Python Note Three

2013-09-05 331 words 2 minutes

Contents

Iterator protocol
- Objects that implements the __iter__ method, __iter__ returns an iterator object, which has a method called next (in PY3 __next__) return its “next value” And StopIteration raised when no more values to return.

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class Fib:
    def __init__(self):
        self.a = 0
        self.b = 1

    def next(self):
        self.a, self.b = self.b, self.a + self.b
        return self.a
    def __iter__(self):
        return self

The built-in function iter can be used to get an iterator from an iterable object.
A generator is a kind of iterator that is defined with normal function syntax. Any function contains a yield statement is called a generator.

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def flatten(nested):
    try:
        try:
            nested +''
        except TypeError: pass
        else: raise TypeError
        for sublist in nested:
            for element in flatten(sublist):
                yield element
    except TypeError:
        yield nested

Generator method
- send works just like next, except it takes a single argument(the ‘message’ to send) yield may now used as an expression: value = (yield oldvalue)
- rather than a statement, when the generator returned, yield returns a value, the value send from the outside through send. If next was used, yield returns None.
Eight Queens: state[0]=3 row 1 column 4 has a queen

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def conflict(state, nextX):
    nextY = len(state)
    for i in range(nextY):
        if abs(state[i]-next]]) in (0, nextY-i):
            return True

    return False


def queens(num = 8, state=()):
    for pos in range(num):
        if not conflict(state, pos):
            if len(state) == num-1:
                yield (pos,)
            else:
                for result in queens(num, state+(pos,)):
                    yield (pos,) + result

Tell interpreter where to look module

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>>>import sys
>>>sys.path.append('test')

“main program” __name__ == "__main__"
Pretty print

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>>>import pprint
>>>pprint.pprint(text)

__all__ variable: define the public interface of the module, tells the interpreter what it menas to import all the names from this module: eg; from copy import *

__file__ property tells module location