Session state changes only after clicking button twice

Using Streamlit
,
1

I have to click the login button twice initially to show the welcome message


if 'username' not in st.session_state:
        st.session_state.username = ' '

if st.session_state.username:
    st.write('welcome ',st.session_state.username)
else:
    st.title('Login/Signup')
    email = st.text_input('Email')
    password = st.text_input('Password',type='password')
    if st.button('Login'):
            try:
                userinfo = sign_in_with_email_and_password(email,password)
                st.session_state.username = userinfo['username']
                st.session_state.useremail = userinfo['email']
                
            except:
                st.warning('Login Failed')

I am trying to create a login form. Once a person is logged in using correct user name and password, only then userinfo[‘username’] will contain a value. Not sure how to display the welcome message on the first button click itself. If I click the button twice, then it is working fine.

2

You can use st.rerun() if the log in succeeds, and that will run the page from top to bottom, and the welcome message will then be displayed properly.

            try:
                userinfo = sign_in_with_email_and_password(email,password)
                st.session_state.username = userinfo['username']
                st.session_state.useremail = userinfo['email']
                st.rerun()
            except:
                st.warning('Login Failed')
3

I have already tried it. st.rerun() fails inside the try except blocks.
Error:
RerunData(page_script_hash=‘d575aef6f69f53c7a18ce42c5b281cbb’, is_fragment_scoped_rerun=True)

4

Ah yes, I should have thought of that. The problem is that the way that st.rerun works is that it raises a special exception, and that tells Streamlit that it needs to rerun the page.

It’s a bad idea for this very reason (and others) to a “bare except” like try:....except: without any specific error (Avoid using bare except in Python - 30 seconds of code)

The best situation would be to figure out what sort of error the sign_in_with_email_and_password would raise, or just raise one yourself, and handle that specific error.

For example:

def sign_in_with_email_and_password(email, password):
    # Replace with real implementation
    if (email, password) not in get_logins():
        raise ValueError("Invalid login")

And then

try:
    userinfo = ...
    st.rerun()
except ValueError:
    st.warning("Login failed")
5

Thanks a lot for the detailed explanation. It worked :grinning::+1: