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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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  • 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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  • Possible to capture the returned value from a Python list comprehension for use a condition?

    - by Joe
    I want to construct a value in a list comprehension, but also filter on that value. For example: [expensive_function(x) for x in generator where expensive_function(x) < 5] I want to avoid calling expensive_function twice per iteration. The generator may return an infinite series, and list comprehensions aren't lazily evaluated. So this wouldn't work: [y in [expensive_function(x) for x in generator where expensive_function(x)] where y < 5] I could write this another way, but it feels right for a list comprehension and I'm sure this is a common usage pattern (possible or not!).

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  • Is it possible to use 'else' in a python list comprehension?

    - by Josh
    Here is the code I was trying to turn into a list comprehension: table = '' for index in xrange(256): if index in ords_to_keep: table += chr(index) else: table += replace_with Is there a way to add the else statement to this comprehension? table = ''.join(chr(index) for index in xrange(15) if index in ords_to_keep) Also, would I be right in concluding that a list comprehension is the most efficient way to do this?

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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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  • What's the life-time of a thread-local value in Python?

    - by Carlos Valiente
    import threading mydata = threading.local() def run(): # When will the garbage collector be able to destroy the object created # here? After the thread exits from ``run()``? After ``join()`` is called? # Or will it survive the thread in which it was created, and live until # ``mydata`` is garbage-collected? mydata.foo = object() t = threading.Thread(target=run) t.start() t.join()

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  • How can I specify a relative path in a Python logging config file?

    - by ClaudioA
    I've the following file to config logging: [loggers] keys=root [handlers] keys = root [formatters] keys = generic # Loggers [logger_root] level = DEBUG handlers = root # Handlers [handler_root] class = handlers.RotatingFileHandler args = ("test.log", "maxBytes=1*1024*1024", "backupCount=10") level = NOTSET formatter = generic # Formatters [formatter_generic] format = %(asctime)s,%(msecs)03d %(levelname)-5.5s [%(name)s] %(message)s datefmt = %Y-%m-%d %H:%M:%S In Development this works great, but when I deploy the application test.log is trying to be written in a path in which I don't have the necessary permission. So my question is, How can I do to specify a relative path in this configuration file.

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  • Where is a good place/way to store Windows config files for Python scripts?

    - by thornomad
    I have a script/program I am working on that requires a configuration file (I am using ConfigParser). On linux, I will default to store these variables in ~/.myscript using the os.getenv('HOME') function. With Windows, I know I can use os.getenv('USERPROFILE') to find the User's "home" directory, however, is it a good idea to save a hidden file that way (ie, with the name .myscript)? I don't use Windows, obviously, but wanted to be smart about it for those who do. Is there a standard place/way to store these config variables on Windows?

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  • how to use @ in python.. and the @property and the @classmethods

    - by zjm1126
    this is my code: def a(): print 'sss' @a() def b(): print 'aaa' b() and the Traceback is: sss Traceback (most recent call last): File "D:\zjm_code\a.py", line 8, in <module> @a() TypeError: 'NoneType' object is not callable so how to use the '@' thanks updated class a: @property def b(x): print 'sss' aa=a() print aa.b it print : sss None how to use @property thanks updated2 and the classmethods: class a: @classmethods def b(x): print 'sss' aa=a() print aa.b the Traceback is : Traceback (most recent call last): File "D:\zjm_code\a.py", line 5, in <module> class a: File "D:\zjm_code\a.py", line 6, in a @classmethods NameError: name 'classmethods' is not defined

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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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  • Python: Find X to Y in a list of strings.

    - by TheLizardKing
    I have a list of maybe a 100 or so elements that is actually an email with each line as an element. The list is slightly variable because lines that have a \n in them are put in a separate element so I can't simply slice using fixed values. I essentially need a variable start and stop phrase (needs to be a partial search as well because one of my start phrases might actually be Total Cost: $13.43 so I would just use Total Cost:.) Same thing with the end phrase. I also do not wish to include the start/stop phrases in the returned list. In summary: email = ['apples','bananas','cats','dogs','elephants','fish','gee'] start = 'ban' stop = 'ele' the magic here new_email = ['cats','dogs'] NOTES While not perfect formatting of the email, it is fairly consistent so there is a slim chance a start/stop phrase will occur more than once. There are also no blank elements.

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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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  • What are the elegant ways to do MixIns in Python?

    - by Slava Vishnyakov
    I need to find an elegant way to do 2 kinds of MixIns. First: class A(object): def method1(self): do_something() Now, a MixInClass should make method1 do this: do_other() - A.method1() - do_smth_else() - i.e. basically "wrap" the older function. I'm pretty sure there must exist a good solution to this. Second: class B(object): def method1(self): do_something() do_more() In this case, I want MixInClass2 to be able to inject itself between do_something() and do_more(), i.e.: do_something() - MixIn.method1 - do_more(). I understand that probably this would require modifying class B - that's ok, just looking for simplest ways to achieve this. These are pretty trivial problems and I actually solved them, but my solution is tainted. Fisrt one by using self._old_method1 = self.method1(); self.method1() = self._new_method1(); and writing _new_method1() that calls to _old_method1(). Problem: multiple MixIns will all rename to _old_method1 and it is inelegant. Second MixIn one was solved by creating a dummy method call_mixin(self): pass and injecting it between calls and defining self.call_mixin(). Again inelegant and will break on multiple MixIns.. Any ideas?

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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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