can't import custum message in python

edit

Hi guys! I get this mistake.

 Traceback (most recent call last):
      File "prova.py", line 6, in <module>
        from immagine.msg import pixel_ned
    ImportError: No module named immagine.msg

It seems that my code doesn't find the custum message I created. immagine is my package and pixel_ned is the name of my .msg file. If I type rosmsg show pixel_ned it exists and I have no problem using it with a c++ code. What is going on?

My listener is this:

#!/usr/bin/env python
import numpy as np

import matplotlib.pyplot as plt

import rospy
from immagine.msg import pixel_ned
from std_msgs.msg import Float64MultiArray

def callback(data):
    rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.data)


def listener():

    # In ROS, nodes are uniquely named. If two nodes with the same
    # name are launched, the previous one is kicked off. The
    # anonymous=True flag means that rospy will choose a unique
    # name for our 'listener' node so that multiple listeners can
    # run simultaneously.
    rospy.init_node('ascolta_immagine', anonymous=True)
    rospy.Subscriber("/scrivi_immagine", pixel_ned, callback)
    rospy.spin()# spin() simply keeps python from exiting until this node is stopped

if __name__ == '__main__':
    listener()

And this my Cmake

cmake_minimum_required(VERSION 2.8.3)
project(immagine)

## Compile as C++11, supported in ROS Kinetic and newer
add_compile_options(-std=c++11)

## Find catkin macros and libraries
## if COMPONENTS list like find_package(catkin REQUIRED COMPONENTS xyz)
## is used, also find other catkin packages
find_package(catkin REQUIRED COMPONENTS

  roscpp
  rospy
  std_msgs
  marta_msgs

 ## DI QUESTI NONSONO SICURA 


    geometry_msgs

  message_generation
  sensor_msgs
  auv_lib
  genmsg
)

## System dependencies are found with CMake's conventions
# find_package(Boost REQUIRED COMPONENTS system)
find_package(cmake_modules REQUIRED)

find_package(Eigen REQUIRED)



## Generate messages in the 'msg' folder
 add_message_files(
   FILES
   pixel_ned.msg
#   Message1.msg
#   Message2.msg
)


## Generate added messages and services with any dependencies listed here
generate_messages(
   DEPENDENCIES
   std_msgs
   marta_msgs
   )

catkin_install_python(PROGRAMS scripts/prova.py
  DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
)
###########
## Build ##
###########

if(NOT DEFINED CMAKE_CXX_STANDARD)
  set(CMAKE_CXX_STANDARD 14)
endif()

## Specify additional locations of header files
## Your package locations should be listed before other locations
include_directories(
  include
  ${catkin_INCLUDE_DIRS}
  ${Eigen_INCLUDE_DIRS}
)

## Declare a C++ library
# add_library(${PROJECT_NAME}
#   src/${PROJECT_NAME}/immagine.cpp
# ) 
add_library(immagine src/funzioni.cpp)

    ## Declare a C++ executable
    ## With catkin_make all packages are built within a single CMake context
    ## The recommended prefix ensures that target names across packages don't collide
    add_executable(gestisco_dati src/gestisco_dati.cpp)


## Add cmake target dependencies of the executable
## same as for the library above
# add_dependencies(${PROJECT_NAME}_node ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})
add_dependencies(gestisco_dati ${catkin_EXPORTED_TARGETS})
add_dependencies(gestisco_dati immagine_gencpp)#perchè dipende da un messaggio costumizzato

## Specify libraries to link a library or executable target against
target_link_libraries(gestisco_dati immagine ${catkin_LIBRARIES})