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From my knowledge of python, this means that std_msgs.msg is a module name (i.e. a .py file) and there should be a class definition for String inside that file.

[..]

I looked through the documentation and it says that rospy do create python source code for msg files, so there must be this std_msgs.msg.py file somewhere [..]

While it is true that Python modules can be contained in a single file (ie: one with the same name as the module + .py), that is not the only way Python modules can be created.

Any directory with a __init__.py file in it will be considered a module by the Python interpreter. Any files inside that directory (apart from __init__.py) are roughly considered classes, and sub directories of a module's directory will be considered sub modules (as long as they also contain a __init__.py file).

In the case of std_msgs, this is what that looks like:

└── /opt/ros/kinetic/lib/python2.7/dist-packages
  └── std_msgs
    ├── __init__.py
    ├── __init__.pyc
    └── msg
      ├── _Bool.py
      ├── _ByteMultiArray.py
      ├── _Byte.py
      ├── ..
      ├── __init__.py
      ├── ..
      ├── _String.py
      ├── ..
      ├── _UInt8MultiArray.py
      └── _UInt8.py

So all ROS Python (2) modules are located in /opt/ros/kinetic/lib/python2.7/dist-packages, in which there is a sub directory called std_msgs. This contains a __init__.py file, marking it as a module. This has a sub directory msg, which then contains both a __init__.py as well as all the files that contain the implementations of all the message classes (and which were generated by genpy (not rospy).

When you write from std_msgs.msg import String, the Python interpreter will traverse this directory structure and end up at _String.py and load it.

From my knowledge of python, this means that std_msgs.msg is a module name (i.e. a .py file) and there should be a class definition for String inside that file.

[..]

I looked through the documentation and it says that rospy do create python source code for msg files, so there must be this std_msgs.msg.py file somewhere [..]

While it is true that Python modules can be contained in a single file (ie: one with the same name as the module + .py), that is not the only way Python modules can be created.

Any directory with a __init__.py file in it will be considered a module by the Python interpreter. Any files inside that directory (apart from __init__.py) are roughly considered classes, and sub directories of a module's directory will be considered sub modules (as long as they also contain a __init__.py file).

In the case of std_msgs, this is what that looks like:

└── /opt/ros/kinetic/lib/python2.7/dist-packages
  └── std_msgs
    ├── __init__.py
    ├── __init__.pyc
    └── msg
      ├── _Bool.py
      ├── _ByteMultiArray.py
      ├── _Byte.py
      ├── ..
      ├── __init__.py
      ├── ..
      ├── _String.py
      ├── ..
      ├── _UInt8MultiArray.py
      └── _UInt8.py

So all ROS Python (2) modules are located in /opt/ros/kinetic/lib/python2.7/dist-packages, in which there is a sub directory called std_msgs. This contains a __init__.py file, marking it as a module. This has a sub directory msg, which then contains both a __init__.py as well as all the files that contain the implementations of all the message classes (and which were generated by genpy (not rospy).

When you write from std_msgs.msg import String, the Python interpreter will traverse this directory structure and end up at _String.py and load it.

From my knowledge of python, this means that std_msgs.msg is a module name (i.e. a .py file) and there should be a class definition for String inside that file.

[..]

I looked through the documentation and it says that rospy do create python source code for msg files, so there must be this std_msgs.msg.py file somewhere [..]

While it is true that Python modules can be are contained in a single file (ie: one with the same name as the module + .py), that is not the only way Python modules can be created.there is a second construct called a package that is what is used here in this case.

Any directory with a __init__.py file in it will be considered a module package by the Python interpreter. Any files inside that directory (apart from __init__.py) are roughly considered classes, again modules (which can contain classes and other Python entities), and sub directories of a module's package's directory will be considered sub modules packages (as long as they also contain a __init__.py file).

In the case of std_msgs, this is what that looks like:

└── /opt/ros/kinetic/lib/python2.7/dist-packages
  └── std_msgs
    ├── __init__.py
    └── msg
      ├── _Bool.py
      ├── _ByteMultiArray.py
      ├── _Byte.py
      ├── ..
      ├── __init__.py
      ├── ..
      ├── _String.py
      ├── ..
      ├── _UInt8MultiArray.py
      └── _UInt8.py

So all ROS Python (2) modules packages (note: not necessarily ROS packages, as a single ROS package may contain multiple Python packages) are located in /opt/ros/kinetic/lib/python2.7/dist-packages, in which there is a sub directory called std_msgs. This contains a __init__.py file, marking it as a module. sub package. This has a sub directory msg, which then contains both a __init__.py as well as all the files that contain the implementations of all the message classes (and which were generated by genpy (not rospy).

When you write from std_msgs.msg import String, the Python interpreter will traverse this directory structure and end up at _String.py and , load it.