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  • What is the difference between type and type.__new__ in python?

    - by Jason Baker
    I was writing a metaclass and accidentally did it like this: class MetaCls(type): def __new__(cls, name, bases, dict): return type(name, bases, dict) ...instead of like this: class MetaCls(type): def __new__(cls, name, bases, dict): return type.__new__(cls, name, bases, dict) What exactly is the difference between these two metaclasses? And more specifically, what caused the first one to not work properly (some classes weren't called into by the metaclass)?

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  • Re-order list in Python to ensure it starts with check values.

    - by S_Swede
    Dear all, I'm reading in serial data using Pyserial, to populate a list of 17 values (1byte each) at a sampling rate of 256Hz. The bytes I ultimately want to use are the 5th to 8th in the list. Providing no bytes are dropped, the first two values of the stream are always the same ('165','90'). I'm getting quite a few dropped values though, and my list values are shifting, so when I read the 5th-8th bytes, they aren't the correct values. I've partially combatted this by ensuring that before the wanted segement is captured, the first few values are checked against what they should be (i.e. if mylist[0]==165 &....). This is crude but ok since the chances of these two values appearing adjacent to each other in the list elsewhere is small. The problem is that this means as soon as the bytes shift, I'm losing a load of values, until it eventually realigns. My question is: what code can I use to either: a) Force the list to realign once it has been detected that it no longer starts with 165,90. (elif....). b) Detect where '165' & '90' are (next to each other) in the list and extract the values I want in relation to their position (next but one, onwards). Thanks in advance S_S Just noticed from the related Qs that I could use mylist.append(mylist.pop(0)) multiple times until they are in the right place. Is there a better way that anyone can suggest?

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  • How do I add a method with a decorator to a class in python?

    - by Timmy
    How do I add a method with a decorator to a class? I tried def add_decorator( cls ): @dec def update(self): pass cls.update = update usage add_decorator( MyClass ) MyClass.update() but MyClass.update does not have the decorator @dec did not apply to update I'm trying to use this with orm.reconstructor in sqlalchemy.

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  • How do I modify this download function in Python?

    - by TIMEX
    Right now, it's iffy. Gzip, images, sometimes it doesn't work. How do I modify this download function so that it can work with anything? (Regardless of gzip or any header?) How do I automatically "Detect" if it's gzip? I don't want to always pass True/False, like I do right now. def download(source_url, g = False, correct_url = True): try: socket.setdefaulttimeout(10) agents = ['Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0)','Mozilla/4.0 (compatible; MSIE 7.0b; Windows NT 5.1)','Microsoft Internet Explorer/4.0b1 (Windows 95)','Opera/8.00 (Windows NT 5.1; U; en)'] ree = urllib2.Request(source_url) ree.add_header('User-Agent',random.choice(agents)) ree.add_header('Accept-encoding', 'gzip') opener = urllib2.build_opener() h = opener.open(ree).read() if g: compressedstream = StringIO(h) gzipper = gzip.GzipFile(fileobj=compressedstream) data = gzipper.read() return data else: return h except Exception, e: return ""

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  • Should Python import statements always be at the top of a module?

    - by Adam J. Forster
    PEP 08 states: Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants. However if the class/method/function that I am importing is only used in rare cases, surely it is more efficient to do the import when it is needed? Isn't this: class SomeClass(object): def not_often_called(self) from datetime import datetime self.datetime = datetime.now() more efficient than this? from datetime import datetime class SomeClass(object): def not_often_called(self) self.datetime = datetime.now()

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  • In Python, can I single line a for loop over iterator with an IF filter?

    - by Tal Weiss
    Silly question: I have a simple for loop followed by a simple if statement: for airport in airports: if airport.is_important: and I was wondering if I can write this as a single line somehow. So, yes, I can do this: for airport in (airport for airport in airports if airport.is_important): but it reads so silly and redundant ("for airport in airport for airport in airports..."). Is there a better way?

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  • How do you iterate through each email in your inbox using python?

    - by djblue2009
    I'm completely new to programming and I'm trying to build an autorespoder to send a msg to a specific email address. Using an if statement, I can check if there is an email from a certain address in the inbox and I can send an email, but if there are multiple emails from that address, how can I make a for loop to send an email for every email from that specific address. I tried to do use this as a loop: for M.search(None, 'From', address) in M.select(): but I get the error: "can't assign to function call" on that line

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  • Python: Why Does a Method Behave Differently with an Added Parameter?

    - by SteveStifler
    I have a method in a Pygame Sprite subclass, defined as such: def walk(self): """move across screen""" displacement = self.rect.move((self.move, 0)) if self.rect.left < self.area.left or self.rect.right > self.area.right: self.move = -self.move displacement = self.rect.move((self.move, 0)) self.rect = displacement I modified it, adding a parameter speed_x, and now the program is broken. def walk(self, speed_x): """move across screen""" displacement = self.rect.move((speed_x, 0)) if self.rect.left < self.area.left or self.rect.right > self.area.right: speed_x = -speed_x displacement = self.rect.move((speed_x, 0)) self.rect = displacement Before I called the method like this: def update(self): self.walk() Now I do: def update(self): self.walk(self.move) Why doesn't this work?

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  • How to implement python to find value between xml tags?

    - by Harshit Sharma
    I am using google site to retrieve weather information , I want to find values between XML tags. Following code give me weather condition of a city , but I am unable to obtain other parameters such as temperature and if possible explain working of split function implied in the code: import urllib def getWeather(city): #create google weather api url url = "http://www.google.com/ig/api?weather=" + urllib.quote(city) try: # open google weather api url f = urllib.urlopen(url) except: # if there was an error opening the url, return return "Error opening url" # read contents to a string s = f.read() # extract weather condition data from xml string weather = s.split("<current_conditions><condition data=\"")[-1].split("\"")[0] # if there was an error getting the condition, the city is invalid if weather == "<?xml version=": return "Invalid city" #return the weather condition return weather def main(): while True: city = raw_input("Give me a city: ") weather = getWeather(city) print(weather) if __name__ == "__main__": main() Thank You

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  • Python fCGI + sqlAlchemy = malformed header from script. Bad header=FROM tags : index.py

    - by crgwbr
    I'm writing an Fast-CGI application that makes use of sqlAlchemy & MySQL for persistent data storage. I have no problem connecting to the DB and setting up ORM (so that tables get mapped to classes); I can even add data to tables (in memory). But, as soon as I query the DB (and push any changes from memory to storage) I get a 500 Internal Server Error and my error.log records malformed header from script. Bad header=FROM tags : index.py, when tags is the table name. Any idea what could be causing this? Also, I don't think it matters, but its a Linux development server talking to an off-site (across the country) MySQL server.

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  • How to determine what user and group a Python script is running as?

    - by Chirael
    I have a CGI script that is getting an "IOError: [Errno 13] Permission denied" error in the stack trace in the web server's error log. As part of debugging this problem, I'd like to add a little bit of code to the script to print the user and (especially) group that the script is running as, into the error log (presumably STDERR). I know I can just print the values to sys.stderr, but how do I figure out what user and group the script is running as? (I'm particularly interested in the group, so the $USER environment variable won't help; the CGI script has the setgid bit set so it should be running as group "list" instead of the web server's "www-data" - but I need code to see if that's actually happening.)

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  • Python Pickle: what can cause stack index out of range error?

    - by Rosarch
    I'm getting this error: File "C:\Python26\lib\pickle.py", line 1374, in loads return Unpickler(file).load() File "C:\Python26\lib\pickle.py", line 858, in load dispatch[key](self) File "C:\Python26\lib\pickle.py", line 1075, in load_obj k = self.marker() File "C:\Python26\lib\pickle.py", line 874, in marker while stack[k] is not mark: k = k-1 IndexError: list index out of range Why could this be happening?

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  • Retrieving information with Python's urllib from a page that is done via __doPostBack()?

    - by Omar
    I'm trying to parse a page that has different sections that are loaded with a Javascript __doPostBack() function. An example of a link is: javascript:__doPostBack('ctl00$cphMain$ucOemSchPicker$dlSch$ctl03$btnSch','') As soon as this is clicked, the browser doesn't fetch a new URL but a section of webpage is updated to reflect new information. What would I pass into a urllib function to complete the operation?

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  • How should I write this string-prefix check so that it's idiomatic Python?

    - by Kevin Stargel
    I have a couple of lists of items: specials = ['apple', 'banana', 'cherry', ...] smoothies = ['banana-apple', 'mocha mango', ...] I want to make a new list, special_smoothies, consisting of elements in smoothies that start with the elements in specials. However, if specials is blank, special_smoothies should be identical to smoothies. What's the most Pythonic way to do this? Is there a way to do this without a separate conditional check on whether specials is blank?

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  • how to set a fixed color bar for pcolor in python matplotlib?

    - by user248237
    I am using pcolor with a custom color map to plot a matrix of values. I set my color map so that low values are white and high values are red, as shown below. All of my matrices have values between 0 and 20 (inclusive) and I'd like 20 to always be pure red and 0 to always be pure white, even if the matrix has values that don't span the entire range. For example, if my matrix only has values between 2 and 7, I don't want it to plot 2 as white and 7 as red, but rather color it as if the range is still 0 to 20. How can I do this? I tried using the "ticks=" option of colorbar but it did not work. Here is my current code (assume "my_matrix" contains the values to be plotted): cdict = {'red': ((0.0, 1.0, 1.0), (0.5, 1.0, 1.0), (1.0, 1.0, 1.0)), 'green': ((0.0, 1.0, 1.0), (0.5, 1.0, 1.0), (1.0, 0.0, 0.0)), 'blue': ((0.0, 1.0, 1.0), (0.5, 1.0, 1.0), (1.0, 0.0, 0.0))} my_cmap = matplotlib.colors.LinearSegmentedColormap('my_colormap', cdict, 256) colored_matrix = plt.pcolor(my_matrix, cmap=my_cmap) plt.colorbar(colored_matrix, ticks=[0, 5, 10, 15, 20]) any idea how I can fix this to get the right result? thanks very much.

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  • Is it possible to create a python iterator over pre-defined mutable data?

    - by Wilduck
    I might be doing this wrong, if I am, let me know, but I'm curious if the following is possible: I have a class that holds a number of dictionaries, each of which pairs names to a different set of objects of a given class. For example: items = {"ball" : ItemInstance1, "sword" : ItemInstance2} people = {"Jerry" : PersonInstance1, "Bob" : PersonInstance2, "Jill" : PersonInstance3} My class would then hold the current items and people that are availible, and these would be subject to change as the state changes: Class State: def __init__(self, items, people): self.items = items self.people = people I would like to define a iter() and next() method such that it iterates through all of the values in its attributes. My first question is whether or not this is possible. If it is, will it be able to support a situation as follows: I define items and people as above then: state = State(items, people) for names, thing in state: print name + " is " + thing.color items[cheese] = ItemInstance3 for names, thing in state: print name + " weighs " + thing.weight While I feel like this would be usefull in the code I have, I don't know if it's either possible or the right approach. Everything I've read about user defined iterators has suggested that each instance of them is one use only.

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  • fastest way to search through this data object? (python)

    - by victor
    I have a data object that looks like this: { 'node-16': { 'tags': ['cuda'], 'localNodes': [ { 'name': 'nC', 'consumesFrom': ['nA', 'nB'], 'classType': 'VectorAdder.VectorAdder' }, { 'name': 'nB', 'consumesFrom': None, 'classType': 'RandomVector' } ] }, 'node-17': { 'tags': ['boring'], 'localNodes': [ { 'name': 'nA', 'consumesFrom': None, 'classType': 'RandomVector' } ] } } Notice that node nA is a producer for nC. What's the fastest way to find out if a given localNode is a producer for another localnode in the data structure (and not within the same list)? For example, I would like to know that nA (node-17) produces for nC (exists on node-16). But I don't need to know that nB produces for nC, since they exist in the same localNodes list.

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