python module加载
init.py的作用
ros2 加载过程
ros2pkg 文件目录

|-- CHANGELOG.rst
|-- package.xml
|-- resource
|   `-- ros2pkg
|-- ros2pkg
|   |-- __init__.py
|   |-- api
|   |   |-- __init__.py
|   |   `-- create.py
|   |-- command
|   |   |-- __init__.py  //说明这是一个包
|   |   `-- pkg.py
|   |-- resource
|   |   |-- __init__.py
|   |   |-- ament_cmake
|   |   |-- ament_python
|   |   |-- cmake
|   |   |-- cpp
|   |   `-- package_environment
|   `-- verb
|       |-- __init__.py
|       |-- create.py
|       |-- executables.py
|       |-- list.py
|       |-- prefix.py
|       `-- xml.py
|-- setup.py
`-- test
    |-- test_api.py
    |-- test_cli.py
    |-- test_copyright.py
    |-- test_flake8.py
    |-- test_pep257.py
    `-- test_xmllint.py

当我们运行ros2 pkg list时，会调用到rospkg包中，进入点，就是entry_points.txt中定义的

[ros2cli.command]
pkg = ros2pkg.command.pkg:PkgCommand

[ros2cli.extension_point]
ros2pkg.verb = ros2pkg.verb:VerbExtension

[ros2pkg.verb]
create = ros2pkg.verb.create:CreateVerb
executables = ros2pkg.verb.executables:ExecutablesVerb
list = ros2pkg.verb.list:ListVerb
prefix = ros2pkg.verb.prefix:PrefixVerb
xml = ros2pkg.verb.xml:XmlVerb

当运行ros2 pkg list,首先是进入这个函数

from ros2cli.command import add_subparsers_on_demand  #导入方法用于命令解决
from ros2cli.command import CommandExtension #导入类 

class PkgCommand(CommandExtension):
    """Various package related sub-commands."""

    def add_arguments(self, parser, cli_name):
        self._subparser = parser
        # add arguments and sub-commands of verbs
        add_subparsers_on_demand(
            parser, cli_name, '_verb', 'ros2pkg.verb', required=False)

    def main(self, *, parser, args):
        if not hasattr(args, '_verb'): #hasattr() 函数用于判断对象是否包含对应的属性。
            # in case no verb was passed
            self._subparser.print_help()
            return 0

        extension = getattr(args, '_verb') #函数用于返回一个对象属性值。

        # call the verb's main method
        return extension.main(args=args)

‘list = ros2pkg.verb.list:ListVerb’

from ros2pkg.api import get_package_names
from ros2pkg.verb import VerbExtension

class ListVerb(VerbExtension):
    """Output a list of available packages."""
    def main(self, *, args):
        for pkg_name in sorted(get_package_names()):
            print(pkg_name) #打印出包名

get_package_names实现在ros2pkg/api/init.py 文件

def get_package_names():
    return get_packages_with_prefixes().keys()
#定义在ros2下面install/lib/python3.6/site-packages/ament_index_python/packages.py文件里 
def get_packages_with_prefixes():
    """
    Return a dict of package names to the prefixes in which they are found.

    :returns: dict of package names to their prefixes
    :rtype: dict
    """
    return get_resources('packages')
    
#install/lib/python3.6/site-packages/ament_index_python/resources.py
def get_resources(resource_type):
    """
    Get the resource names of all resources of the specified type.

    :param resource_type: the type of the resource
    :type resource_type: str
    :returns: dict of resource names to the prefix path they are in
    :raises: :exc:`EnvironmentError`
    """
    assert resource_type, 'The resource type must not be empty'
    resources = {}
    for path in get_search_paths():
        resource_path = os.path.join(path, RESOURCE_INDEX_SUBFOLDER, resource_type)
        if os.path.isdir(resource_path):
            for resource in os.listdir(resource_path):
                # Ignore subdirectories, and anything starting with a dot
                if os.path.isdir(os.path.join(resource_path, resource)) \
                        or resource.startswith('.'):
                    continue
                if resource not in resources:
                    resources[resource] = path
    return resources