import pytz


def get_obj_type(obj):
    """Determines the string representation of object's type."""

    # Get default type value.
    obj_type = type(obj).__name__

    # Special handling for certain types.
    if obj_type == 'NoneType':
        obj_type = 'null'
    elif isinstance(obj, pytz.BaseTzInfo):
        obj_type = 'pytz_timezone'

    return obj_type