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10-28-2011 , 11:02 AM

#276

Madmax2000

journeyman

Join Date: Aug 2010 Posts: 244

Quote:

Originally Posted by TheIrishThug

Can we see the code you ran and the actual error message? This will make it easier for us to see what is actually going wrong and why it is happening.

I wrote the code in python command line, then saved it as a .py, then ran it through IDLE, I clicked run --> check module-- then I get the syntax error.

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10-28-2011 , 11:29 AM

#277

TheIrishThug

Pooh-Bah

Join Date: Jan 2005 Posts: 5,223

Ok, there is the problem. Saving the text from the command line includes text that is not part of the code that runs.

Delete any line that doesn't have ">>>" before it. Then delete all of the instances of ">>>". You should end up with something very close to what Gazillion posted. That is the actual Python code. Everything else is there as part of the interactive python shell.

Quote

10-28-2011 , 12:47 PM

#278

Gazillion

Carpal \'Tunnel

Join Date: Feb 2008 Posts: 8,721

Lol, yes - very much what Irishthug said

It would be a really good idea to not get into the habit of copying the output of the command line into a text file and saving it as a .py

Think of them as two seperate entities. Just use the command line if you want to quickly experiment with new commands etc. For actually writing a piece of code that you wish to execute at will, always do it from a blank file in a text editor or IDLE and save it as a .py

The command line will have a tonne of stuff that is specific to it and it alone, including all the stuff that you've already discovered to be problematic.

Quote

10-31-2011 , 05:34 AM

#279

m_hags

enthusiast

Join Date: Jun 2008 Posts: 95

madmax, I'm just learning now to program and am working through the tutorial here: http://learnpythonthehardway.org/ the book is free if you view it online. what you're doing made me cringe. I think you are a step or two ahead of yourself and should maybe check this out.

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10-31-2011 , 11:00 PM

#280

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

That MIT class ~is~ the hard way to learn Python.

I haven't read the code in that book but the author is pretty entertaining.

Quote

11-01-2011 , 01:23 AM

#281

m_hags

enthusiast

Join Date: Jun 2008 Posts: 95

what course number are you referring to? I found it earlier, but it looked like I missed the start date or something and didn't investigate past that. Now I can't find the link.

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11-03-2011 , 07:30 PM

#282

calm

banned

Join Date: Jul 2006 Posts: 2,087

Pretty cool trick I just learned, using an else statement with a for loop...

Instead of:

Code:

found = False
for count in xrange(10):
    if count == number_wanted:
	    # do something
	    found = True
	    break
if not found:
    # do something else

It's easier and cleaner like this:

Code:

for count in xrange(10):
    if count == number_wanted:
	    # do something
	    break
else:
    # do something else

Quote

11-03-2011 , 08:00 PM

#283

RoundTower

ɹǝʍoʇpunoɹ

Join Date: Feb 2005 Posts: 14,562

I always mix up how that works, I can't decide if I should claim it's bad style or just get more used to it.

Quote

11-03-2011 , 08:36 PM

#284

TheIrishThug

Pooh-Bah

Join Date: Jan 2005 Posts: 5,223

If more languages supported it, it would be fine. The fact that most people don't know what it means, is what causes the issue.

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11-05-2011 , 03:58 PM

#285

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

Quote:

Originally Posted by calm

Pretty cool trick I just learned, using an else statement with a for loop...

Instead of:

Code:

found = False
for count in xrange(10):
    if count == number_wanted:
	    # do something
	    found = True
	    break
if not found:
    # do something else

It's easier and cleaner like this:

Code:

for count in xrange(10):
    if count == number_wanted:
	    # do something
	    break
else:
    # do something else

The second option looks best to me. Not sure how I feel about the first version. I think it was probably done that way because whoever is teaching you the language is trying to make things explicit, but not sure if that is a good thing or not, tbh. Sort of like using 'try:' for every single variable your function takes.

Just an interesting tidbit about xrange():

xrange is syntatically replaced by range() in Python3 so that range() has the same function as xrange().

range is (in many cases) supposed to be written as list(range()).

Quote

11-05-2011 , 04:57 PM

#286

calm

banned

Join Date: Jul 2006 Posts: 2,087

Quote:

Originally Posted by daveT

The first version is just an example of how I used to write that loop before learning the else case existed.

I'm still using Python 2.6, and there xrange() is more efficient memory-wise than range(). It always confused me though, so I'm glad to hear in 3.x that the two get consolidated.

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11-05-2011 , 09:02 PM

#287

Xhad

Carpal \'Tunnel

Join Date: Jul 2005 Posts: 10,962

I love the loop-else construct so much that I use it even when it's not necessary (because the "break" was actually a function return statement). That way I find it easier to remember what I did for some reason.

Quote

11-05-2011 , 10:41 PM

#288

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

Quote:

Originally Posted by calm

Python3 doesn't actually consolidate them.

range() works by turning the iterator into a list object first and then computing, and considering that many programmers don't know about xrange(), this only serves to slow down the programs written with python. xrange() simply counts out the iterators and computes on it one after another.

So, unless you really need a list of values, range() isn't very useful. What Python3 did is default range to act like xrange(). So now if you want your iterators to become lists (make range() act like the old range()), you have to make it explicit that you do.

list(range())

Of course, this opens up a whole can of worms about the "best" way to do things, but for the most part, the difference in range() in 2 and 3 is superficial and I don't think most programmers moving from 2 to 3 will ever know the difference. I suspect that this was done on purpose by the Python overlords.

Quote

11-07-2011 , 05:25 PM

#289

Xhad

Carpal \'Tunnel

Join Date: Jul 2005 Posts: 10,962

Quote:

Originally Posted by daveT

Code:

#this no longer works

def myFun(a, b):
    one = a
    two = b
    myVar = one, two
    return myVar

So, I actually use Python 3 exclusively and this has been bugging me for awhile. In what sense does this function "not work"? It looks and runs fine to me.

Also, in response to a different discussion, either they didn't remove reduce from Python3 or they changed their minds: http://docs.python.org/release/3.1.3...nctools.reduce

Quote

11-09-2011 , 09:43 PM

#290

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

Quote:

Originally Posted by Xhad

So, I actually use Python 3 exclusively and this has been bugging me for awhile. In what sense does this function "not work"? It looks and runs fine to me.

Can't really remember what it was I was thinking about when I wrote that. Maybe there is something different in the output or there was something wrong with the original install I had. I did reinstall 3.

Sorry, I'm already forgetting 2.

Quote

11-12-2011 , 10:46 PM

#291

Xhad

Carpal \'Tunnel

Join Date: Jul 2005 Posts: 10,962

np, I was just puzzled more than anything

btw, in case you're still using int() to fix division results, a shorter way to get around the integer division problem is to use // (integer division) instead of / (division). So, this:

Code:

x = 10//2

for i in range(x):
    print(i, end=' ')

will print "0 1 2 3 4"

Quote

11-13-2011 , 02:22 AM

#292

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

Quote:

Originally Posted by Xhad

Code:

x = 10//2

for i in range(x):
    print(i, end=' ')

will print "0 1 2 3 4"

Nice one. I didn't know this. ty.

Quote

11-13-2011 , 02:53 PM

#293

calm

banned

Join Date: Jul 2006 Posts: 2,087

I've been programming in Python for a few years now and never learned list comprehensions. They seem to pop up a lot in other people's code, but so far I've never had a problem getting things done other ways. What are their benefits over just a standard for-loop to process data? Are there any times where I *really* should be using list comprehensions?

Edit: Map and filter too, I've never used those. Worth learning?

Quote

11-13-2011 , 03:50 PM

#294

Neko

Pooh-Bah

Join Date: Jul 2005 Posts: 4,000

You should be using them anywhere that their use makes your code shorter/more concise without sacrificing clarity of purpose/readability.

I'd rather see:

Code:

rands = [random.random() for x in range(10000)]

then

Code:

rands = []
for x in range(10000):
    rands.append(random.random())

The former reduces the code from 3 lines to 1, without sacrificing any clarity. The first example will also run faster then the second code which is another benefit.

I'd often use map for something like:

Code:

some_string = "1,2,3,4"
new_list = map(int,some_string.split(','))

But this could also be achieved with a list comprehension:

Code:

some_string = "1,2,3,4"
new_list = [int(x) for x in some_string.split(',')]

With filter I often find a list comprehension is a little nicer to look at:

Code:

odds = [x for x in range(10) if x%2 != 0]

vs:

Code:

odds = filter(lambda x: x%2!=0,range(10))

That said, map, filter & reduce are all worth learning about even if you don't use them too often!

Quote

11-13-2011 , 04:02 PM

#295

calm

banned

Join Date: Jul 2006 Posts: 2,087

Thanks.

So looking at my code, this seems like a good candidate...

Code:

header = ['Column A','Column B','Column C']
for book in book_list:
    if book in settings['books']:
        header.append(book)

I've translated the loop to this list comprehension:

Code:

header = [book for book in book_list if book in settings['book']]

but how do i make sure Column A, B and C get put at the front of the list (before the books)?

Code:

header = ['Column A','Column B','Column C'].extend([book for book in book_list if book in settings['book']])

maybe?

Last edited by calm; 11-13-2011 at 04:27 PM.

Quote

11-13-2011 , 04:27 PM

#296

Neko

Pooh-Bah

Join Date: Jul 2005 Posts: 4,000

Quote:

Originally Posted by calm

Thanks.

So looking at my code, this seems like a good candidate...

Code:

header = ['Column A','Column B','Column C']
for book in book_list:
    if book in settings['books']:
        header.append(book)

I've translated the loop to this list comprehension:

Code:

header = [book for book in book_list if book in settings['book']]

but how do i make sure Column A, B and C get put at the front of the list (before the books)?

You could do either:

Code:

header = ['Column A','Column B','Column C']  + [book for book in book_list if book in settings['book']]

Code:

header =  [book for book in book_list if book in settings['book']]
header = ['Column A','Column B','Column C']  + header

Code:

header = ['Column A','Column B','Column C']
header = header.extend([book for book in book_list if book in settings['book']])

Last edited by Neko; 11-13-2011 at 04:38 PM.

Quote

11-13-2011 , 04:38 PM

#297

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

Quote:

Originally Posted by calm

Thanks.

So looking at my code, this seems like a good candidate...

Code:

header = ['Column A','Column B','Column C']
for book in book_list:
    if book in settings['books']:
        header.append(book)

I've translated the loop to this list comprehension:

Code:

header = [book for book in book_list if book in settings['book']]

but how do i make sure Column A, B and C get put at the front of the list (before the books)?

Code:

header = ['Column A','Column B','Column C'].extend([book for book in book_list if book in settings['book']])

maybe?

What do your output to look like?

Are you saying you want:

[columnA [bookA, bookB], columnB....]

or

[columnA, columnB, columnC, bookA, bookB, bookC...]

Because these are two very different questions, but if you want option 2, you shouldn't have to do anything. List are ordered objects, so if you put 1 in, then you put it in spot 1.

Code:

>>> a = []
>>> for i in range(1, 10):
	a.append(i)
	print(a)

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

Quote

11-13-2011 , 04:53 PM

#298

calm

banned

Join Date: Jul 2006 Posts: 2,087

Yes I want option 2, but as a list comprehension instead of a for loop. Think I got it figured out thanks to Neko.

Here's another for loop I'm trying to substitute with a list comprehension:

Code:

row_data = []
row_color = []
for key in dict:
    row_data.append([])
    row_color.append([])

Now I could easily do this with two separate list comprehensions:

Code:

row_data = [[] for key in dict]
row_color = [[] for key in dict]

But since I'm then looping over the dict twice, I guess the for loop would be faster? Is there a way to combine the two in one list comprehension?

Last edited by calm; 11-13-2011 at 05:02 PM.

Quote

11-13-2011 , 04:58 PM

#299

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

But he didn't tell you which one is the 'Pythonic' way.

Quote

11-13-2011 , 05:12 PM

#300

calm

banned

Join Date: Jul 2006 Posts: 2,087

Code:

row_data,row_color = [[[] for key in dict] for x in xrange(2)]

Any better?

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