Authentication problem

Anvil Q&A
1

What I’m trying to do:
I’m trying to authenticate my users with anvil.users.login_with_email
What I’ve tried and what’s not working:
I currently have 3 users (gmwebman, nickb, martinc) with their passwords the same as their usernames for testing. If on my login form I provide correct credentials the user is successfully logged in. If I supply an incorrect password I get a TypeError instead of AuthenticationFailed : TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types
at frmLogin, line 43

Code Sample:

# this is a formatted code snippet.
  def btnLogin_click(self, **event_args):
    pass
    if self.txtUsername.text == '':
      alert('Please enter your username')
    elif self.txtPassword.text == '':
      alert('Please enter your password')
    else:
      uu = app_tables.users.search()
      un = ''
      em = ''
      for u in uu:
        if u['username'] == self.txtUsername.text:
          un = u['username']
          em = u['email']
      if un == '':
        alert('Login failed. No such username.')
      else:
        pw = self.txtPassword.text
        try:
43      user = anvil.users.login_with_email(em, pw)
          if user['isadmin']:
            alert('Administrator has logged in.')
          else:
            alert('User has logged in.')
        except 'AuthenticationFailed':
          alert('Login failed. Incorrect password.')
# paste your code between ```

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2

could it simply be that the line needs to be indented within the try block

3

Thanks for that dsweet. I eventually spotted the indentation error myself and corrected it but the TypeError: isinstance() is still occuring.

4

That’s pretty strange.

When I cloned your app and played with it, I regularly saw the error you’re seeing when I entered in a username with a garbage password. If I entered the correct password, the user logged in successfully.

I created a new app to test with, and in that one entering the wrong password gets the expected “wrong password” error message, not the error you’re seeing.

5

Yes, that’s exactly the behaviour I’m seeing. Weird.

6

I created a new app with nothing but a custom login form. Initially I did not put the “anvil.users.login_with_email” in a try block. A correct username and password produced a successful login, an incorrect password produced the expected “AuthenticationFailed” exception.
I then put the login in a try block with an except block to catch the above:
except ‘AuthenticationFailed’:
As before a correct username+password produced a successful login but an incorrect password produced the weird TypeError: isinstance() exception. The weirdness only appears when the login is in a try!

7

the error message isn’t so nice

it should be more like:

TypeError: catching classes that do not inherit from BaseException is not allowed

because you have

        except 'AuthenticationFailed':

which is not a valid python exception type

this should fix your issue

        except anvil.users.AuthenticationFailed:
2 Likes
8

Thanks again stucork!

I assume that the try block was working perfectly but the invalid exception type created an uncaught exception which was the weird one I was seeing.

9

Yeah. If you run the following code in Python locally you’ll get a similar, but more complete error traceback.

try:
  raise Exception("foo")
except "Exception":
  pass

for each except block python will check if the error is an instance of that line.