Algorithm.co.il

Various Small Python Helpers

Posted on by 5 Comments

I’ve been working lately with arkon on various projects related to diStorm. One of these projects involved writing a solid Python API around an existing C API. This C API uses a lot of flags as arguments and return values of functions.
Since I want my code to be easy to use in an interactive shell, I’d like to return something that’s not just a number. The obvious solution is to use a Python Enum. There are many implementations going around, most of them equivalent. I’ve got the one I use. What I recently added though, is the SymbolInt:

def SymbolInt(value, name):
    class _SymbolInt(int):
        def __str__(self):
            return name
        def __repr__(self):
            return 'SymbolInt(%d, "%s")' % (value, name)
        def __eq__(self, other):
            if isinstance(other, str):
                other = other.lower()
            return int(self)==other or name.lower() == other
        def __ne__(self, other):
            return not self == other
    return _SymbolInt(value)

Short, and very much to the point, this makes exploratory API’s much more readable and usable. It is useful as a return value from enum functions as well.

Along with SymbolInt I wrote hexint:

class hexint(long):
    def __str__(self):
        return hex(self)[2:-1]
    __repr__ = __str__
    def __repr__(self):
        return hex(self)[:-1]

This one makes program addresses readable.

Lastly, here’s a little function that I found missing from itertools:

def head(iterable, num_items):
    for obj,i in zip(iterable,xrange(num_items)):
        yield obj

UPDATE: Don’t use this head() function, use itertools.islice instead. Thanks go to Erez for pointing that out.

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Comments

  1. Neat @ first two.

    The last one isn’t missing (unless I misunderstood your intent): “enumerate” is a built-in since Python 2.4, I believe.

    Reply

  2. Yoni: You misunderstood my intent. Head takes the first num_items from iterable.
    I’m piggy-backing on zip’s behavior. zip always takes the shorter of the input sequences.
    It also works for infinite sequences (such as itertools.count()).

    Maybe I should have used itertools.izip instead though, to avoid generating the num_items of iterable at once.

    And thanks!

    Reply

  3. Hi, two suggestions:
    1) Replace hex(self)[2:-1] with ‘%x’%self. It’s prettier.
    Also, the ‘L’ postfix gets abandoned in Python 3.0 .

    2) Your ‘head’ function is just a special case of ‘itertools.islice’ .

    Reply

  4. 1. You are correct. Using %x is more elegant, and will work in Python 3.
    2. You are correct on this one as well. In fact, my implementation of head might cause a potential bug. This is because zip (and izip) leave long iterators in an uncertain state, by taking more elements than they should.

    Reply

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