使用GeckoDriver和Python的Selenium firefox webdriver未知错误

Question

I've created many test cases using:

• Firefox v55
• Python 2.7.3
• GeckoDriver v0.19.0

This scenario is beeing used by around 50 test cases

try:
    caps = DesiredCapabilities.FIREFOX
    caps["wires"] = True
    driver = webdriver.Firefox(capabilities=caps)
    driver.maximize_window()

    self.log("Selenium driver created for %s" % browser)
    return self.STATUS.PASS, driver

except Exception, e:
    self.log("Error when creating selenium driver")
    self.log(traceback.format_exc())
    return self.STATUS.FAIL 

This code simple creates a Firefox web driver, and it is working fine in all the test cases but 1, in one in particular when I execute it in a daily execution i got the following error

Traceback (most recent call last):

File "C:\QA Tools\selenium\CreateSeleniumDriver.py", line 26, in run

driver = webdriver.Firefox(capabilities=caps)

File "c:\python27\lib\site-packages\selenium-3.5.0-py2.7.egg\selenium\webdriver\firefox\webdriver.py", line 154, in __init__
keep_alive=True)

File "c:\python27\lib\site-packages\selenium-3.5.0-py2.7.egg\selenium\webdriver\remote\webdriver.py", line 140, in __init__
self.start_session(desired_capabilities, browser_profile)

File "c:\python27\lib\site-packages\selenium-3.5.0-py2.7.egg\selenium\webdriver\remote\webdriver.py", line 229, in start_session
response = self.execute(Command.NEW_SESSION, parameters)

File "c:\python27\lib\site-packages\selenium-3.5.0-py2.7.egg\selenium\webdriver\remote\webdriver.py", line 295, in execute
response = self.command_executor.execute(driver_command, params)

File "c:\python27\lib\site-packages\selenium-3.5.0-py2.7.egg\selenium\webdriver\remote\remote_connection.py", line 464, in execute
return self._request(command_info[0], url, body=data)

File "c:\python27\lib\site-packages\selenium-3.5.0-py2.7.egg\selenium\webdriver\remote\remote_connection.py", line 538, in _request
body = data.decode('utf-8').replace('\x00', '').strip()

File "c:\python27\lib\encodings\utf_8.py", line 16, in decode
return codecs.utf_8_decode(input, errors, True)

AttributeError: 'NoneType' object has no attribute 'utf_8_decode'

Selenium driver created for Firefox
Error when creating selenium driver
Traceback (most recent call last):
File "C:\QA Tools\selenium\CreateSeleniumDriver.py", line 63, in run
return self.STATUS.PASS, driver

UnboundLocalError: local variable 'driver' referenced before assignment

So, this is breaking a part of selenium web driver, but i have no idea why!

• I've tried everything to make it work
• This works in all other scenarios I'm using it
• This only happens on Firefox
• This works if it is executed manualy, but not working when executed on a daily execution with all others test cases (any related to this kind of execution was checked and not problem found)

Any ideas what could be going on wrong?

Answer 1

This looks like you're running into this bug filed on the official Python bug tracker: https://bugs.python.org/issue14847

The comments there suggest that the issue might be related to one of two things:

1. Installing packages with distribute , due to a bug known to exist at least in distribute 0.6.26 . If your "daily execution" relies on installing with this bugged version of distribute , that may explain your issue. Do you know if you do this? And can you update the version of distribute used in your environment, or perhaps use virtual environments to avoid it?

2. Some other way of manipulating sys.modules such that codecs is not found and gives None instead. Does your code do anything like this in some kind of fancy test setup?

Judging by the trace, you've modified your code for brevity, but in a way that might not completely capture the original logic. The UnboundLocalError suggests that your assignment of driver was skipped, and you didn't return right away:

try:
    caps = DesiredCapabilities.FIREFOX
    caps["wires"] = True
    # This line raises an AttributeError deep inside a standard python lib:
    driver = webdriver.Firefox(capabilities=caps)
    driver.maximize_window()

    self.log("Selenium driver created for %s" % browser)

except Exception, e:
    # The driver variable was never actually created or assigned due to the
    # AttributeError above.
    self.log("Error when creating selenium driver")
    self.log(traceback.format_exc())

# Some omitted code...

# The driver variable does not exist past the try if the exception happened
# before assignment to driver. So this raises the UnboundLocalError:
return self.STATUS.PASS, driver

You can fix UnboundLocalError pretty easily by doing the return inside the except, or pre-assigning driver to None or something, but that won't fix the underlying issue creating the driver. That sounds like an issue with your execution environment. Maybe something in your run infrastructure is doing something fancy (what are you using as your test runner?) or perhaps there's something corrupt in the Python environment of your daily execution runner. Depending on what the issue is, you might be able to solve the problem by defining a specific virtualenv for all test execution, so you can be sure it runs the same manually as it does in the automatic daily execution.