diff --git a/signxml/xades/xades.py b/signxml/xades/xades.py
index 030abc0..5632b2e 100644
--- a/signxml/xades/xades.py
+++ b/signxml/xades/xades.py
@@ -272,7 +272,7 @@ class XAdESVerifier(XAdESProcessor, XMLVerifier):
     def _verify_signing_time(self, verify_result: VerifyResult):
         pass
 
-    def _verify_cert_digest(self, signing_cert_node, expect_cert,idx):
+    def _verify_cert_digest(self, signing_cert_node, expect_cert, idx):
         cert = self._find(signing_cert_node, "xades:Cert[{0}]".format(idx))
         cert_digest = self._find(cert, "xades:CertDigest")
         digest_alg = DigestAlgorithm(self._find(cert_digest, "DigestMethod").get("Algorithm"))
@@ -285,21 +285,23 @@ def _verify_cert_digest(self, signing_cert_node, expect_cert,idx):
 
     def _verify_cert_digests(self, verify_result: VerifyResult):
         x509_data = verify_result.signature_xml.find("ds:KeyInfo/ds:X509Data", namespaces=namespaces)
-        for idx,x_cert in enumerate(self._findall(x509_data, "X509Certificate")):
-            cert_from_key_info = load_certificate(
-                FILETYPE_PEM, add_pem_header(x_cert.text)
-            )
+        for idx, x_cert in enumerate(self._findall(x509_data, "X509Certificate")):
+            cert_from_key_info = load_certificate(FILETYPE_PEM, add_pem_header(x_cert.text))
             signed_signature_props = self._find(verify_result.signed_xml, "xades:SignedSignatureProperties")
             signing_cert = self._find(signed_signature_props, "xades:SigningCertificate", require=False)
             signing_cert_v2 = self._find(signed_signature_props, "xades:SigningCertificateV2", require=False)
             if signing_cert is None and signing_cert_v2 is None:
-                raise InvalidInput("Expected to find XML element xades:SigningCertificate or xades:SigningCertificateV2")
+                raise InvalidInput(
+                    "Expected to find XML element xades:SigningCertificate or xades:SigningCertificateV2"
+                )
             if signing_cert is not None and signing_cert_v2 is not None:
-                raise InvalidInput("Expected to find exactly one of xades:SigningCertificate or xades:SigningCertificateV2")
+                raise InvalidInput(
+                    "Expected to find exactly one of xades:SigningCertificate or xades:SigningCertificateV2"
+                )
             if signing_cert is not None:
-                self._verify_cert_digest(signing_cert, expect_cert=cert_from_key_info,idx=idx+1)
+                self._verify_cert_digest(signing_cert, expect_cert=cert_from_key_info, idx=(idx+1))
             elif signing_cert_v2 is not None:
-                self._verify_cert_digest(signing_cert_v2, expect_cert=cert_from_key_info,idx=idx+1)
+                self._verify_cert_digest(signing_cert_v2, expect_cert=cert_from_key_info, idx=(idx+1))
 
     def _verify_signature_policy(self, verify_result: VerifyResult, expect_signature_policy: XAdESSignaturePolicy):
         signed_signature_props = self._find(verify_result.signed_xml, "xades:SignedSignatureProperties")