class SHA1Field(BaseField):
    """A field that validates input as resembling an SHA1 hash.
    """
    hash_length = 40

    def validate(self, value):
        if len(value) != SHA1Field.hash_length:
            raise DictPunch('SHA1 value is wrong length')
        try:
            x = int(value, 16)
        except:
            raise DictPunch('SHA1 value is not hex')