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This code is a suggested answer to question asked on Stackoverflow:

http://stackoverflow.com/questions/4827965/can-i-change-djangos-auth-user-username-field-to-be-100-chars-long-without-break/

authhack.models.py

# authhacks/models.py
from django.conf import settings
from authhacks import username_length


USERNAME_MAXLENGTH = getattr(settings, 'USERNAME_MAXLENGTH', 72)


username_length.hack_models()
username_length.hack_forms()

authhack.username_length.py

# authhacks/username_length.py
import sys


def hack_models(length=72):
    from django.contrib.auth.models import User
    username = User._meta.get_field("username")
    username.max_length = length
    hack_validators(username.validators)


def hack_forms(length=72, forms=[
        'django.contrib.auth.forms.UserCreationForm',
        'django.contrib.auth.forms.UserChangeForm',
        'django.contrib.auth.forms.AuthenticationForm',
    ]):
    """
    Hacks username length in django forms.
    """
    for form in forms:
        modulename, sep, classname = form.rpartition('.')
        if not modulename in sys.modules:
            __import__(modulename)
        module = sys.modules[modulename]
        klass = getattr(module, classname)
        hack_single_form(klass, length)


def hack_single_form(form_class, length=72):
    if hasattr(form_class, 'declared_fields'):
        fields = form_class.declared_fields
    elif hasattr(form_class, 'base_fields'):
        fields = form_class.base_fields
    else:
        raise TypeError('Provided object: %s doesnt seem to be a valid Form or '
                        'ModelForm class.' % form_class)
    username = fields['username']
    hack_validators(username.validators)
    username.max_length = length
    username.widget.attrs['maxlength'] = length


def hack_validators(validators, length=72):
    from django.core.validators import MaxLengthValidator
    for key, validator in enumerate(validators):
        if isinstance(validator, MaxLengthValidator):
            validators.pop(key)
    validators.insert(0, MaxLengthValidator(length))

authhacks.__init__.py

# authhacks/__init__.py

settings.py

# settings.py

INSTALLED_APPS = [
    # ...
    'django.contrib.auth',
    'authhacks'
    # ...
]