/Blog/BlogVenv/Lib/site-packages/django/contrib/auth/views.py 의 form_class = AuthenticationForm 에서
AuthenticationForm 정의 따라가면 아래의 소스에서

1. /Blog/BlogVenv/Lib/site-packages/django/contrib/auth/forms.py 아래와 같이 placeholder 속성추가
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class AuthenticationForm(forms.Form):
    """
    Base class for authenticating users. Extend this to get a form that accepts
    username/password logins.
    """
    username = UsernameField(widget=forms.TextInput(attrs={'autofocus': True, "placeholder": "User ID"}))
    password = forms.CharField(
        label=_("Password"),
        strip=False,
        widget=forms.PasswordInput(attrs={'autocomplete': 'current-password', "placeholder": "Password"}),
    )

2. /Blog/BlogVenv/Lib/site-packages/django/contrib/auth/forms.py 에서 Sign up Form에 placeholder추가와 email field추가
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class UserCreationForm(forms.ModelForm):
    """
    A form that creates a user, with no privileges, from the given username and
    password.
    """
    error_messages = {
        'password_mismatch': _('The two password fields didn’t match.'),
    }
    password1 = forms.CharField(
        label=_("Password"),
        strip=False,
        widget=forms.PasswordInput(attrs={'autocomplete': 'new-password', "placeholder": "Enter Password"}),
        help_text=password_validation.password_validators_help_text_html(),
    )
    password2 = forms.CharField(
        label=_("Password confirmation"),
        widget=forms.PasswordInput(attrs={'autocomplete': 'new-password', "placeholder": "Repeat Password"}),
        strip=False,
        help_text=_("Enter the same password as before, for verification."),
    )

    email = forms.EmailField(label=_("Email"), max_length=254, widget=forms.TextInput(attrs={"placeholder": "Enter Email"}))

    class Meta:
        model = User
        fields = ("username", "email")
        field_classes = {'username': UsernameField}

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        if self._meta.model.USERNAME_FIELD in self.fields:
            self.fields[self._meta.model.USERNAME_FIELD].widget.attrs.update({'autofocus': True, "placeholder":"Enter User's ID"})