core.py 文件源码

def head(self, n=5, npartitions=1, compute=True):
        """ First n rows of the dataset

        Parameters
        ----------
        n : int, optional
            The number of rows to return. Default is 5.
        npartitions : int, optional
            Elements are only taken from the first ``npartitions``, with a
            default of 1. If there are fewer than ``n`` rows in the first
            ``npartitions`` a warning will be raised and any found rows
            returned. Pass -1 to use all partitions.
        compute : bool, optional
            Whether to compute the result, default is True.
        """
        if npartitions <= -1:
            npartitions = self.npartitions
        if npartitions > self.npartitions:
            raise ValueError("only %d partitions, received "
                             "%d" % (self.npartitions, npartitions))

        name = 'head-%d-%d-%s' % (npartitions, n, self._name)

        if npartitions > 1:
            name_p = 'head-partial-%d-%s' % (n, self._name)
            dsk = {(name_p, i): (M.head, (self._name, i), n)
                   for i in range(npartitions)}
            dsk[(name, 0)] = (M.head, (gd.concat, sorted(dsk)), n)
        else:
            dsk = {(name, 0): (M.head, (self._name, 0), n)}

        res = new_dd_object(merge(self.dask, dsk), name, self._meta,
                            (self.divisions[0], self.divisions[npartitions]))

        return res.compute() if compute else res