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def currency(value, group_sep=' ', decimal_sep='.'):
    """Format currency by digits.

    >>> currency(100)
    '100.00'
    >>> currency(3.1415)
    '3.14'
    >>> currency(31415)
    '31 415.00'
    >>> currency(3141500.1)
    '3 141 500.10'
    >>> currency(31415000.1)
    '31 415 000.10'
    """
    head,tail = ('%.2f'%value).split('.')
    l = len(head)
    return group_sep.join([head[r-3:l+r] for r in xrange(0,-l,-3)][::-1]) + decimal_sep + tail

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