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• How to define and use Python generators appropriately

  - by Morlock

  I want to define a generator from a list that will output the elements one at a time, then use this generator object in an appropriate manner. a = ["Hello", "world", "!"] b = (x for x in a) c = next(b, None) while c != None: print c, c = next(b, None) Is there anything wrong or improvable with the while approach here? Is there a way to avoid having to assign 'c' before the loop? Thanks!

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• Python Django Global Variables

  - by Joe J

  Hi all, I'm looking for simple but recommended way in Django to store a variable in memory only. When Apache restarts or the Django development server restarts, the variable is reset back to 0. More specifically, I want to count how many times a particular action takes place on each model instance (database record), but for performance reasons, I don't want to store these counts in the database. I don't care if the counts disappear after a server restart. But as long as the server is up, I want these counts to be consistent between the Django shell and the web interface, and I want to be able to return how many times the action has taken place on each model instance. I don't want the variables to be associated with a user or session because I might want to return these counts without being logged in (and I want the counts to be consistent no matter what user is logged in). Am I describing a global variable? If so, how do I use one in Django? I've noticed the files like the urls.py, settings.py and models.py seems to be parsed only once per server startup (by contrast to the views.py which seems to be parsed eache time a request is made). Does this mean I should declare my variables in one of those files? Or should I store it in a model attribute somehow (as long as it sticks around for as long as the server is running)? This is probably an easy question, but I'm just not sure how it's done in Django. Any comments or advice is much appreciated. Thanks, Joe

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• python urllib2 thread safety

  - by jldupont

  Is urllib2 thread safe? I find that using urllib2.urlopen sometimes results in the thread issuing the call to stop functioning. Yes I have used the timeout option to no avail. Is there a thread safe HTTP GET functionality I could use? NOTE: I am not interested in using Twisted to solve this problem. I have used Twisted in the past and I love it but this time I need a simpler solution. NOTE2: I also tried httplib with the same result (blocking).

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• Django/Python best practice template_dict

  - by fredrik

  Hi, After just been coding for about 6-9 months. I probably changed my coding style a number of times after reading some code or read best practices. But one thing I haven't yet come a cross is a good why to populate the template_dict. As of now I pass the template_dict across a number of methods (that changes/modifies it) and returns is. The result is that every methods takes template_dict as first argument and the returns it and this in my eyes doesn't seems to be the best solution. An idea is to have a method that handles all the changes. But I'm curios if there's a best practice for this? Or is it "do what you feel like"-type of thing? The 2 things I think is pretty ugly is to send as an argument and return it in all methods. And the just the var name is written xxx number of times in the code :) ..fredrik

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• What Great Python Web Developers Ought To Know?

  - by Anry

  What should know the Entry Level, Mid-level, Senior Developer? Theoretical knowledge, development tools, frameworks, languages (Ruby, Java and etc.) and more. What issues are usually at the interview?

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• Python Expand Tabs Length Calculation

  - by Mithrill

  I'm confused by how the length of a string is calculated when expandtabs is used. I thought expandtabs replaces tabs with the appropriate number of spaces (with the default number of spaces per tab being 8). However, when I ran the commands using strings of varying lengths and varying numbers of tabs, the length calculation was different than I thought it would be (i.e., each tab didn't always result in the string length being increased by 8 for each instance of "/t"). Below is a detailed script output with comments explaining what I thought should be the result of the command executed above. Would someone please explain the how the length is calculated when expand tabs is used? IDLE 2.6.5 >>> s = '\t' >>> print len(s) 1 >>> #the length of the string without expandtabs was one (1 tab counted as a single space), as expected. >>> print len(s.expandtabs()) 8 >>> #the length of the string with expandtabs was eight (1 tab counted as eight spaces). >>> s = '\t\t' >>> print len(s) 2 >>> #the length of the string without expandtabs was 2 (2 tabs, each counted as a single space). >>> print len(s.expandtabs()) 16 >>> #the length of the string with expandtabs was 16 (2 tabs counted as 8 spaces each). >>> s = 'abc\tabc' >>> print len(s) 7 >>> #the length of the string without expandtabs was seven (6 characters and 1 tab counted as a single space). >>> print len(s.expandtabs()) 11 >>> #the length of the string with expandtabs was NOT 14 (6 characters and one 8 space tabs). >>> s = 'abc\tabc\tabc' >>> print len(s) 11 >>> #the length of the string without expandtabs was 11 (9 characters and 2 tabs counted as a single space). >>> print len(s.expandtabs()) 19 >>> #the length of the string with expandtabs was NOT 25 (9 characters and two 8 space tabs). >>>

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• Python Mechanize select a form with no name

  - by mvid

  I am attempting to have mechanize select a form from a page, but the form in question has no "name" attribute in the html. What should I do? when I try to use br.select_form(name = "") i get errors that no form is declared with that name, and the function requires a name input. There is only one form on the page, is there some other way I can select that form?

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• python socket.recv/sendall call blocking

  - by fsm

  Hi everyone. This post is incorrectly tagged 'send' since I cannot create new tags. I have a very basic question about this simple echo server. Here are some code snippets. client while True: data = raw_input("Enter data: ") mySock.sendall(data) echoedData = mySock.recv(1024) if not echoedData: break print echoedData server while True: print "Waiting for connection" (clientSock, address) = serverSock.accept() print "Entering read loop" while True: print "Waiting for data" data = clientSock.recv(1024) if not data: break clientSock.send(data) clientSock.close() Now this works alright, except when the client sends an empty string (by hitting the return key in response to "enter data: "), in which case I see some deadlock-ish behavior. Now, what exactly happens when the user presses return on the client side? I can only imagine that the sendall call blocks waiting for some data to be added to the send buffer, causing the recv call to block in turn. What's going on here? Thanks for reading!

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• Web-based game in Python + Django and client browser polling

  - by ty

  I am creating a text-based game that implements a basic model in which multiple (10+) players interact with data and one moderator watches them and sets certain environmental statistics that affect gameplay. Recently I have begun to familiarize myself with Django. It seems to me that it would be an excellent tool for creating a game quickly, particularly because the nature of my game depends largely on sets of data (which lends itself quite well to a database). I am wondering how to "push" changes made by the game moderator to the players (for example, the moderator can decide to display an image to all players). The game is turn-based, not real-time, but certain messages need to be pushed out in roughly real-time. My thoughts: I could have each player's browser poll a status periodically (say, every 30 seconds) to see if there is a message from a moderator. But this forces a lag and means different players might receive it at different times. And reducing this interval to <10 seems like a bad idea for the server. Is there a better way to inform clients of changes? Would you suggest something other than using a web framework like Django? Thanks!

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• python: sorting

  - by nabizan

  hi im doing a loop so i could get dict of data, but since its a dict it's sorting alphabetical and not as i push it trought the loop ... is it possible to somehow turn off alphabetical sorting? here is how do i do that data = {} for item in container: data[item] = {} ... for key, val in item_container.iteritems(): ... data[item][key] = val whitch give me something like this data = { A : { K1 : V1, K2 : V2, K3 : V3 }, B : { K1 : V1, K2 : V2, K3 : V3 }, C : { K1 : V1, K2 : V2, K3 : V3 } } and i want it to be as i was going throught the loop, e.g. data = { B : {K2 : V2, K3 : V3, K1 : V1}, A : {K1 : V1, K2 : V2, K3 : V3}, C : {K3 : V3, K1 : V1, K2 : V2} }

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• In python: how to apply itertools.product to elements of a list of lists

  - by Guilherme Rocha

  I have a list of arrays and I would like to get the cartesian product of the elements in the arrays. I will use an example to make this more concrete... itertools.product seems to do the trick but I am stuck in a little detail. arrays = [(-1,+1), (-2,+2), (-3,+3)]; If I do cp = list(itertools.product(arrays)); I get cp = cp0 = [((-1, 1),), ((-2, 2),), ((-3, 3),)] But what I want to get is cp1 = [(-1,-2,-3), (-1,-2,+3), (-1,+2,-3), (-1,+2,+3), ..., (+1,+2,-3), (+1,+2,+3)]. I have tried a few different things: cp = list(itertools.product(itertools.islice(arrays, len(arrays)))); cp = list(itertools.product(iter(arrays, len(arrays)))); They all gave me cp0 instead of cp1. Any ideas? Thanks in advance.

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• unevenly centered subplots in matplotlib in Python?

  - by user248237

  I am plotting a simple pair of subplots in matplotlib that are for some reason unevenly centered. I plot them as follows: plt.figure() # first subplot s1 = plt.subplot(2, 1, 1) plt.bar([1, 2, 3], [4, 5, 6]) # second subplot s2 = plt.subplot(2, 1, 2) plt.pcolor(rand(5,5)) # add colorbar plt.colorbar() # square axes axes_square(s1) axes_square(s2) where axes_square is simply: def axes_square(plot_handle): plot_handle.axes.set_aspect(1/plot_handle.axes.get_data_ratio()) The plot I get is attached. The top and bottom plots are unevenly centered. I'd like their yaxis to be aligned and their boxes to be aligned. If I remove the plt.colorbar() call, the plots become centered. How can I have the plots centered while the colorbar of pcolor is still shown? I want the axes to be centered and have the colorbar be outside of that alignment, either to the left or to the right of the pcolor matrix. image of plots link thanks.

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• Python: OSX Library for fast full screen jpg/png display

  - by Parand

  Frustrated by lack of a simple ACDSee equivalent for OS X, I'm looking to hack one up for myself. I'm looking for a gui library that accommodates: Full screen image display High quality image fit-to-screen (for display) Low memory usage Fast display Reasonable learning curve (the simpler the better) Looks like there are several choices, so which is the best? Here are some I've run across: PyOpenGL PyGame PyQT wxpython I don't have any particular experience with any of these, nor any strong desire to become an expert - I'm looking for the simplest solution. What do you recommend? [Update] For those not familiar with ACDSee, here's what it does that I care about: Simple list/thubmnail display of images in a directory Sort by name/size/type Ability to view images full screen Single-key delete while viewing full screen Move to next/previous image while viewing full screen Ability to select a group of images for: move to / copy to directory delete resize ACDSee has a bunch of niceties as well, such as remembering directories you've moved images to in the past, remembering your resize settings, displaying the total size of the images you've selected, etc. I've tried most of the options I could find (including Xee) and none of them quite get there. Please keep in mind that this is a programming/library question, not a criticism of any of the existing tools.

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• what is this code mean in python,

  - by zjm1126

  def a(): print 'sss' print getattr(a, "_decorated_function", a).__name__ it print : a thanks

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• Python: How efficient is subtring extraction?

  - by Cameron

  I've got the entire contents of a text file (at least a few KB) in string myStr. Will the following code create a copy of the string (less the first character) in memory? myStr = myStr[1:] I'm hoping it just refers to a different location in the same internal buffer. If not, is there a more efficient way to do this? Thanks!

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• Sorting objects in Python

  - by Curious2learn

  I want to sort objects using by one of their attributes. As of now, I am doing it in the following way USpeople.sort(key=lambda person: person.utility[chosenCar],reverse=True) This works fine, but I have read that using operator.attrgetter() might be a faster way to achieve this sort. First, is this correct? Assuming that it is correct, how do I use operator.attrgetter() to achieve this sort? I tried, keyFunc=operator.attrgetter('utility[chosenCar]') USpeople.sort(key=keyFunc,reverse=True) However, I get an error saying that there is no attribute 'utility[chosenCar]'. The problem is that the attribute by which I want to sort is in a dictionary. For example, the utility attribute is in the following form: utility={chosenCar:25000,anotherCar:24000,yetAnotherCar:24500} I want to sort by the utility of the chosenCar using operator.attrgetter(). How could I do this? Thanks in advance.

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• Simplest way to handle and display errors in a Python Pylons controller without a helper class

  - by ensnare

  I have a class User() that throw exceptions when attributes are incorrectly set. I am currently passing the exceptions from the models through the controller to the templates by essentially catching exceptions two times for each variable. Is this a correct way of doing it? Is there a better (but still simple) way? I prefer not to use any third party error or form handlers due to the extensive database queries we already have in place in our classes. Furthermore, how can I "stop" the chain of processing in the class if one of the values is invalid? Is there like a "break" syntax or something? Thanks. >>> u = User() >>> u.name = 'Jason Mendez' >>> u.password = '1234' Traceback (most recent call last): File "<stdin>", line 1, in <module> File "topic/model/user.py", line 79, in password return self._password ValueError: Your password must be greater than 6 characters In my controller "register," I have: class RegisterController(BaseController): def index(self): if request.POST: c.errors = {} u = User() try: u.name = c.name = request.POST['name'] except ValueError, error: c.errors['name'] = error try: u.email = c.email = request.POST['email'] except ValueError, error: c.errors['email'] = error try: u.password = c.password = request.POST['password'] except ValueError, error: c.errors['password'] = error try: u.commit() except ValueError, error: pass return render('/register.mako')

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• Converting python objects for rpy2

  - by bgbg

  The following code is supposed to created a heatmap in rpy2 import numpy as np from rpy2.robjects import r data = np.random.random((10,10)) r.heatmap(data) However, it results in the following error Traceback (most recent call last): File "z.py", line 8, in <module> labRow=rowNames, labCol=colNames) File "C:\Python25\lib\site-packages\rpy2\robjects\__init__.py", line 418, in __call__ new_args = [conversion.py2ri(a) for a in args] File "C:\Python25\lib\site-packages\rpy2\robjects\__init__.py", line 93, in default_py2ri raise(ValueError("Nothing can be done for the type %s at the moment." %(type(o)))) ValueError: Nothing can be done for the type <type 'numpy.ndarray'> at the moment. From the documentation I learn that r.heatmap expects "a numeric matrix". How do I convert np.array to the required data type?

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• Threading in python: retrieve return value when using target=

  - by Philipp Keller

  I want to get the "free memory" of a bunch of servers like this: def get_mem(servername): res = os.popen('ssh %s "grep MemFree /proc/meminfo | sed \'s/[^0-9]//g\'"' % servername) return res.read().strip() since this can be threaded I want to do something like that: import threading thread1 = threading.Thread(target=get_mem, args=("server01", )) thread1.start() But now: how can I access the return value(s) of the get_mem functions? Do I really need to go the full fledged way creating a class MemThread(threading.Thread) and overwriting __init__ and __run__?

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• python regular expression for domain names

  - by user230911

  I am trying use the following regression to extract domain name from a text, but it just produce nothing, what's wrong with it? I don't know if this is suitable to ask this "fix code" question, maybe I should read more. I just want to save some time. Thanks pat_url = re.compile(r''' (?:https?://)* (?:[\w]+[\-\w]+[.])* (?P<domain>[\w\-]*[\w.](com|net)([.](cn|jp|us))*[/]*) ''') print re.findall(pat_url,"http://www.google.com/abcde") I want the output to be google.com

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• Passing list and dictionary type parameter with Python

  - by prosseek

  When I run this code def func(x, y, *w, **z): print x print y if w: print w if z: print z else: print "None" func(10,20, 1,2,3,{'k':'a'}) I get the result as follows. 10 20 (1, 2, 3, {'k': 'a'}) None But, I expected as follows, I mean the list parameters (1,2,3) matching *w, and dictionary matching **z. 10 20 (1,2,3) {'k':'a'} Q : What went wrong? How can I pass the list and dictionary as parameters? Added func(10,20, 10,20,30, k='a') seems to be working

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• Python key word arguments

  - by pythonic metaphor

  I have several layers of function calls, passing around a common dictionary of key word arguments: def func1(**qwargs): func2(**qwargs) func3(**qwargs) I would like to supply some default arguments in some of the subsequent function calls, something like this: def func1(**qwargs): func2(arg = qwargs.get("arg", default), **qwargs) func3(**qwargs) The problem with this approach is that if arg is inside qwargs, a TypeError is raised with "got multiple values for keyword argument". I don't want to set qwargs["arg"] to default, because then func3 gets this argument without warrant. I could make a copy.copy of the qwargs and set "arg" in the copy, but qwargs could have large data structures in it and I don't want to copy them (maybe copy.copy wouldn't, only copy.deepcopy?). What's the pythonic thing to do here?

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• Fast iterating over first n items of an iterable in python

  - by martinthenext

  Hello! I'm looking for a pythonic way of iterating over first n items of a list, and it's quite important to do this as fast as possible. This is how I do it now: count = 0 for item in iterable: do_somethin(item) count += 1 if count >= n: break Doesn't seem neat to me. Another way of doing this is: for item in itertools.islice(iterable, n): do_something(item) This looks good, the question is it fast enough to use with some generator(s)? For example: pair_generator = lambda iterable: itertools.izip(*[iter(iterable)]*2) for item in itertools.islice(pair_generator(iterable), n): so_something(item) Will it run fast enough as compared to the first method? Is there some easier way to do it?

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• Python - How to pickle yourself?

  - by Mark

  I want my class to implement Save and Load functions which simply do a pickle of the class. But apparently you cannot use 'self' in the fashion below. How can you do this? self = cPickle.load(f) cPickle.dump(self,f,2)

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• Xml comparison in Python

  - by Gregg Lind

  Building on another SO question, how can one check whether two well-formed XML snippets are semantically equal. All I need is "equal" or not, since I'm using this for unit tests. In the system I want, these would be equal (note the order of 'start' and 'end'): <?xml version='1.0' encoding='utf-8' standalone='yes'?> <Stats start="1275955200" end="1276041599"> </Stats> # Reodered start and end <?xml version='1.0' encoding='utf-8' standalone='yes'?> <Stats end="1276041599" start="1275955200" > </Stats> I have lmxl and other tools at my disposal, and a simple function that only allows reordering of attributes would work fine as well!

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