django add placeholder text to form field

You must use the placeholder properties

class LoginForm(forms.Form):
    username = forms.CharField(label='username')
    password = forms.CharField(label='password')


    def __init__(self, *args, **kwargs):
        super(LoginForm, self).__init__(*args, **kwargs)
        self.fields['username'].widget.attrs['placeholder'] = 'username'
        self.fields['password '].widget.attrs['placeholder'] = 'password'

or

class LoginForm(forms.Form):
    username = forms.CharField(label='username',widget=forms.TextInput(attrs={'placeholder':'username'}))
    password = forms.CharField(label='password',widget=forms.PasswordInput(attrs={'placeholder':'password'}))

In case it might help someone, I wanted to use the help_text property of a model as the placeholder. This is the simplest way I could figure it out, based on aziminia's answer:

class MyForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        for _, value in self.fields.items():
            value.widget.attrs['placeholder'] = value.help_text

    class Meta:
        model = models.MyModel
        fields = (...)

I hope you do have a forms.py file in your project. While creating your form, you can use following to set placeholder for your fields:

username = forms.CharField(label='username', 
                widget=forms.TextInput(attrs={'placeholder': 'username'}))

If you have ModelForm in your project you can implement as:

 class MyForm(forms.ModelForm):
    class Meta:
        model = User
        widgets = {
            'username': forms.TextInput(attrs={'placeholder': 'username'}),
             ..........
        }