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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_signature(payment_request):
 """ Returns the signature for the transaction. To compute the signature, first you have to get the value of all the fields that starts by 'vads_', ordering them alphabetically. All the values are separated by the '+' character. Then you add the value of the payzen certificate. Finaly you hash the string using sha1."""


    vads_args = {}
    for field in payment_request._meta.fields:
        if field.name[:5] == 'vads_':
            field_value = field.value_from_object(payment_request)
            if field_value:
                vads_args.update({
                    field.name: field_value
                })
    base_str = ''
    for key in sorted(vads_args):
        base_str += str(vads_args[key]) + '+'
    base_str += app_settings.VADS_CERTIFICATE
    return hashlib.sha1(base_str.encode("utf-8")).hexdigest()