Take this very simple form for example:

class SearchForm(Form):
    q = forms.CharField(label='search')

This gets rendered in the template:

<input type="text" name="q" id="id_q" />

However, I want to add the placeholder attribute to this field with a value of Search so that the HTML would look something like:

<input type="text" name="q" id="id_q" placeholder="Search" />

Preferably I would like to pass the placeholder value in to CharField in the form class through a dictionary or something like:

q = forms.CharField(label='search', placeholder='Search')

What would be the best way to accomplish this?

Look at the widgets documentation. Basically it would look like:

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

More writing, yes, but the separation allows for better abstraction of more complicated cases.

You can also declare a widgets attribute containing a <field name> => <widget instance> mapping directly on the Meta of your ModelForm sub-class.

For a ModelForm, you can use the Meta class thus:

from django import forms

from .models import MyModel

class MyModelForm(forms.ModelForm):
    class Meta:
        model = MyModel
        widgets = {
            'name': forms.TextInput(attrs={'placeholder': 'Name'}),
            'description': forms.Textarea(
                attrs={'placeholder': 'Enter description here'}),
        }

The other methods are all good. However, if you prefer to not specify the field (e.g. for some dynamic method), you can use this:

def __init__(self, *args, **kwargs):
    super(MyForm, self).__init__(*args, **kwargs)
    self.fields['email'].widget.attrs['placeholder'] = self.fields['email'].label or 'email@address.nl'

It also allows the placeholder to depend on the instance for ModelForms with instance specified.

You can use this code to add placeholder attr for every TextInput field in you form. Text for placeholders will be taken from model field labels.

class PlaceholderDemoForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super(PlaceholderDemoForm, self).__init__(*args, **kwargs)
        for field_name in self.fields:
            field = self.fields.get(field_name)  
            if field:
                if type(field.widget) in (forms.TextInput, forms.DateInput):
                    field.widget = forms.TextInput(attrs={'placeholder': field.label})

    class Meta:
        model = DemoModel

Great question. There are three solutions I know about:

Solution #1

Replace the default widget.

class SearchForm(forms.Form):  
    q = forms.CharField(
            label='Search',
            widget=forms.TextInput(attrs={'placeholder': 'Search'})
        )

Solution #2

Customize the default widget. If you’re using the same widget that the field usually uses then you can simply customize that one instead of instantiating an entirely new one.

class SearchForm(forms.Form):  
    q = forms.CharField(label='Search')

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['q'].widget.attrs.update({'placeholder': 'Search'})

Solution #3

Finally, if you’re working with a model form then (in addition to the previous two solutions) you have the option to specify a custom widget for a field by setting the widgets attribute of the inner Meta class.

class CommentForm(forms.ModelForm):  
    class Meta:
        model = Comment
        widgets = {
            'body': forms.Textarea(attrs={'cols': 80, 'rows': 20})
        }

It’s undesirable to have to know how to instantiate a widget when you just want to override its placeholder.

    q = forms.CharField(label='search')
    ...
    q.widget.attrs['placeholder'] = "Search"

Most of the time I just wish to have all placeholders equal to the verbose name of the field defined in my models

I’ve added a mixin to easily do this to any form that I create,

class ProductForm(PlaceholderMixin, ModelForm):
    class Meta:
        model = Product
        fields = ('name', 'description', 'location', 'store')

And

class PlaceholderMixin:
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        field_names = [field_name for field_name, _ in self.fields.items()]
        for field_name in field_names:
            field = self.fields.get(field_name)
            field.widget.attrs.update({'placeholder': field.label})

After looking at your method, I used this method to solve it.

class Register(forms.Form):
    username = forms.CharField(label='用户名', max_length=32)
    email = forms.EmailField(label='邮箱', max_length=64)
    password = forms.CharField(label="密码", min_length=6, max_length=16)
    captcha = forms.CharField(label="验证码", max_length=4)

def __init__(self, *args, **kwargs):
    super().__init__(*args, **kwargs)
    for field_name in self.fields:
        field = self.fields.get(field_name)
        self.fields[field_name].widget.attrs.update({
            "placeholder": field.label,
            'class': "input-control"
        })