Show errors in login form

Question

I've created a user login page, and it works great! But when the user inputs the wrong credentials, how can I show an error on the login page? I tried usign an if and a variable, but that resulted in the error being shown no matter what.

In my views, I've set the "error_message" variable like so:

def LoginView(request):
    error_message = ''
    logout(request)
    context = RequestContext(request)
    username = request.POST.get('username')
    password = request.POST.get('password')
    user = authenticate(username=username, password=password)
    if user is not None:
        if user.is_active:
            login(request, user)
            return HttpResponseRedirect("/")
        else:
            # Return a 'disabled account' error message
            return HttpResponseRedirect("/")
    else:
        # Return an 'invalid login' error message
  --->  error_message = "Incorrect username or password."
    return render(request, 'profile/login.html', {'error_message': error_message}, context)

However, in my template, this variable is displayed no matter what, even if I set the variable inside the "if" statement.

Here's my template:

{% include '__header.html' %}
<style>
main {
    width: 920px;
    background: none;
    margin-top: calc(50vh - 280px)
}
</style>

<div class="form-container">
<form id="user" method="post" action="/login/" 
enctype="multipart/form-data" class="text">
    {% csrf_token %}
    <div class="title">Login to your account</div>
    <div class="form-col">
        <div class="field-container">
            <div class="field-input-container">
                <div class="field-label">Username</div>
                <div class="field-input">
                    <input class="field-input-element" type="text" name="username"/>
                </div>
            </div>
        <div class="field-help"></div>
        </div>
    </div>
    <div class="form-col">
        <div class="field-container" style="height: 110px">
            <div class="field-input-container">
                <div class="field-label">Password</div>
                <div class="field-input">
                <input class="field-input-element" type="password" name="password"/>
                </div>
            </div>
        <div class="field-error">{{ error_message }}</div>
        <div class="field-help"></div>
        </div>
    </div>
    <div class="form-button-container">
    <input type="submit" name="submit" value="Login" class="button form-button"/>
    </div>
</form> 
</div>

{% include '__footer.html' %}

So, how can I show an error message in the template only if there has been an error?

---

Updated code based off answers (still does same thing)

def LoginView(request):
    template_name='profile/login.html'
    logout(request)
    context = RequestContext(request)

    if request.method == 'POST':
        username = request.POST.get('username')
        password = request.POST.get('password')
        user = authenticate(username=username, password=password)
        if user is not None:
            if user.is_active:
                login(request, user)
                return HttpResponseRedirect("/")
            else:
                # Return a 'disabled account' error message
                return HttpResponseRedirect("/")
        else:
            # Return an 'invalid login' error message
            error_message = "Incorrect username or password."
    else:
        error_message = ''

    return render(request, 'profile/login.html', {'error_message': error_message}, context)

Answer 1

The logic that you have for checking the users details should only be done if they've posted the form so you can wrap this part in an if request.method == 'POST'

if request.method == 'POST':
    username = request.POST.get('username')
    password = request.POST.get('password')
    user = authenticate(username=username, password=password)
    if user is not None:
        if user.is_active:
            login(request, user)
            return HttpResponseRedirect("/")
        else:
            # Return a 'disabled account' error message
            return HttpResponseRedirect("/")
    else:
        # Return an 'invalid login' error message
        error_message = "Incorrect username or password."

You might also want to wrap your error div in a

{% if error_message %}