Mailman 3 September 2012 - Python-ideas

itertools.chunks(iterable, size, fill=None)
by anatoly techtonik 03 Sep '12

03 Sep '12

Now that Python 3 is all about iterators (which is a user killer feature for Python according to StackOverflow - http://stackoverflow.com/questions/tagged/python) would it be nice to introduce more first class functions to work with them? One function to be exact to split string into chunks. itertools.chunks(iterable, size, fill=None) Which is the 33th most voted Python question on SO - http://stackoverflow.com/questions/312443/how-do-you-split-a-list-into-even… P.S. CC'ing to python-dev@ to notify about the thread in python-ideas.

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sequence.apply(function)
by anatoly techtonik 02 Sep '12

02 Sep '12

Idea: Apply function to every element of a sequence and return new sequence. It's more pythonic than map(), because clearly works only as a list method. -- anatoly t.

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Verbose traceback formatting
by Mike Graham 01 Sep '12

01 Sep '12

It's possible to give a lot more on error than the default traceback gives you. I propose that Python should ship a more verbose formatter and a command line switch to use it. Here's an example of IPython's verbose formatter. I wrote a buggy program: > def f(a): > x = a * 4 > y = a - 4 > return x / y > > def main(): > for i in xrange(100): > f(i) > > main() and then ran it in IPython with verbose tracebacks and got the following output: > ZeroDivisionError Traceback (most recent call last) > > /home/mike/foo.py in <module>() > 8 f(i) > 9 > ---> 10 main() > global main = <function main at 0x10bd7d0> > 11 > 12 > > /home/mike/foo.py in main() > 6 def main(): > 7 for i in xrange(100): > ----> 8 f(i) > global f = <function f at 0x10bd758> > i = 4 > 9 > 10 main() > > /home/mike/foo.py in f(a=4) > 2 x = a * 4 > 3 y = a - 4 > ----> 4 return x / y > x = 16 > y = 0 > 5 > 6 def main(): > > ZeroDivisionError: integer division or modulo by zero This is very handy! The reprs of all locals are input so I can see what the values of a, x, and y were when I had my error and there are a few lines of code on either side of the line that matters to help me get oriented. The former feature is the more powerful one, although enabling this by default is a bad idea; (first and foremost, this can be a security hazard). I can't count how many trips into pdb this would have saved me. I think having this feature be part of Python itself would be very helpful to new learners and to those helping them. I constantly deal with learners seeking help who are unable to clearly provide the actual values and types of the objects in the code they're having trouble with; it would be nice simply to say, "Show me a verbose traceback" and might even help them to debug their code without assistance. Mike

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Add a "hasmethod()" builtin?
by Guido van Rossum 01 Sep '12

01 Sep '12

There's a concept that's sometimes useful when explaining behavior of certain Python operations in terms of simpler ones, and it is "does the class of x define a method m?". This currently cannot expressed using hasattr(): hasattr(x, 'm') might find an instance variable named m, but that is not a method (and there are contexts where they are not interchangeable); hasattr(type(x), 'm') might find a *metaclass* method. Example of the former (assume Python 3): class C: def __add__(self, other): return 42 c = C() c.__add__ = lambda *args: 0 c + 1 # prints 42, not 0 Example of the latter: class C: pass c = C() hasattr(C, 'mro') # prints True, since mro() is a method of the standard metaclass ('type'). c.mro() # raises AttributeError The use case I am mainly thinking of is the formal explanation of the semantics of binary (and other) operators, e.g. def __add__(a, b): r = NotImplemented if hasmethod(a, '__add__'): r = a.__add__(b) if r is NotImplemented and hasmethod(b, '__radd__'): r = b.__radd__(a) if r is NotImplemented: raise TypeError return r (Caveat: it's even more complicated if type(b) is a subclass of type(a).) I'm not sure if it would be better if the first argument of hasmethod() was a type instead of an instance (so the example would use hasattr(type(a), '__add__') etc.). It's also interesting to figure out what should happen for proxy types. (Proxy types in general need a better foundation.) Thoughts? -- --Guido van Rossum (python.org/~guido)

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Unpack of sequences
by Carlo Pires 01 Sep '12

01 Sep '12

Hi, I was just wondering why unpack of sequences did not follow same behavior of functions parameters. I mean: first, *rest = 'a b c'.split() should work in python, why doesn't it? -- Carlo Pires

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