Python帮助：从另一个类访问静态成员变量

Question

I'll do my best to describe the issue I am having. I am building a Python program that is built on multiple classes and uses the unittest framework. In a nutshell, the Main.py file has a "ValidateDriver" class that defines a "driver" variable as an ElementTree type. If I point this directly to the XML file I need to parse, (ie driver = ElementTree.parse(rC:\\test.xml)) then I can access it from another class. However, in reality I don't have the actual XML file that is passed in from the command-line until you get to the Main function in the ValidateDriver class. So under the ValidateDriver class driver would really be driver = ElementTree and then in the main function I would reassign that variable to ValidateDriver.driver = ElementTree.parse(args.driver). However, this is the crux. When I go to the other class and try to call ValidateDriver.driver I don't have the "findall" method/attribute available. Again, the only way it will work is to do something like: ElementTree.parse(rC:\\test.xml)). If I did this in C# it would work, but I am new to Python and this is kicking my butt. Any help/suggestions is appreciated. I've included the code for both classes.

Main Function:

import sys
import argparse
import xml.etree.ElementTree as ElementTree
import unittest
import Tests.TestManufacturer


class ValidateDriver:
    driver = ElementTree

    def main(argv):
        parser = argparse.ArgumentParser(description='Validation.')
        parser.add_argument('-d', '--driver', help='Path and file name xml file', required=True)
        parser.add_argument('-v', '--verbosity',
                            help='Verbosity for test output.  1 for terse, 2 for verbose.  Default is verbose',
                            default=2, type=int)
        #args = parser.parse_args()
        args = r'C:\test.c4i'
        #print ("Validate Driver: %s" % args.driver)
        #print ("Verbosity Level: %s" % args.verbosity)

        ValidateDriver.driver = ElementTree.parse(r'C:\test.c4i')

        loader = unittest.TestLoader()
        suite = loader.loadTestsFromModule(Tests.TestManufacturer)


        runner = unittest.TextTestRunner(verbosity=2) # TODO Remove this...
        # TODO Uncomment this...  
        runner = unittest.TextTestRunner(verbosity=args.verbosity)
        result = runner.run(suite)


    if __name__ == "__main__":
        main(sys.argv[1:])

Other Class, Test Manufacturer:

import unittest
import Main


manufacturer = ['']


class Tests(unittest.TestCase):

    # Test to see if Manufacturer exists.
    def test_manufacturer_exists(self):
        for m in Main.ValidateDriver.driver.findall('./manufacturer'):
            print m.text

Producing the following error:

C:\Python27\python.exe C:\Users\test\PycharmProjects\Validator\Main.py
Traceback (most recent call last):
  File "C:\Users\test\PycharmProjects\Validator\Main.py", line 22, in <module>
    class ValidateDriver:
  File "C:\Users\test\PycharmProjects\Validator\Main.py", line 65, in ValidateDriver
    main(sys.argv[1:])
  File "C:\Users\test\PycharmProjects\Validator\Main.py", line 36, in main
    ValidateDriver.driver = ElementTree.parse(r'C:\test.c4i')
NameError: global name 'ValidateDriver' is not defined

Process finished with exit code 1

Answer 1

The main problem seems to be that your main script is wrapped in a class. There's really no reason for this, and is quite confusing.

if __name__ == "__main__":
    main_object = ValidateDriver()
    main_object.main(sys.argv[1:])

This should go outside the class definition

Answer 2

This has nothing to do with "findall" being available. The issue is that the class itself hasn't yet been completely declared at the time you try to access it. In python, the file is read top to bottom. For example, this is not allowed:

if __name__ == "__main__":
    f()

def f():
    ...

The call to f must happen at the bottom of the file after it is declared.

What you're doing with ValidateDriver is similar, because the class isn't defined until the statements directly in its body are executed (this is different from functions, whose bodies of course aren't executed until they are called). You call main(sys.argv[1:]) inside the class body, which in turn tries to access ValidateDriver.driver , which doesn't exist yet.

Preferably, the main function, as well as the code which calls it, should be outside the class. As far as I can tell, the class doesn't need to exist at all (this isn't C# or Java -- you can put code directly at the module level without a class container). If you insist on putting it in a class as a static method, it must be defined as a class method:

@classmethod
def main(cls, argv):
    ...

which can then be called (outside the class definition) like:

ValidateDriver.main(sys.argv[1:])

But I stress that this is non-standard and should not be necessary.