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class UserSerializer(serializers.Serializer):
username = serializers.CharField(
max_length=100,
min_length=4,
allow_blank=False,
trim_whitespace=True,
required=True,
default='guest',
write_only=False,
read_only=False,
validators=[validate_no_space],
error_messages={'blank': 'Please provide a username.'},
label='User Name',
help_text='Enter a unique username.',
style={'input_type': 'text'},
source='user.username',
allow_null=False,
initial='guest'
)