[Tutor] Python Tutorials: How to create useful programs after learning the syntax?

[Luis Galvan]

Hello all, this is my first time using a mailing list, so I'm not sure if
I'm doing this right!  Anyway, I have a wee bit of a problem.  I've recently
completed watching a Youtube video series on Python 2.6 by thenewboston
which helped me a TON with learning Python's syntax and how to use some of
the various builtin modules.  I am very thankful for his tutorials, but the
only thing that I need is something to help me really grasp onto the world
of programming. (I'm new to programming)  What I'm looking for is a tutorial
series that walks you through the steps of writing a useful program in
Python.  Whether it be a text editor, a simple browser, etc, it doesn't
really matter.  I learned the syntax (at least most of it), but I guess I'm
just looking for something to help me learn to "utilize it".  Something to
teach me how I can use everything I learned about manipulating strings,
creating classes, and importing modules and how to stir it all up into
something meaningful.  Most people's answer to this kind of question is that
the best way to learn is to "play around with it", but personally I believe
that you can't really play with it if it you don't know where to start.  If
anyone has any idea on how I can really get started with programming useful
programs, please do let me know!  Any help would be immensely appreciated!
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Re: [Tutor] append question

[Rich Lovely]

2009/7/6 Steven Buck :
> Thanks for the previous responses.  This isn't homework--I'm beyond
> coursework, although I am a newbie to Python (and I've never had to do much
> real programming since I've just used Stata for econometric analysis).  I'm
> testing Python as a more powerful alternative to Stata.
>
> I've learned from the responses I received, although now see my problem
> differently.  The data structure I have uses a dictionary and know now
> that the append command doesn't work.  Having said that, perhaps my
> variables of interest have already been created--perhaps I just don't know
> how to identify them.  I've been using some borrowed code to get me started;
> my modified version is below:
>
>
> import sys
>
> # The modules below help me get a .dta file into Python.
> # Although I'm not sure what form they take; I suppose a list of lists???
> from StataTools import Reader
> from StataTypes import MissingValue
>
> # I call my data set the psid (Panel Study of Income Dynamics)
> # In Stata this would look like and NXK matrix (N observations and K
> variables)
> psid=Reader(file('data3.dta'))
>
> # I gather this next just creates a list of the variable names.
> varnames=[x.name for x in psid.variables()]
>
> # It's not clear what these next two lines gain me.
> labels=psid.file_headers()['vlblist']
> Labels=dict(zip(varnames,labels))
>
>
>  From here, I'd like Python to identify the Nx1 vectors (or n-tuples) that
> correspond to the varnames list defined above.  I can't seem grab the
> vectors representing age, wage, etc..  I've tried things like
> age, psid['age'], psid.age.  My last email was an attempt to create the
> vectors myself, although the Reader module puts the data in a dictionary
> structure so the append command I was trying to use doesn't work.
>
> Hopefully once I learn to create and call on my own vectors and matrices
> I'll be better off--I'm comfortable working with these in MATLAB and Stata.
>
> Bottom line:  Given the above data I've imported/extracted from Stata .dta
> file, how do I create an Nx1 vector  which I call 'age'?
>
> Thanks for your patience with this newbie.
> Steve

Sorry about suggesting this was homework... I noticed the word
University, but not the line above it saying "PhD student"...

If you're new to python, the standard path we recommend is to take an
online tutorial.

Dive into Python (http://www.diveintopython.org/) has been recommended
for people with some programming experience, and Think Python for
those without, although I'm sure that if you ask 10 members of this
list, you'll get 20 different suggestions.  Real veterans get pointed
at the standard libraries, which are extremly well commented in the
most part.  I personally learned from the tutorial in the python
documentation, but it does leave a fair bit of the thought processes
involved in programming out.

See what I mean?  One person, three recomendations.

-- 
Richard "Roadie Rich" Lovely, part of the JNP|UK Famile
www.theJNP.com
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Re: [Tutor] Poor style to use list as "array"?

[bob gailer]

Angus Rodgers wrote:

The problem this time is, given an amount of US currency in cents
(and less than a dollar), to print out a description of the coins
needed to represent it, using the smallest number of coins.  E.g.:

How many cents this time? 59
2 quarters, 1 nickel and 4 pennies.

How many cents this time? 55
2 quarters and 1 nickel.

How many cents this time? 75
3 quarters.

My program relaxes the restriction to amounts less than a dollar,
and initialises the necessary data structures as follows:

denom = ['dollar', 'quarter', 'dime', 'nickel', 'penny']
value = [100, 25, 10, 5, 1]
LEN = 5
plural = [None] * LEN
plural[-1] = 'pennies'
count = [None] * LEN   # Kludge
  


I'm inclined to use a class. I omitted the input and loop for "simplicity".

class Coin:

 def __init__(self, name, value, plural=None):
   self.name = name
   self.value = value
   if plural:
 self.plural = plural
   else:
 self.plural = self.name + 's'
   self.count = 0

 def display(self):
   if self.count == 0:
 return None
   if self.count == 1:
 return "%d %s" % (self.count, name)
   else:
 return "%d %s" % (self.count, self.plural)
   
coins = Coin('dollar', 100), Coin('quarter', 25), Coin('dime', 10), 
Coin('nickel', 5), Coin('penny', 1, 'pennies')

amnt = 99
buff = []
for coin in coins:
 (coin.count, amnt) = divmod(amnt, coin.value)
 d = coin.display()
 if d:
   buff.append(d)
if len(buff) < 2:
 print buff
else:
 print ', '.join(buff[:-1]) + " and " + buff[-1]


I use the list 'count' as a kind of array, so that e.g. count[-1]
is the number of pennies needed, count[2] is the number of dimes
needed, and so on.  Any initial values I might store as elements
of the list are never used, hence the use of 'None'. (The use of
'None' in the list 'plural' is much more defensible, I think, as
it is used to denote the formation of a regular plural by adding
's' to the name.) Instead, the actual values are assigned in a
'for' loop, as follows:

for i in range(LEN - 1):
(count[i], amnt) = divmod(amnt, value[i])

This feels like a bit of a cheat, as if I am trying to program in
Python as if it were some other more familiar language.  Should I
have coded this differently?

The full source code is here, in case anyone wants to look at it
(but I'm not soliciting any more attention, as I've already been
given quite a lot of it, and fear being offered the comfy chair!):
   (retention: 1 day)

Could I have used dictionaries instead, with the denomination names
as keys?  Is it possible to guarantee a sequence in which the keys
of a dictionary are iterated through? (If not, I suppose I could
keep the list 'denom' as it is here, and iterate through it with
"for key in denom:", although this seems a bit redundant.)
  



--
Bob Gailer
Chapel Hill NC
919-636-4239
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Re: [Tutor] Poor style to use list as "array"?

[Rich Lovely]

>                if name in plural:
>                    name = plural[name]
>                else:
>                    name += 's'
This could be written more cleanly (although arguably not as readably) as

name = plural.get(name, name + "s")

d.get(key, default) returns the value from d mapped to key if it
exists, or default otherwise.

You might also want to split your calculation and display code into
two separate loops.  This might seem wasteful, but it will make your
code easier to read and maintain, and the waste is only marginal with
the loops you're running - there is a maximum of only 17 passes (once
for each value of coin and note)

-- 
Richard "Roadie Rich" Lovely, part of the JNP|UK Famile
www.theJNP.com
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Re: [Tutor] append question

[Robert Berman]

In [1]: test=[[1,2,3],[4,5,6],[7,8,9]]
In [3]: testvar2 = []

In [16]: for i in range(len(test)):
   : testvar2.append(test[i][1])
   : 
   : 

In [17]: testvar2
Out[17]: [2, 5, 8]

Robert


On Sun, 2009-07-05 at 15:57 -0700, Steven Buck wrote:
> Hi Python Tutors:
>  
> I have a data structure that looks like:
>  
> >>> test=[[1,2,3],[4,5,6],[7,8,9]]
>  
> I want to define a new variable that captures the second element of
> each sublist from above:
>  
> >>> testvar2 = []
>  
> Next I try to capture the aforementioned elements:
>  
> >>> for i in len(test):
> testvar2.append(test[i][2])
>  
> I want testvar2 = [2,5,8] but instead I get the following error
> message:
>  
> Traceback (most recent call last):
>   File "", line 1, in 
> for i in len(test):
> TypeError: 'int' object is not iterable
>  
> Any insight would be appreciated.
> Thanks
> Steve
>  
>  
>  
>  
>  
> 
>  
> 
> 
> -- 
> Steven Buck
> Ph.D. Student
> Department of Agricultural and Resource Economics
> University of California, Berkeley
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Re: [Tutor] append question

[Rich Lovely]

2009/7/5 Steven Buck :
 for i in len(test):
>     testvar2.append(test[i][2])
>
> I want testvar2 = [2,5,8] but instead I get the following error message:
>
> Traceback (most recent call last):
>   File "", line 1, in 
>     for i in len(test):
> TypeError: 'int' object is not iterable
>
> Any insight would be appreciated.
> Thanks
> Steve
> --
> Steven Buck
> Ph.D. Student
> Department of Agricultural and Resource Economics
> University of California, Berkeley

This sounds like a homework assignment, and we're not supposed to give
out answers to homework.

The error message and the docs explain what you're doing wrong if you
take a moment to look.
from http://www.python.org/doc/2.6/reference/compound_stmts.html#for

"""for_stmt ::=  "for" target_list "in" expression_list ":" suite
  ["else" ":" suite]

The expression list is evaluated once; it should yield an iterable
object. An iterator is created for the result of the expression_list.
The suite is then executed once for each item provided by the
iterator, in the order of ascending indices. Each item in turn is
assigned to the target list using the standard rules for assignments,
and then the suite is executed."""

As Luke said, len returns an int, which as your error tells you, is
not iterable.  From the same page:
"""The for statement is used to iterate over the elements of a
sequence (such as a string, tuple or list) or other iterable
object:"""

Therefore you have an iterable, there is no need to try and construct a new one.

Does that help?

It is extremly unpythonic to iterate over range(len(...)), as it adds
in the overhead of two function calls, and ruins the readability of
code.  The latter is probably the most important of the two.

An even more pythonic way to do this would be a list comprehension,
http://www.python.org/doc/2.6/tutorial/datastructures.html#list-comprehensions

If it's not homework, let us know, and we'll be more than willing to
give you code if you still need it.

-- 
Richard "Roadie Rich" Lovely, part of the JNP|UK Famile
www.theJNP.com
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Re: [Tutor] Poor style to use list as "array"?

[Angus Rodgers]

On Mon, 06 Jul 2009 01:02:10 +0100, I hastily wrote:

>Incidentally, I forgot to quote the next line:
>
>count[-1] = m

That was copied-and-pasted from an older version of the program,
with less descriptive identifiers.  'm' should be 'amnt'.

>Of course, this is much better incorporated into the loop, thus:
>
>for i in range(LEN):
>(count[i], amnt) = divmod(amnt, value[i])
>
>[...]
>
>So it looks like we should have something like:
>
>plural = {'penny':'pennies'}
>
>counts = {}
>for val in value:
>(count, amnt) = divmod(amnt, val)
>counts[val] = count

Better, of course (I still haven't run it, but it should work):

counts = {}
for val in value:
(counts[val], amnt) = divmod(amnt, val)

>I'm much happier now, thanks!

... but also rather annoyed with myself.
-- 
Angus Rodgers
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Re: [Tutor] append question

[Luke Paireepinart]

Read your error message... It highlighted the first line of your for
loop ansd said ints aren't iterable. len(list) returns an integer. You
want a list of items... for i in range(len(list)):

On 7/5/09, Steven Buck  wrote:
> Hi Python Tutors:
>
> I have a data structure that looks like:
>
 test=[[1,2,3],[4,5,6],[7,8,9]]
>
> I want to define a new variable that captures the second element of each
> sublist from above:
>
 testvar2 = []
>
> Next I try to capture the aforementioned elements:
>
 for i in len(test):
> testvar2.append(test[i][2])
>
> I want testvar2 = [2,5,8] but instead I get the following error message:
>
> Traceback (most recent call last):
>   File "", line 1, in 
> for i in len(test):
> TypeError: 'int' object is not iterable
>
> Any insight would be appreciated.
> Thanks
> Steve
>
>
>
>
>
>
>
>
> --
> Steven Buck
> Ph.D. Student
> Department of Agricultural and Resource Economics
> University of California, Berkeley
>

-- 
Sent from my mobile device
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Re: [Tutor] Poor style to use list as "array"?

[Angus Rodgers]

On Sun, 5 Jul 2009 18:49:32 -0400, Kent Johnson wrote:

>On Sun, Jul 5, 2009 at 2:48 PM, Angus Rodgers wrote:
>
>> for i in range(LEN - 1):
>>    (count[i], amnt) = divmod(amnt, value[i])

Incidentally, I forgot to quote the next line:

count[-1] = m

Of course, this is much better incorporated into the loop, thus:

for i in range(LEN):
(count[i], amnt) = divmod(amnt, value[i])

and this lends itself to being rewritten in terms of some other
kind of iteration (as below).

>How about this:
>counts = []
>for val in value:
>  count, amnt = divmod(amnt, val)
>  counts.append(count)

I like that very much, because it is in the nature of the problem
that the numbers in the list 'value' are all distinct, and so can
be used as keys.  However, as this remark suggests, I think I am
going to need 'count' to be a dictionary, rather than a list, and
the same goes for 'denom', and 'plural' (although this should be
keyed by the strings in 'denom', and we don't need the 'None's).

So it looks like we should have something like:

plural = {'penny':'pennies'}

counts = {}
for val in value:
(count, amnt) = divmod(amnt, val)
counts[val] = count

and, later in the program, something like this (comments stripped
for brevity, and one new comment added):

for val in value:
amnt = counts[val]
name = denom[val]
if amnt:
to_print -= 1
if printed:
if to_print:
buff += ", "
else:
buff += " and "
printed += 1
if amnt > 1:
# This should become a function
if name in plural:
name = plural[name]
else:
name += 's'
buff += "%d %s" % (amnt, name)

I haven't run this, but I'll try rewriting the program tomorrow.
It looks like there's nothing to it.

I'm much happier now, thanks!
-- 
Angus Rodgers
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[Tutor] append question

[Steven Buck]

Hi Python Tutors:

I have a data structure that looks like:

>>> test=[[1,2,3],[4,5,6],[7,8,9]]

I want to define a new variable that captures the second element of each
sublist from above:

>>> testvar2 = []

Next I try to capture the aforementioned elements:

>>> for i in len(test):
testvar2.append(test[i][2])

I want testvar2 = [2,5,8] but instead I get the following error message:

Traceback (most recent call last):
  File "", line 1, in 
for i in len(test):
TypeError: 'int' object is not iterable

Any insight would be appreciated.
Thanks
Steve








-- 
Steven Buck
Ph.D. Student
Department of Agricultural and Resource Economics
University of California, Berkeley
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Re: [Tutor] Poor style to use list as "array"?

[Alan Gauld]

"Angus Rodgers"  wrote 


as keys?  Is it possible to guarantee a sequence in which the keys
of a dictionary are iterated through? 


Indirectly yes:

for key in sorted( dct ):
   print key

I would definitely tend to go with a dictionary for the 
denominations/values in this case  (Actually I'd probably 
go for a class but that's another ball game entirely!)



--
Alan Gauld
Author of the Learn to Program web site
http://www.alan-g.me.uk/

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Re: [Tutor] Poor style to use list as "array"?

[Kent Johnson]

On Sun, Jul 5, 2009 at 2:48 PM, Angus Rodgers wrote:

> for i in range(LEN - 1):
>    (count[i], amnt) = divmod(amnt, value[i])

How about this:
counts = []
for val in value:
  count, amnt = divmod(amnt, val)
  counts.append(count)

> This feels like a bit of a cheat, as if I am trying to program in
> Python as if it were some other more familiar language.  Should I
> have coded this differently?

Generally it's more straighforward to iterate over a list directly
rather than using an index.

> The full source code is here, in case anyone wants to look at it
> (but I'm not soliciting any more attention, as I've already been
> given quite a lot of it, and fear being offered the comfy chair!):

Don't worry, we have retired the comfy chair.

> Could I have used dictionaries instead, with the denomination names
> as keys?  Is it possible to guarantee a sequence in which the keys
> of a dictionary are iterated through?

In general no. Python 2.7 and 3.1 do have an ordered dictionary:
http://www.python.org/dev/peps/pep-0372/

Kent
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Re: [Tutor] Stack unwind using exceptions.

[Eike Welk]

On Sunday 05 July 2009, Noufal Ibrahim wrote:
> Kent Johnson wrote:
> [..]
>
> > Why not just return the value from the function and pass it up
> > the call chain? If a call fails return None. Something like this:
>
> That's what I ended up doing but the first thing occurred to me and
> I was just wondering if there's any production code that relies on
> the technique.

The Pyparsing library uses exceptions a lot internally. 

If I understood it right, exceptions are used to tell that a pattern 
does not match. If the pattern matches the results are transported 
with a regular 'return'. As it happens quite often that a pattern 
does not match, exceptions can be considered a regular mechanism for 
information transport in Pyparsing.

There are two types of exceptions in Pyparsing. An exception that 
means: try the next pattern; and there are exceptions that mean: 
there was a fatal error, stop parsing.

Pyparsing:
http://pyparsing.wikispaces.com/


Kind regards,
Eike.
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Re: [Tutor] Is my style OK in this elementary student exercise?

[Tim Peters]

[Angus Rogers, suffering eval-angst]
> ...
> On the other hand, so long as I AM only executing the function
> myself, I am no more at risk than I already am every single time
> I type a command into a Python interpreter, of any description.
> (A somewhat Existentialist thought, perhaps!  Virtual suicide
> is always a possibility.) >->
>
> Does that seem reasonable?  You've made me clearly aware of a
> risk that I was only vaguely aware of before (I ruminated only
> briefly as to whether any harm might come from entering general
> Python expressions, but my main feeling about this facility was
> that it would probably be useful - in some "future exercise"),
> but isn't there a role for functions that one can use oneself,
> but never ever distribute to the "general public"?

Certainly!  I use eval() and input() all the time in my private code
(but never in released code), and you're never going to screw yourself
"by accident" doing so.  Especially if you have an interest in writing
mathematical code, it's a tremendous convenience for prompted input to
support arbitrary computation.


> If so, are the precautions I have suggested above sufficient?

In private code, any precautions are probably just a waste of time.
You really don't, for example, need to remind /yourself/ not to enter
a convoluted expression that emails your credit card info to a hacker
site in Nigeria.  Or if you do need to remind yourself not to do
things like that, you probably shouldn't be using a computer to begin
with ;-)
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Re: [Tutor] Poor style to use list as "array"?

[Noufal Ibrahim]

Angus Rodgers wrote:
[..]


This feels like a bit of a cheat, as if I am trying to program in
Python as if it were some other more familiar language.  Should I
have coded this differently?


I can't offer anything concrete but here are some things that occur to 
me at a glance.


0. You can try to use the collections module in 2.6 (especially the 
namedtuple object which gives you access to things through attributes as 
well as through order.
1. You have 3 lists (denom, plural and count) which you will iterate 
through in lockstep as far as I can tell. It's an option to use a list 
of 3-tuples or a list of dictionaries with three keys (denomination, 
plural and count).


Thanks.

--
~noufal
http://nibrahim.net.in/
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[Tutor] Poor style to use list as "array"?

[Angus Rodgers]

The problem this time is, given an amount of US currency in cents
(and less than a dollar), to print out a description of the coins
needed to represent it, using the smallest number of coins.  E.g.:

How many cents this time? 59
2 quarters, 1 nickel and 4 pennies.

How many cents this time? 55
2 quarters and 1 nickel.

How many cents this time? 75
3 quarters.

My program relaxes the restriction to amounts less than a dollar,
and initialises the necessary data structures as follows:

denom = ['dollar', 'quarter', 'dime', 'nickel', 'penny']
value = [100, 25, 10, 5, 1]
LEN = 5
plural = [None] * LEN
plural[-1] = 'pennies'
count = [None] * LEN   # Kludge

I use the list 'count' as a kind of array, so that e.g. count[-1]
is the number of pennies needed, count[2] is the number of dimes
needed, and so on.  Any initial values I might store as elements
of the list are never used, hence the use of 'None'. (The use of
'None' in the list 'plural' is much more defensible, I think, as
it is used to denote the formation of a regular plural by adding
's' to the name.) Instead, the actual values are assigned in a
'for' loop, as follows:

for i in range(LEN - 1):
(count[i], amnt) = divmod(amnt, value[i])

This feels like a bit of a cheat, as if I am trying to program in
Python as if it were some other more familiar language.  Should I
have coded this differently?

The full source code is here, in case anyone wants to look at it
(but I'm not soliciting any more attention, as I've already been
given quite a lot of it, and fear being offered the comfy chair!):
   (retention: 1 day)

Could I have used dictionaries instead, with the denomination names
as keys?  Is it possible to guarantee a sequence in which the keys
of a dictionary are iterated through? (If not, I suppose I could
keep the list 'denom' as it is here, and iterate through it with
"for key in denom:", although this seems a bit redundant.)
-- 
Angus Rodgers
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Re: [Tutor] Stack unwind using exceptions.

[bob gailer]

Noufal Ibrahim wrote:

Kent Johnson wrote:
[..]

Why not just return the value from the function and pass it up the
call chain? If a call fails return None. Something like this:


That's what I ended up doing but the first thing occurred to me and I 
was just wondering if there's any production code that relies on the 
technique.




I use the exception technique in my Python Pipelines parser. The 
recursive routine has 6 raise statements. The exceptions are caught by 
the program that calls the recursive routine. There would be more code 
and complexity to work back up the call chain.



--
Bob Gailer
Chapel Hill NC
919-636-4239
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Re: [Tutor] reading variables in a data set?

[Kent Johnson]

On Sat, Jul 4, 2009 at 12:09 PM, Steven Buck wrote:

> I've used a module (StataTools) from (http://presbrey.mit.edu/PyDTA ) to get
> a Stata ".dta" file into Python. In Stata the data set is an NXK matrix
> where N is the number of observations (households) and K is the number of
> variables.
> I gather it's now a list where each element of the list is an observation (a
> vector) for one household.  The name of my list is "data"; I gather Python
> recognizes the first observation by: data[1] .
> Example,
> data = [X_1, X_2, X_3, . . . . , X_N]  where each X_i for all i, is vector
> of household characteristics, eg X_1 = (age_1, wage_1, . . . , residence_1).
>
> I also have a list for variable names called "varname"; although I'm not
> sure the module I used to extract the ".dta" into Python also created a
> correspondence between the varname list and the data list--the python
> interpreter won't print anything when I type one of the variable names, I
> was hoping it would print out a vector of ages or the like.

varname is probably just a list of strings without any direct
connection to the data.

> In anycase, I'd like to make a scatter plot in pylab, but don't know how to
> identify a variable in "data" (i.e.  I'd like a vector listing the ages and
> another vector listing the wages of  households).  Perhaps, I need to run
> subroutine to collect each relevant data point to create a new list which I
> define as my variable of interest?  From the above example, I'd like to
> create a list such as: age = [age_1, age_2, . . . , age_N] and likewise for
> wages.

You can use a list comprehension to collect columns from the data. If
age is the first element of each observation (index 0), and wages the
second (index 1), then
ages = [ observation[0] for observation in data ]
wages = [ observation[1] for observation in data ]

> Any help you could offer would be very much appreciated.  Also, this is my
> first time using the python tutor, so let me know if I've used it
> appropriately or if I should change/narrow the structure of my question.

It's very helpful if you show us the code you have so far.

Kent
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Re: [Tutor] Stack unwind using exceptions.

[Noufal Ibrahim]

Kent Johnson wrote:
[..]

Why not just return the value from the function and pass it up the
call chain? If a call fails return None. Something like this:


That's what I ended up doing but the first thing occurred to me and I 
was just wondering if there's any production code that relies on the 
technique.


--
~noufal
http://nibrahim.net.in/
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