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from pydap.client import open_url
from netcdftime import utime
import requests
import numpy as np
from ocw.dataset import Dataset

def load(url, variable, name=''):
    '''Load a Dataset from an OpenDAP URL

    :param url: The OpenDAP URL for the dataset of interest.
    :type url: :mod:`string`

    :param variable: The name of the variable to read from the dataset.
    :type variable: :mod:`string`

    :param name: (Optional) A name for the loaded dataset.
    :type name: :mod:`string`

    :returns: A :class:`dataset.Dataset` containing the dataset pointed to by
        the OpenDAP URL.

    :raises: ServerError
    '''
    # Grab the dataset information and pull the appropriate variable
    d = open_url(url)
    dataset = d[variable]

    # Grab the lat, lon, and time variable names.
    # We assume the variable order is (time, lat, lon)
    dataset_dimensions = dataset.dimensions
    time = dataset_dimensions[0]
    lat = dataset_dimensions[1]
    lon = dataset_dimensions[2]

    # Time is given to us in some units since an epoch. We need to convert
    # these values to datetime objects. Note that we use the main object's
    # time object and not the dataset specific reference to it. We need to 
    # grab the 'units' from it and it fails on the dataset specific object.
    times = np.array(_convert_times_to_datetime(d[time]))

    lats = np.array(dataset[lat][:])
    lons = np.array(dataset[lon][:])
    values = np.array(dataset[:])

    origin = {
        'source': 'dap',
        'url': url
    }

    return Dataset(lats, lons, times, values, variable,
                   name=name, origin=origin)

def _convert_times_to_datetime(time):
    '''Convert the OpenDAP time object's values to datetime objects

    The time values are stores as some unit since an epoch. These need to be 
    converted into datetime objects for the OCW Dataset object.

    :param time: The time object's values to convert
    :type time: pydap.model.BaseType

    :returns: list of converted time values as datetime objects
    '''
    units = time.units
    # parse the time units string into a useful object.
    # NOTE: This assumes a 'standard' calendar. It's possible (likely?) that
    #   users will want to customize this in the future.
    parsed_time = utime(units)
    return [parsed_time.num2date(x) for x in time[:]]