import bpy

item='MESH'
bpy.ops.object.select_all(action='DESELECT')
bpy.ops.object.select_by_type(type=item)

# Subcontexts.
subcontexts_list = ['INVOKE_DEFAULT', 'INVOKE_REGION_WIN', 'INVOKE_REGION_CHANNELS',
                    'INVOKE_REGION_PREVIEW', 'INVOKE_AREA', 'INVOKE_SCREEN',
                    'EXEC_DEFAULT', 'EXEC_REGION_WIN', 'EXEC_REGION_CHANNELS',
                    'EXEC_REGION_PREVIEW', 'EXEC_AREA', 'EXEC_SCREEN']

# Subcontext.
for sub_context in subcontexts_list:
    try:
        bpy.ops.object.join(sub_context)

    except Exception as e:
        print('{}: {}'.format(sub_context, e))

INVOKE_DEFAULT: Operator bpy.ops.object.join.poll() failed, context is incorrect
INVOKE_REGION_WIN: Operator bpy.ops.object.join.poll() failed, context is incorrect
INVOKE_REGION_CHANNELS: Operator bpy.ops.object.join.poll() failed, context is incorrect
INVOKE_REGION_PREVIEW: Operator bpy.ops.object.join.poll() failed, context is incorrect
INVOKE_AREA: Operator bpy.ops.object.join.poll() failed, context is incorrect
INVOKE_SCREEN: Operator bpy.ops.object.join.poll() failed, context is incorrect
EXEC_DEFAULT: Operator bpy.ops.object.join.poll() failed, context is incorrect
EXEC_REGION_WIN: Operator bpy.ops.object.join.poll() failed, context is incorrect
EXEC_REGION_CHANNELS: Operator bpy.ops.object.join.poll() failed, context is incorrect
EXEC_REGION_PREVIEW: Operator bpy.ops.object.join.poll() failed, context is incorrect
EXEC_AREA: Operator bpy.ops.object.join.poll() failed, context is incorrect
EXEC_SCREEN: Operator bpy.ops.object.join.poll() failed, context is incorrect