diff --git a/silkaj/blocks.py b/silkaj/blocks.py
index 6aa1c76188a6c64643c1307b8eedbc5dab8bd102..81197941a69da4c48e6b00f37e1d37c27828daf9 100644
--- a/silkaj/blocks.py
+++ b/silkaj/blocks.py
@@ -44,14 +44,12 @@ def verify_blocks_signatures(from_block, to_block):
         for chunk_from in bar:
             chunk_size = get_chunk_size(from_block, to_block, chunks_from, chunk_from)
             logging.info(
-                "Processing chunk from block {} to {}".format(
-                    chunk_from, chunk_from + chunk_size
-                )
+                f"Processing chunk from block {chunk_from} to {chunk_from + chunk_size}"
             )
             chunk = get_chunk(client, chunk_size, chunk_from)
 
             for block in chunk:
-                block = Block.from_signed_raw(block["raw"] + block["signature"] + "\n")
+                block = Block.from_signed_raw(f'{block["raw"]}{block["signature"]}\n')
                 verify_block_signature(invalid_blocks_signatures, block)
 
     display_result(from_block, to_block, invalid_blocks_signatures)
@@ -63,8 +61,7 @@ def check_passed_blocks_range(client, from_block, to_block):
         to_block = head_number
     if to_block > head_number:
         message_exit(
-            "Passed TO_BLOCK argument is bigger than the head block: "
-            + str(head_number)
+            f"Passed TO_BLOCK argument is bigger than the head block: {str(head_number)}"
         )
     if from_block > to_block:
         message_exit("TO_BLOCK should be bigger or equal to FROM_BLOCK")
diff --git a/silkaj/cert.py b/silkaj/cert.py
index de07e2ec8ade79f89c0c242f0a2db610ea32b755..4f4a010d4d5906c98c35530c9c01fdd3e5870237 100644
--- a/silkaj/cert.py
+++ b/silkaj/cert.py
@@ -99,7 +99,7 @@ def pre_checks(client, issuer_pubkey, pubkey_to_certify):
             renewable = cert["expiresIn"] - params["sigValidity"] + params["sigReplay"]
             if renewable > 0:
                 renewable_date = now().add(seconds=renewable).format(DATE)
-                sys.exit("Certification renewable from " + renewable_date)
+                sys.exit(f"Certification renewable from {renewable_date}")
 
     # Check if the certification is already in the pending certifications
     for pending_cert in req["pendingCerts"]: