control characters
> 2. I cannot read data streaming from the serial port
>
>
> I have been doing fine with:
>
> os.system("echo '5' >/dev/tty00")
> os.system("echo '8' >/dev/tty00")
>
>
> and the like to the remote machine, but I need to be able to send control
> characters the same way to scroll through menus. I have tried pyserial and
> pexpect with mixed (mostly horrible) results (not the fault of the module
> builders!), thus far the simple act of echoing the strings to the serial
> port is the only reliable method I have found.
>
> Reading from the serial port has also been problematic. I find that simply
> opening the port /dev/tty00 file and using readlines(), read() or whatever
> gives me empty strings or lists. Pbth.
>
> So first: how should I go about adding the hex escape sequences (\x0a and
> \x50) to the above strings in their raw form?
> second: what method should I use to read a raw stream from the serial port
> for a given period?
>
> I have looked carefully at the neat and tidy miniterm.py example in the
> pyserial examples directory, and see how one can continuously read via a
> thread from the port, but have not been able to replicate it thus far.
>
> Any help whatsoever would be great! I'm beginning to feel a little
> lightheaded. Thanks!
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> http://mail.python.org/mailman/listinfo/tutor
>
>
> End of Tutor Digest, Vol 26, Issue 56
> *************************************
>
I want to build a function which will reverse my input sentance.if i try to
use the rervese it does not work
here is my function:
def reversor():
for i in string(a):
l=[]
if a==string:
l.reversed(a)
print l
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