(seq, keyfunc)
return m
Sift the sequence, ``seq`` into a dictionary according to keyfunc.

    OUTPUT: each element in expr is stored in a list keyed to the value
    of keyfunc for the element.

    Examples
    ========

    >>> from sympy.utilities import sift
    >>> from sympy.abc import x, y
    >>> from sympy import sqrt, exp

    >>> sift(range(5), lambda x: x % 2)
    {0: [0, 2, 4], 1: [1, 3]}

    sift() returns a defaultdict() object, so any key that has no matches will
    give [].

    >>> sift([x], lambda x: x.is_commutative)
    {True: [x]}
    >>> _[False]
    []

    Sometimes you won't know how many keys you will get:

    >>> sift([sqrt(x), exp(x), (y**x)**2],
    ...      lambda x: x.as_base_exp()[0])
    {E: [exp(x)], x: [sqrt(x)], y: [y**(2*x)]}

    If you need to sort the sifted items it might be better to use
    ``ordered`` which can economically apply multiple sort keys
    to a squence while sorting.

    See Also
    ========
    ordered
Sift the sequence, ``seq`` into a dictionary according to keyfunc.
Sift the sequence seq into a dictionary according to keyfunc .
def sift(seq, keyfunc):
    """
    Sift the sequence, ``seq`` into a dictionary according to keyfunc.

    OUTPUT: each element in expr is stored in a list keyed to the value
    of keyfunc for the element.

    Examples
    ========

    >>> from sympy.utilities import sift
    >>> from sympy.abc import x, y
    >>> from sympy import sqrt, exp

    >>> sift(range(5), lambda x: x % 2)
    {0: [0, 2, 4], 1: [1, 3]}

    sift() returns a defaultdict() object, so any key that has no matches will
    give [].

    >>> sift([x], lambda x: x.is_commutative)
    {True: [x]}
    >>> _[False]
    []

    Sometimes you won't know how many keys you will get:

    >>> sift([sqrt(x), exp(x), (y**x)**2],
    ...      lambda x: x.as_base_exp()[0])
    {E: [exp(x)], x: [sqrt(x)], y: [y**(2*x)]}

    If you need to sort the sifted items it might be better to use
    ``ordered`` which can economically apply multiple sort keys
    to a squence while sorting.

    See Also
    ========
    ordered
    """
    m = defaultdict(list)
    for i in seq:
        m[keyfunc(i)].append(i)
    return m
def sift ( seq , keyfunc ) : m = defaultdict ( list ) for i in seq : m [ keyfunc ( i ) ] . append ( i ) return m