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• python conditional list creation from 2D lists

  - by dls

  Say I've got a list of lists. Say the inner list of three elements in size and looks like this: ['apple', 'fruit', 1.23] The outer list looks like this data = [['apple', 'fruit', 1.23], ['pear', 'fruit', 2.34], ['lettuce', 'vegetable', 3.45]] I want to iterate through the outer list and cull data for a temporary list only in the case that element 1 matches some keyword (aka: 'fruit'). So, if I'm matching fruit, I would end up with this: tempList = [('apple', 1.23), ('pear', 2.34)] This is one way to accomplish this: tempList = [] for i in data: if i[1] == 'fruit': tempList.append(i[0], i[2]) is there some 'Pythonic' way to do this in fewer lines?

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• How should I declare default values for instance variables in Python?

  - by int3

  Should I give my class members default values like this: class Foo: num = 1 or like this? class Foo: def __init__(self): self.num = 1 In this question I discovered that in both cases, bar = Foo() bar.num += 1 is a well-defined operation. I understand that the first method will give me a class variable while the second one will not. However, if I do not require a class variable, but only need to set a default value for my instance variables, are both methods equally good? Or one of them more 'pythonic' than the other? One thing I've noticed is that in the Django tutorial, they use the second method to declare Models. Personally I think the second method is more elegant, but I'd like to know what the 'standard' way is.

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• Can I turn off implicit Python unicode conversions to find my mixed-strings bugs?

  - by Tal Weiss

  When profiling our code I was surprised to find millions of calls to C:\Python26\lib\encodings\utf_8.py:15(decode) I started debugging and found that across our code base there are many small bugs, usually comparing a string to a unicode or adding a sting and a unicode. Python graciously decodes the strings and performs the following operations in unicode. How kind. But expensive! I am fluent in unicode, having read Joel Spolsky and Dive Into Python... I try to keep our code internals in unicode only. My question - can I turn off this pythonic nice-guy behavior? At least until I find all these bugs and fix them (usually by adding a u'u')? Some of them are extremely hard to find (a variable that is sometimes a string...). Python 2.6.5 (and I can't switch to 3.x).

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• Divide numpy array

  - by BandGap

  Hi all I have some data represented in a 1300x1341 matrix. I would like to split this matrix in several pieces (e.g. 9) so that I can loop over and process them. The data needs to stay ordered in the sense that x[0,1] stays below (or above if you like) x[0,0] and besides x[1,1]. Just like if you had imaged the data, you could draw 2 vertical and 2 horizontal lines over the image to illustrate the 9 parts. If I use numpys reshape (eg. matrix.reshape(9,260,745) or any other combination of 9,260,745) it doesn't yield the required structure since the above mentioned ordering is lost... Did I misunderstand the reshape method or can it be done this way? What other pythonic/numpy way is there to do this?

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• idiomatic way to take groups of n items from a list in Python?

  - by Wang

  Given a list A = [1 2 3 4 5 6] Is there any idiomatic (Pythonic) way to iterate over it as though it were B = [(1, 2) (3, 4) (5, 6)] other than indexing? That feels like a holdover from C: for a1,a2 in [ (A[i], A[i+1]) for i in range(0, len(A), 2) ]: I can't help but feel there should be some clever hack using itertools or slicing or something. (Of course, two at a time is just an example; I'd like a solution that works for any n.) Edit: related http://stackoverflow.com/questions/1162592/iterate-over-a-string-2-or-n-characters-at-a-time-in-python but even the cleanest solution (accepted, using zip) doesn't generalize well to higher n without a list comprehension and *-notation.

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• Python - List and Loop in one def

  - by Dunwitch

  I'm trying to get the def wfsc_pod1 and wfsc_ip into the same def. I'm not quite sure how to approach the problem. I want wfsc_pod1 to display all the information for name, subnet and gateway. Then wfsc_ip shows the ip addresses below it. I also get a None value when I run it as it. Not sure why. Anything more pythonic is more appreciated. class OutageAddress: subnet = ["255.255.255.0", "255.255.255.1"] # Gateway order is matched with names gateway = ["192.168.1.1", "192.168.1.2", "192.168.1.3", "192.168.1.4", "192.168.1.5", "192.168.1.6", "192.168.1.7", "192.168.1.8", "192.168.1.9"] name = ["LOC1", "LOC2", "LOC3", "LOC4", "LOC5", "LOC6", "LOC7", "LOC8", "LOC9"] def wfsc_pod1(self): wfsc_1 = "%s\t %s\t %s\t" % (network.name[0],network.subnet[0],network.gateway[0]) return wfsc_1 def wfsc_ip(self): for ip in range(100,110): ip = "192.168.1."+str(ip) print ip network = OutageAddress() print network.wfsc_pod1() print network.wfsc_ip()

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• Python: How can I override one module in a package with a modified version that lives outside the pa

  - by zlovelady

  I would like to update one module in a python package with my own version of the module, with the following conditions: I want my updated module to live outside of the original package (either because I don't have access to the package source, or because I want to keep my local modifications in a separate repo, etc). I want import statements that refer to original package/module to resolve to my local module Here's an example of what I'd like to do using specifics from django, because that's where this problem has arisen for me: Say this is my project structure django/ ... the original, unadulterated django package ... local_django/ conf/ settings.py myproject/ __init__.py myapp/ myfile.py And then in myfile.py # These imports should fetch modules from the original django package from django import models from django.core.urlresolvers import reverse # I would like this following import statement to grab a custom version of settings # that I define in local_django/conf/settings.py from django.conf import settings def foo(): return settings.some_setting Can I do some magic with the __import__ statement in myproject/__init__.py to accomplish this? Is there a more "pythonic" way to achieve this?

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• Python - Is there a better/efficient way to find a node in tree?

  - by Sej P

  I have a node data structure defined as below and was not sure the find_matching_node method is pythonic or efficient. I am not well versed with generators but think there might be better solution using them. Any ideas? class HierarchyNode(): def __init__(self, nodeId): self.nodeId = nodeId self.children = {} # opted for dictionary to help reduce lookup time def addOrGetChild(self, childNode): return self.children.setdefault(childNode.nodeId,childNode) def find_matching_node(self, node): ''' look for the node in the immediate children of the current node. if not found recursively look for it in the children nodes until gone through all nodes ''' matching_node = self.children.get(node.nodeId) if matching_node: return matching_node else: for child in self.children.itervalues(): matching_node = child.find_matching_node(node) if matching_node: return matching_node return None

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• Right design to validate attributes of a class instance

  - by systempuntoout

  Having a simple Python class like this: class Spam(object): __init__(self, description, value): self.description = description self.value = value Which is the correct approach to check these constraints: "description cannot be empty" "value must be greater than zero" Should i: 1.validate data before creating spam object ? 2.check data on __init__ method ? 3.create an is_valid method on Spam class and call it with spam.isValid() ? 4.create an is_valid static method on Spam class and call it with Spam.isValid(description, value) ? 5.check data on setters? 6.... Could you recommend a well designed\Pythonic\not verbose (on class with many attributes)\elegant approach?

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• Inspiration and influence of the else clause of loop statements?

  - by Aristide

  Python offers an optional loop-else clause which is executed if and only if the loop is not terminated by a break. (In other words, the condition fails for a while-loop or the iterator is exhausted for a for-loop.) Does this loop-else construct originate from another language? (Either theoretical or actually implemented.) Has it been taken up in any newer language? Maybe I should ask the former of Guido, but surely he is too busy for such a futile inquiry. ;-) Related discussion and examples: Pythonic ways to use ‘else’ in a for loop

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• List filtering: list comprehension vs. lambda + filter

  - by Agos

  I happened to find myself having a basic filtering need: I have a list and I have to filter it by an attribute of the items. My code looked like this: list = [i for i in list if i.attribute == value] But then i thought, wouldn't it be better to write it like this? filter(lambda x: x.attribute == value, list) It's more readable, and if needed for performance the lambda could be taken out to gain something. Question is: are there any caveats in using the second way? Any performance difference? Am I missing the Pythonic Way™ entirely and should do it in yet another way (such as using itemgetter instead of the lambda)? Thanks in advance

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• Parsing a string representing a float *with an exponent* in Python

  - by Lucas

  Hi, I have a large file with numbers in the form of 6,52353753563E-7. So there's an exponent in that string. float() dies on this. While I could write custom code to pre-process the string into something float() can eat, I'm looking for the pythonic way of converting these into a float (something like a format string passed somewhere). I must say I'm surprised float() can't handle strings with such an exponent, this is pretty common stuff. I'm using python 2.6, but 3.1 is an option if need be.

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• In Python, is it better to use list comprehensions or for-each loops?

  - by froadie

  Which of the following is better to use and why? Method 1: for k, v in os.environ.items() print "%s=%s" % (k, v) Method 2: print "\n".join(["%s=%s" % (k, v) for k,v in os.environ.items()]) I tend to lead towards the first as more understandable, but that might just be because I'm new to Python and list comprehensions are still somewhat foreign to me. Is the second way considered more Pythonic? I'm assuming there's no performance difference, but I may be wrong. What would be the advantages and disadvantages of these 2 techniques? (Code taken from Dive into Python)

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• String contains all the elements of a list

  - by CSSS

  I am shifting to Python, and am still relatively new to the pythonic approach. I want to write a function that takes a string and a list and returns true if all the elements in the list occur in the string. This seemed fairly simple. However, I am facing some difficulties with it. The code goes something like this: def myfun(str,list): for a in list: if not a in str: return False return True Example : myfun('tomato',['t','o','m','a']) should return true myfun('potato',['t','o','m','a']) should return false myfun('tomato',['t','o','m']) should return true Also, I was hoping if someone could suggest a possible regex approach here. I am trying out my hands on them too.

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• How to optimize this script

  - by marks34

  I have written the following script. It opens a file, reads each line from it splitting by new line character and deleting first character in line. If line exists it's being added to array. Next each element of array is splitted by whitespace, sorted alphabetically and joined again. Every line is printed because script is fired from console and writes everything to file using standard output. I'd like to optimize this code to be more pythonic. Any ideas ? import sys def main(): filename = sys.argv[1] file = open(filename) arr = [] for line in file: line = line[1:].replace("\n", "") if line: arr.append(line) for line in arr: lines = line.split(" ") lines.sort(key=str.lower) line = ''.join(lines) print line if __name__ == '__main__': main()

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• Correct approach to validate attributes of an instance of class

  - by systempuntoout

  Having a simple Python class like this: class Spam(object): __init__(self, description, value): self.description = description self.value = value Which is the correct approach to check these constraints: "description cannot be empty" "value must be greater than zero" Should i: 1.validate data before creating spam object ? 2.check data on __init__ method ? 3.create an is_valid method on Spam class and call it with spam.isValid() ? 4.create an is_valid static method on Spam class and call it with Spam.isValid(description, value) ? 5.check data on setters declaration ? 6.... Could you recommend a well designed\Pythonic\not verbose (on class with many attributes)\elegant approach?

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• Filtering across two ManyToMany fields

  - by KVISH

  I have a User model and an Event model. I have the following for both: class Event(models.Model): ... timestamp = models.DateTimeField() organization_map = models.ManyToManyField(Organization) class User(AuthUser): ... subscribed_orgs = models.ManyToManyField('Organization') I want to find all events that were created in a certain timeframe and find the users who are subscribed to those organizations. I know how to write SQL for this (it's very easy), but whats the pythonic way of doing this using Django ORM? I'm trying as per below: orgs = Organization.objects.all() events = Event.objects.filter(timestamp__gt=min_time) # Min time is the time I want to start from events = events.filter(organization_map__in=orgs) But from there, how do I map to users who have that organization as a subscription? I'm trying to map it like so: users = User.objects.filter(subscribed_orgs__in=...

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• What's the best way to create a static utility class in python? Is using metaclasses code smell?

  - by rsimp

  Ok so I need to create a bunch of utility classes in python. Normally I would just use a simple module for this but I need to be able to inherit in order to share common code between them. The common code needs to reference the state of the module using it so simple imports wouldn't work well. I don't like singletons, and classes that use the classmethod decorator do not have proper support for python properties. One pattern I see used a lot is creating an internal python class prefixed with an underscore and creating a single instance which is then explicitly imported or set as the module itself. This is also used by fabric to create a common environment object (fabric.api.env). I've realized another way to accomplish this would be with metaclasses. For example: #util.py class MetaFooBase(type): @property def file_path(cls): raise NotImplementedError def inherited_method(cls): print cls.file_path #foo.py from util import * import env class MetaFoo(MetaFooBase): @property def file_path(cls): return env.base_path + "relative/path" def another_class_method(cls): pass class Foo(object): __metaclass__ = MetaFoo #client.py from foo import Foo file_path = Foo.file_path I like this approach better than the first pattern for a few reasons: First, instantiating Foo would be meaningless as it has no attributes or methods, which insures this class acts like a true single interface utility, unlike the first pattern which relies on the underscore convention to dissuade client code from creating more instances of the internal class. Second, sub-classing MetaFoo in a different module wouldn't be as awkward because I wouldn't be importing a class with an underscore which is inherently going against its private naming convention. Third, this seems to be the closest approximation to a static class that exists in python, as all the meta code applies only to the class and not to its instances. This is shown by the common convention of using cls instead of self in the class methods. As well, the base class inherits from type instead of object which would prevent users from trying to use it as a base for other non-static classes. It's implementation as a static class is also apparent when using it by the naming convention Foo, as opposed to foo, which denotes a static class method is being used. As much as I think this is a good fit, I feel that others might feel its not pythonic because its not a sanctioned use for metaclasses which should be avoided 99% of the time. I also find most python devs tend to shy away from metaclasses which might affect code reuse/maintainability. Is this code considered code smell in the python community? I ask because I'm creating a pypi package, and would like to do everything I can to increase adoption.

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• Is there any functional-like unix shell?

  - by Caruccio

  I'm (really) newbie to functional programming (in fact only had contact with it using python) but seems to be a good approach for some list-intensive tasks in a shell environment. I'd love to do something like this: $ [ git clone $host/$repo for repo in repo1 repo2 repo3 ] Is there any Unix shell with these kind of feature? Or maybe some feature to allow easy shell access (commands, env/vars, readline, etc...) from within python (the idea is to use python's interactive interpreter as a replacement to bash). EDIT: Maybe a comparative example would clarify. Let's say I have a list composed of dir/file: $ FILES=( build/project.rpm build/project.src.rpm ) And I want to do a really simple task: copy all files to dist/ AND install it in the system (it's part of a build process): Using bash: $ cp ${files[*]} dist/ $ cd dist && rpm -Uvh $(for f in ${files[*]}; do basename $f; done)) Using a "pythonic shell" approach (caution: this is imaginary code): $ cp [ os.path.join('dist', os.path.basename(file)) for file in FILES ] 'dist' Can you see the difference ? THAT is what i'm talking about. How can not exits a shell with these kind of stuff build-in yet? It's a real pain to handle lists in shell, even its being a so common task: list of files, list of PIDs, list of everything. And a really, really, important point: using syntax/tools/features everybody already knows: sh and python. IPython seams to be on a good direction, but it's bloated: if var name starts with '$', it does this, if '$$' it does that. It's syntax is not "natural", so many rules and "workarounds" ([ ln.upper() for ln in !ls ] -- syntax error)

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• using python 'with' statement with iterators?

  - by Stephen

  Hi, I'm using Python 2.5. I'm trying to use this 'with' statement. from __future__ import with_statement a = [] with open('exampletxt.txt','r') as f: while True: a.append(f.next().strip().split()) print a The contents of 'exampletxt.txt' are simple: a b In this case, I get the error: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/tmp/python-7036sVf.py", line 5, in <module> a.append(f.next().strip().split()) StopIteration And if I replace f.next() with f.read(), it seems to be caught in an infinite loop. I wonder if I have to write a decorator class that accepts the iterator object as an argument, and define an __exit__ method for it? I know it's more pythonic to use a for-loop for iterators, but I wanted to implement a while loop within a generator that's called by a for-loop... something like def g(f): while True: x = f.next() if test1(x): a = x elif test2(x): b = f.next() yield [a,x,b] a = [] with open(filename) as f: for x in g(f): a.append(x)

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• Download-from-PyPI-and-install script

  - by zubin71

  Hello, I have written a script which fetches a distribution, given the URL. After downloading the distribution, it compares the md5 hashes to verify that the file has been downloaded properly. This is how I do it. def download(package_name, url): import urllib2 downloader = urllib2.urlopen(url) package = downloader.read() package_file_path = os.path.join('/tmp', package_name) package_file = open(package_file_path, "w") package_file.write(package) package_file.close() I wonder if there is any better(more pythonic) way to do what I have done using the above code snippet. Also, once the package is downloaded this is what is done: def install_package(package_name): if package_name.endswith('.tar'): import tarfile tarfile.open('/tmp/' + package_name) tarfile.extract('/tmp') import shlex import subprocess installation_cmd = 'python %ssetup.py install' %('/tmp/'+package_name) subprocess.Popen(shlex.split(installation_cmd) As there are a number of imports for the install_package method, i wonder if there is a better way to do this. I`d love to have some constructive criticism and suggestions for improvement. Also, I have only implemented the install_package method for .tar files; would there be a better manner by which I could install .tar.gz and .zip files too without having to write seperate methods for each of these?

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• why does b'(and sometimes b' ') show up when I split some HTML source[Python]

  - by Oliver

  I'm fairly new to Python and programming in general. I have done a few tutorials and am about 2/3 through a pretty good book. That being said I've been trying to get more comfortable with Python and proggramming by just trying things in the std lib out. that being said I have recently run into a wierd quirk that I'm sure is the result of my own incorrect or un-"pythonic" use of the urllib module(with Python 3.2.2) import urllib.request HTML_source = urllib.request.urlopen(www.somelink.com).read() print(HTML_source) when this bit is run through the active interpreter it returns the HTML source of somelink, however it prefixes it with b' for example b'<HTML>\r\n<HEAD> (etc). . . . if I split the string into a list by whitespace it prefixes every item with the b' I'm not really trying to accomplish something specific just trying to familiarize myself with the std lib. I would like to know why this b' is getting prefixed also bonus -- Is there a better way to get HTML source WITHOUT using a third party module. I know all that jazz about not reinventing the wheel and what not but I'm trying to learn by "building my own tools" Thanks in Advance!

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• python 'with' statement

  - by Stephen

  Hi, I'm using Python 2.5. I'm trying to use this 'with' statement. from __future__ import with_statement a = [] with open('exampletxt.txt','r') as f: while True: a.append(f.next().strip().split()) print a The contents of 'exampletxt.txt' are simple: a b In this case, I get the error: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/tmp/python-7036sVf.py", line 5, in <module> a.append(f.next().strip().split()) StopIteration And if I replace f.next() with f.read(), it seems to be caught in an infinite loop. I wonder if I have to write a decorator class that accepts the iterator object as an argument, and define an __exit__ method for it? I know it's more pythonic to use a for-loop for iterators, but I wanted to implement a while loop within a generator that's called by a for-loop... something like def g(f): while True: x = f.next() if test(x): a = x elif test(x): b = f.next() yield [a,x,b] a = [] with open(filename) as f: for x in g(f): a.append(x)

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• deepcopy and python - tips to avoid using it?

  - by blackkettle

  Hi, I have a very simple python routine that involves cycling through a list of roughly 20,000 latitude,longitude coordinates and calculating the distance of each point to a reference point. def compute_nearest_points( lat, lon, nPoints=5 ): """Find the nearest N points, given the input coordinates.""" points = session.query(PointIndex).all() oldNearest = [] newNearest = [] for n in xrange(nPoints): oldNearest.append(PointDistance(None,None,None,99999.0,99999.0)) newNearest.append(obj2) #This is almost certainly an inappropriate use of deepcopy # but how SHOULD I be doing this?!?! for point in points: distance = compute_spherical_law_of_cosines( lat, lon, point.avg_lat, point.avg_lon ) k = 0 for p in oldNearest: if distance < p.distance: newNearest[k] = PointDistance( point.point, point.kana, point.english, point.avg_lat, point.avg_lon, distance=distance ) break else: newNearest[k] = deepcopy(oldNearest[k]) k += 1 for j in range(k,nPoints-1): newNearest[j+1] = deepcopy(oldNearest[j]) oldNearest = deepcopy(newNearest) #We're done, now print the result for point in oldNearest: print point.station, point.english, point.distance return I initially wrote this in C, using the exact same approach, and it works fine there, and is basically instantaneous for nPoints<=100. So I decided to port it to python because I wanted to use SqlAlchemy to do some other stuff. I first ported it without the deepcopy statements that now pepper the method, and this caused the results to be 'odd', or partially incorrect, because some of the points were just getting copied as references(I guess? I think?) -- but it was still pretty nearly as fast as the C version. Now with the deepcopy calls added, the routine does it's job correctly, but it has incurred an extreme performance penalty, and now takes several seconds to do the same job. This seems like a pretty common job, but I'm clearly not doing it the pythonic way. How should I be doing this so that I still get the correct results but don't have to include deepcopy everywhere?

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• Directly call distutils' or setuptools' setup() function with command name/options, without parsing

  - by Ryan B. Lynch

  I'd like to call Python's distutils' or setuptools' setup() function in a slightly unconventional way, but I'm not sure whether distutils is meant for this kind of usage. As an example, let's say I currently have a 'setup.py' file, which looks like this (lifted verbatim from the distutils docs--the setuptools usage is almost identical): from distutils.core import setup setup(name='Distutils', version='1.0', description='Python Distribution Utilities', author='Greg Ward', author_email='[email protected]', url='http://www.python.org/sigs/distutils-sig/', packages=['distutils', 'distutils.command'], ) Normally, to build just the .spec file for an RPM of this module, I could run python setup.py bdist_rpm --spec-only, which parses the command line and calls the 'bdist_rpm' code to handle the RPM-specific stuff. The .spec file ends up in './dist'. How can I change my setup() invocation so that it runs the 'bdist_rpm' command with the '--spec-only' option, WITHOUT parsing command-line parameters? Can I pass the command name and options as parameters to setup()? Or can I manually construct a command line, and pass that as a parameter, instead? NOTE: I already know that I could call the script in a separate process, with an actual command line, using os.system() or the subprocess module or something similar. I'm trying to avoid using any kind of external command invocations. I'm looking specifically for a solution that runs setup() in the current interpreter. For background, I'm converting some release-management shell scripts into a single Python program. One of the tasks is running 'setup.py' to generate a .spec file for further pre-release testing. Running 'setup.py' as an external command, with its own command line options, seems like an awkward method, and it complicates the rest of the program. I feel like there may be a more Pythonic way.

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