Load numpy array from a long text file

my_loadtxt.py

def my_loadtxt(fname, columns=-1, delimiter=',', dtype='float32'):
    with open(fname) as f:
        if columns == -1:
            line = f.readline()
            columns = np.fromstring(line, sep=delimiter).shape[0]
            f.seek(0)
        rows = 0
        for line in f:
            rows += 1
        f.seek(0)
        array = np.ndarray((rows, columns), dtype=dtype)
        for i, line in enumerate(f):
            array[i] = np.fromstring(line, sep=delimiter)[:columns]
        return array