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  • How can I learn to effectively write Pythonic code?

    - by Matt Fenwick
    I'm tired of getting downvoted and/or semi-rude comments on my Python answers, saying things like "this isn't Pythonic" or "that's not the Python way of doing things". To clarify, I'm not tired of getting corrected and downvoted, and I'm not tired of being wrong: I'm tired of feeling like there's a whole field of Python that I know nothing about, and seems to be implicit knowledge of experienced Python programmers. Doing a google search for "Pythonic" reveals a wide range of interpretations. The wikipedia page says: A common neologism in the Python community is pythonic, which can have a wide range of meanings related to program style. To say that code is pythonic is to say that it uses Python idioms well, that it is natural or shows fluency in the language. Likewise, to say of an interface or language feature that it is pythonic is to say that it works well with Python idioms, that its use meshes well with the rest of the language. It also discusses the term "unpythonic": In contrast, a mark of unpythonic code is that it attempts to write C++ (or Lisp, Perl, or Java) code in Python—that is, provides a rough transcription rather than an idiomatic translation of forms from another language. The concept of pythonicity is tightly bound to Python's minimalist philosophy of readability and avoiding the "there's more than one way to do it" approach. Unreadable code or incomprehensible idioms are unpythonic. I suspect one way to learn the Pythonic way is just to program in Python a whole bunch. But I bet I could write a bunch of crap and not improve that much without some guidance, whereas a good resource might speed up the learning process significantly. PEP 8 might be exactly what I'm looking for, or maybe not. I'm not sure; on the one hand it covers a lot of ground, but on the other hand, I feel like it's more suited as a reference for knowledgeable programmers than a tutorial for fresh 'uns. How do I get my foot in the Pythonic/Python way of doing things door?

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  • Pythonic use of the isinstance function?

    - by Pace
    Whenever I find myself wanting to use the isinstance() function I usually know that I'm doing something wrong and end up changing my ways. However, in this case I think I have a valid use for it. I will use shapes to illustrate my point although I am not actually working with shapes. I am parsing XML configuration files that look like the following: <square> <width>7</width> </square> <rectangle> <width>5</width> <height>7</height> </rectangle> <circle> <radius>4</radius> </circle> For each element I create an instance of the Shape class and build up a list of Shape objects in a class called the ShapeContainer. Different parts of the rest of my application need to refer to the ShapeContainer to get certain shapes. Depending on what the code is doing it might need just rectangles, or it might operate on all quadrangles, or it might operate on all shapes. I have created the following function in the ShapeContainer class (the actual function uses a list comprehension but I have expanded it here for readability): def locate(self, shapeClass): result = [] for shape in self.__shapes: if isinstance(shape,shapeClass): result.append(shape) return result Is this a valid use of the isinstance function? Is there another way I can do this which might be more pythonic?

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  • Questioning pythonic type checking

    - by Pace
    I've seen countless times the following approach suggested for "taking in a collection of objects and doing X if X is a Y and ignoring the object otherwise" def quackAllDucks(ducks): for duck in ducks: try: duck.quack("QUACK") except AttributeError: #Not a duck, can't quack, don't worry about it pass The alternative implementation below always gets flak for the performance hit caused by type checking def quackAllDucks(ducks): for duck in ducks: if hasattr(duck,"quack"): duck.quack("QUACK") However, it seems to me that in 99% of scenarios you would want to use the second solution because of the following: If the user gets the parameters wrong then they will not be treated like a duck and there will be no indication. A lot of time will be wasted debugging why there is no quacking going on until the user finally realizes his silly mistake. The second solution would throw a stack trace as soon the user tried to quack. If the user has any bugs in their quack() method which cause an AttributeError then those bugs will be silently swallowed. Once again time will be wasted digging for the bug when the second solution would simply give a stack trace showing the immediate issue. In fact, it seems to me that the only time you would ever want to use the first method is when: The block of code in question is in an extremely performance critical section of your application. Following the principal of "avoid premature optimization" you would only realize this of course, after you had implemented the safer approach and found it to be a bottleneck. There are many types of quacking objects out there and you are only interested in quacking objects that quack with a very specific set of arguments (this seems to be a very rare case to me). Given this, why is it that so many people prefer the first approach over the second approach? What is it that I am missing? Also, I realize there are other solutions (such as using abcs) but these are the two solutions I seem to see most often for the basic case.

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  • Which of these is pythonic? and Pythonic vs. Speed

    - by Kashyap Nadig
    Hi! I'm new to python and just wrote this module level function: def _interval(patt): """ Converts a string pattern of the form '1y 42d 14h56m' to a timedelta object. y - years (365 days), M - months (30 days), w - weeks, d - days, h - hours, m - minutes, s - seconds""" m = _re.findall(r'([+-]?\d*(?:\.\d+)?)([yMwdhms])', patt) args = {'weeks': 0.0, 'days': 0.0, 'hours': 0.0, 'minutes': 0.0, 'seconds': 0.0} for (n,q) in m: if q=='y': args['days'] += float(n)*365 elif q=='M': args['days'] += float(n)*30 elif q=='w': args['weeks'] += float(n) elif q=='d': args['days'] += float(n) elif q=='h': args['hours'] += float(n) elif q=='m': args['minutes'] += float(n) elif q=='s': args['seconds'] += float(n) return _dt.timedelta(**args) My issue is with the for loop here i.e the long if elif block, and was wondering if there is a more pythonic way of doing it. So I re-wrote the function as: def _interval2(patt): m = _re.findall(r'([+-]?\d*(?:\.\d+)?)([yMwdhms])', patt) args = {'weeks': 0.0, 'days': 0.0, 'hours': 0.0, 'minutes': 0.0, 'seconds': 0.0} argsmap = {'y': ('days', lambda x: float(x)*365), 'M': ('days', lambda x: float(x)*30), 'w': ('weeks', lambda x: float(x)), 'd': ('days', lambda x: float(x)), 'h': ('hours', lambda x: float(x)), 'm': ('minutes', lambda x: float(x)), 's': ('seconds', lambda x: float(x))} for (n,q) in m: args[argsmap[q][0]] += argsmap[q][1](n) return _dt.timedelta(**args) I tested the execution times of both the codes using timeit module and found that the second one took about 5-6 seconds longer (for the default number of repeats). So my question is: 1. Which code is considered more pythonic? 2. Is there still a more pythonic was of writing this function? 3. What about the trade-offs between pythonicity and other aspects (like speed in this case) of programming? p.s. I kinda have an OCD for elegant code. EDITED _interval2 after seeing this answer: argsmap = {'y': ('days', 365), 'M': ('days', 30), 'w': ('weeks', 1), 'd': ('days', 1), 'h': ('hours', 1), 'm': ('minutes', 1), 's': ('seconds', 1)} for (n,q) in m: args[argsmap[q][0]] += float(n)*argsmap[q][1]

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  • More pythonic way to iterate

    - by fmark
    I am using a module that is part of a commercial software API. The good news is there is a python module - the bad news is that its pretty unpythonic. To iterate over rows, the follwoing syntax is used: cursor = gp.getcursor(table) row = cursor.Next() while row: #do something with row row = cursor.next() What is the most pythonic way to deal with this situation? I have considered creating a first class function/generator and wrapping calls to a for loop in it: def cursor_iterator(cursor): row = cursor.Next() while row: yield row row = cursor.next() [...] cursor = gp.getcursor(table) for row in cursor_iterator(cursor): # do something with row This is an improvement, but feels a little clumsy. Is there a more pythonic approach? Should I create a wrapper class around the table type?

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  • What defines "pythonian" or "pythonic"?

    - by Thomas Owens
    I want to begin to learn Python, and I've seen that phrase come up here before, but I don't know exactly what it means. I've read some websites on Python scripting, but I don't recall ever seeing that (but I could have just glanced over it). What exactly makes something "pythonian" or "pythonic"?

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  • pythonic way of selecing a random value that satisfies a certain predicate

    - by MK
    Suppose I have a list of elements and I want to randomly select an element from the list that satisfies a predicate. What is the pythonic way of doing this? I currently do a comprehension followed by a random.choice() but that is unnecessarily inefficient : intlist = [1,2,3,4,5,6,7,8,9] evenlist = [ i for i in intlist if i % 2 == 0 ] randomeven = random.choice(evenlist) Thanks!

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  • Why does the word "Pythonic" exist?

    - by Billy ONeal
    Honestly, I hate the word "Pythonic" -- it's used as a simple synonym of "good" in many circles, and I think that's pretentious. Those who use it are silently saying that good code cannot be written in a language other than Python. Not saying Python is a bad language, but it's certainly not the "end all be all language to solve ALL of everyone's problems forever!" (Because that language does not exist). What it seems like people who use this word really mean is "idiomatic" rather than "Pythonic" -- and of course the word "idiomatic" already exists. Therefore I wonder: Why does the word "Pythonic" exist?

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  • pythonic way to associate list elements with their indices

    - by Dragan Chupacabrovic
    Hello Everybody, I have a list of values and I want to put them in a dictionary that would map each value to it's index. I can do it this way: >>> t = (5,6,7) >>> d = dict(zip(t, range(len(t)))) >>> d {5: 0, 6: 1, 7: 2} this is not bad, but I'm looking for something more elegant. I've come across the following, but it does the opposite of what I need: >>> d = dict(enumerate(t)) >>> d {0: 5, 1: 6, 2: 7} Please share your solutions, Thank you

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  • What's the pythonic way of declaring variables?

    - by the_drow
    Usually declaring variables on assignment is considered a best practice in VBScript or JavaScript , for example, although it is allowed. Why does Python force you to create the variable only when you use it? Since Python is case sensitive can't it cause bugs because you misspelled a variable's name? How would you avoid such a situation?

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  • How to Convert multiple sets of Data going from left to right to top to bottom the Pythonic way?

    - by ThinkCode
    Following is a sample of sets of contacts for each company going from left to right. ID Company ContactFirst1 ContactLast1 Title1 Email1 ContactFirst2 ContactLast2 Title2 Email2 1 ABC John Doe CEO [email protected] Steve Bern CIO [email protected] How do I get them to go top to bottom as shown? ID Company Contactfirst ContactLast Title Email 1 ABC John Doe CEO [email protected] 1 ABC Steve Bern CIO [email protected] I am hoping there is a Pythonic way of solving this task. Any pointers or samples are really appreciated! p.s : In the actual file, there are 10 sets of contacts going from left to right and there are few thousand such records. It is a CSV file and I loaded into MySQL to manipulate the data.

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  • Is it Pythonic to have a class keep track of its instances?

    - by Lightbreeze
    Take the following code snippet class Missile: instances = [] def __init__(self): Missile.instances.append(self) Now take the code: class Hero(): ... def fire(self): Missile() When the hero fires, a missile needs to be created and appended to the main list. Thus the hero object needs to reference the list when it fires. Here are a few solutions, although I'm sure there are others: Make the list a global, Use a class variable (as above), or Have the hero object hold a reference to the list. I didn't post this on gamedev because my question is actually more general: Is the previous code considered okay? Given a situation like this, is there a more Pythonic solution?

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  • Abstracting away the type of a property

    - by L. De Leo
    In Python luckily most of the times you don't have to write getters and setters to get access to class properties. That said sometimes you'll have to remember that a certain property is a list or whatnot and a property would save you there by abstracting the type and providing a setter to add something to such list for example rather than exposing the list directly. Where do you draw the line between exposing the type directly or wrapping its access in a property? What's the general "pythonic" advice?

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  • Python Forgiveness vs. Permission and Duck Typing

    - by darkfeline
    In Python, I often hear that it is better to "beg forgiveness" (exception catching) instead of "ask permission" (type/condition checking). In regards to enforcing duck typing in Python, is this try: x = foo.bar except AttributeError: pass else: do(x) better or worse than if hasattr(foo, "bar"): do(foo.bar) else: pass in terms of performance, readability, "pythonic", or some other important factor?

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  • a more pythonic way to express conditionally bounded loop?

    - by msw
    I've got a loop that wants to execute to exhaustion or until some user specified limit is reached. I've got a construct that looks bad yet I can't seem to find a more elegant way to express it; is there one? def ello_bruce(limit=None): for i in xrange(10**5): if predicate(i): if not limit is None: limit -= 1 if limit <= 0: break def predicate(i): # lengthy computation return True Holy nesting! There has to be a better way. For purposes of a working example, xrange is used where I normally have an iterator of finite but unknown length (and predicate sometimes returns False).

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  • Django Models / SQLAlchemy are bloated! Any truly Pythonic DB models out there?

    - by Luke Stanley
    "Make things as simple as possible, but no simpler." Can we find the solution/s that fix the Python database world? from someAmazingDB import * class Task (model): title = '' isDone = False db.taskList = [] #or db.taskList = expandableTypeCollection(Task) #not sure what this syntax would be db['taskList'].append(Task(title='Beat old sql interfaces',done=False)) db.taskList.append(Task('Illustrate different syntax modes',True)) #at this point it should autosave #we should be able to reload the console and access like: >> from someAmazingDB import * >> print 'Done tasks:' >> for task in db.taskList: >> if task.done: >> print task 'Illustrate different syntax modes' I'm a fan of Python, webPy and Cherry Py, and KISS in general. We're talking automatic Python to SQL type translation or NoSQL. We don't have to totally be SQL compatible! Just a scalable subset or ignore it! Re:model changes, it's ok to ask the developer when they try to change it or have a set of sensible defaults. Here is the challenge: The above code should work with very little modification or thinking required. Why must we put up with compromise when we know better? It's 2010, we should be able to code scalable, simple databases in our sleep. If you think this is important, please upvote!

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  • How do I convert tuple of tuples to list in one line (pythonic)?

    - by ThinkCode
    query = 'select mydata from mytable' cursor.execute(query) myoutput = cursor.fetchall() print myoutput (('aa',), ('bb',), ('cc',)) Why is it (cursor.fetchall) returning a tuple of tuples instead of a tuple since my query is asking for only one column of data? What is the best way of converting it to ['aa', 'bb', 'cc'] ? I can do something like this : mylist = [] myoutput = list(myoutput) for each in myoutput: mylist.append(each[0]) I am sure this isn't the best way of doing it. Please enlighten me!

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  • What's wrong with relative imports in Python?

    - by Oddthinking
    I recently upgraded versions of pylint, a popular Python style-checker. It has gone ballistic throughout my code, pointing out places where I import modules in the same package, without specifying the full package path. The new error message is W0403. W0403: Relative import %r, should be %r Used when an import relative to the package directory is detected. Example For example, if my packages are structured like this: /cake /__init__.py /icing.py /sponge.py /drink and in the sponge package I write: import icing instead of import cake.icing I will get this error. While I understand that not all Pylint messages are of equal importance, and I am not afraid to dismiss them, I don't understand why such a practice is considered a poor idea. I was hoping someone could explain the pitfalls, so I could improve my coding style rather than (as I currently plan to do) turning off this apparently spurious warning.

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  • Pairs from single list

    - by Apalala
    Often enough, I've found the need to process a list by pairs. I was wondering which would be the pythonic and efficient way to do it, and found this on Google: pairs = zip(t[::2], t[1::2]) I thought that was pythonic enough, but after a recent discussion involving idioms versus efficiency, I decided to do some tests: import time from itertools import islice, izip def pairs_1(t): return zip(t[::2], t[1::2]) def pairs_2(t): return izip(t[::2], t[1::2]) def pairs_3(t): return izip(islice(t,None,None,2), islice(t,1,None,2)) A = range(10000) B = xrange(len(A)) def pairs_4(t): # ignore value of t! t = B return izip(islice(t,None,None,2), islice(t,1,None,2)) for f in pairs_1, pairs_2, pairs_3, pairs_4: # time the pairing s = time.time() for i in range(1000): p = f(A) t1 = time.time() - s # time using the pairs s = time.time() for i in range(1000): p = f(A) for a, b in p: pass t2 = time.time() - s print t1, t2, t2-t1 These were the results on my computer: 1.48668909073 2.63187503815 1.14518594742 0.105381965637 1.35109519958 1.24571323395 0.00257992744446 1.46182489395 1.45924496651 0.00251388549805 1.70076990128 1.69825601578 If I'm interpreting them correctly, that should mean that the implementation of lists, list indexing, and list slicing in Python is very efficient. It's a result both comforting and unexpected. Is there another, "better" way of traversing a list in pairs? Note that if the list has an odd number of elements then the last one will not be in any of the pairs. Which would be the right way to ensure that all elements are included? I added these two suggestions from the answers to the tests: def pairwise(t): it = iter(t) return izip(it, it) def chunkwise(t, size=2): it = iter(t) return izip(*[it]*size) These are the results: 0.00159502029419 1.25745987892 1.25586485863 0.00222492218018 1.23795199394 1.23572707176 Results so far Most pythonic and very efficient: pairs = izip(t[::2], t[1::2]) Most efficient and very pythonic: pairs = izip(*[iter(t)]*2) It took me a moment to grok that the first answer uses two iterators while the second uses a single one. To deal with sequences with an odd number of elements, the suggestion has been to augment the original sequence adding one element (None) that gets paired with the previous last element, something that can be achieved with itertools.izip_longest().

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  • Pythonic reading from config files

    - by Adam Matan
    Hi, I have a python class which reads a config file using ConfigParser: Config file: [geography] Xmin=6.6 Xmax=18.6 Ymin=36.6 YMax=47.1 Python code: class Slicer: def __init__(self, config_file_name): config = ConfigParser.ConfigParser() config.read(config_file_name) # Rad the lines from the file self.x_min = config.getfloat('geography', 'xmin') self.x_max = config.getfloat('geography', 'xmax') self.y_min = config.getfloat('geography', 'ymin') self.y_max = config.getfloat('geography', 'ymax') I feel that the last four lines are repetitive, and should somehow be compressed to one Pythonic line that would create a self.item variable for each item in the section. Any ideas? Adam

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  • Pythonic / itertools way to go through a dict?

    - by dmd
    def reportCSV(t): ret = '' for ev in t: for p in t[ev]: for w in t[ev][p]: ret += ','.join((ev, p, w, t[ev][p][w])) + '\n' return ret What is a more pythonic way to do this, e.g. using itertools or the like? In this case I'm just writing it out to a CSV file. t is a dict t[ev] is a dict t[ev][p] is a dict t[ev][p][w] is a float I'm not sure how I'd use itertools.product in this case.

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  • Assigning IDs to instances of a class (Pythonic)

    - by mellort
    I want to have each instance of some class have a unique integer identifier based on the order that I create them, starting with (say) 0. In Java, I could do this with a static class variable. I know I can emulate the same sort of behavior with Python, but what would be the most 'Pythonic' way to do this? Thanks

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