lint: remove E731 Ruff ignore

diff --git a/ruff.toml b/ruff.toml
index a252c2f4..61cf6d24 100644
--- a/ruff.toml
+++ b/ruff.toml
@@ -12,6 +12,5 @@ select = [
 ]
 ignore = [
     "E501", # line too long
-    "E731", # lambda assignment
     "E741", # ambiguous-variable-name
 ]
diff --git a/test/functional/feature_addrman.py b/test/functional/feature_addrman.py
index a80261ec..1930a277 100755
--- a/test/functional/feature_addrman.py
+++ b/test/functional/feature_addrman.py
@@ -52,12 +52,13 @@ class AddrmanTest(BitcoinTestFramework):
 
     def run_test(self):
         peers_dat = os.path.join(self.nodes[0].chain_path, "peers.dat")
-        init_error = lambda reason: (
-            f"Error: Invalid or corrupt peers.dat \\({reason}\\). If you believe this "
-            f"is a bug, please report it to {self.config['environment']['CLIENT_BUGREPORT']}. "
-            f'As a workaround, you can move the file \\("{re.escape(peers_dat)}"\\) out of the way \\(rename, '
-            "move, or delete\\) to have a new one created on the next start."
-        )
+        def init_error(reason):
+            return (
+                f"Error: Invalid or corrupt peers.dat \\({reason}\\). If you believe this "
+                f"is a bug, please report it to {self.config['environment']['CLIENT_BUGREPORT']}. "
+                f'As a workaround, you can move the file \\("{re.escape(peers_dat)}"\\) out of the way \\(rename, '
+                "move, or delete\\) to have a new one created on the next start."
+            )
 
         self.log.info("Check that mocked addrman is valid")
         self.stop_node(0)
diff --git a/test/functional/feature_reindex_readonly.py b/test/functional/feature_reindex_readonly.py
index 889d0c2a..336fab69 100755
--- a/test/functional/feature_reindex_readonly.py
+++ b/test/functional/feature_reindex_readonly.py
@@ -35,13 +35,14 @@ class BlockstoreReindexTest(BitcoinTestFramework):
         filename = self.nodes[0].chain_path / "blocks" / "blk00000.dat"
         filename.chmod(stat.S_IREAD)
 
-        undo_immutable = lambda: None
+        undo_immutable_cmd = None
+        should_reindex = True
         # Linux
         try:
             subprocess.run(['chattr'], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
             try:
                 subprocess.run(['chattr', '+i', filename], capture_output=True, check=True)
-                undo_immutable = lambda: subprocess.check_call(['chattr', '-i', filename])
+                undo_immutable_cmd = ['chattr', '-i', filename]
                 self.log.info("Made file immutable with chattr")
             except subprocess.CalledProcessError as e:
                 self.log.warning(str(e))
@@ -53,14 +54,14 @@ class BlockstoreReindexTest(BitcoinTestFramework):
                     self.log.warning("Return early on Linux under root, because chattr failed.")
                     self.log.warning("This should only happen due to missing capabilities in a container.")
                     self.log.warning("Make sure to --cap-add LINUX_IMMUTABLE if you want to run this test.")
-                    undo_immutable = False
+                    should_reindex = False
         except Exception:
             # macOS, and *BSD
             try:
                 subprocess.run(['chflags'], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
                 try:
                     subprocess.run(['chflags', 'uchg', filename], capture_output=True, check=True)
-                    undo_immutable = lambda: subprocess.check_call(['chflags', 'nouchg', filename])
+                    undo_immutable_cmd = ['chflags', 'nouchg', filename]
                     self.log.info("Made file immutable with chflags")
                 except subprocess.CalledProcessError as e:
                     self.log.warning(str(e))
@@ -70,16 +71,17 @@ class BlockstoreReindexTest(BitcoinTestFramework):
                         self.log.warning(f"stderr: {e.stderr}")
                     if os.getuid() == 0:
                         self.log.warning("Return early on BSD under root, because chflags failed.")
-                        undo_immutable = False
+                        should_reindex = False
             except Exception:
                 pass
 
-        if undo_immutable:
+        if should_reindex:
             self.log.debug("Attempt to restart and reindex the node with the unwritable block file")
             with self.nodes[0].assert_debug_log(["Reindexing finished"], timeout=60):
                 self.start_node(0, extra_args=['-reindex', '-fastprune'])
             assert_equal(block_count, self.nodes[0].getblockcount())
-            undo_immutable()
+            if undo_immutable_cmd is not None:
+                subprocess.check_call(undo_immutable_cmd)
 
         filename.chmod(0o777)
 
diff --git a/test/functional/feature_taproot.py b/test/functional/feature_taproot.py
index 1a41527e..a0c11fd6 100755
--- a/test/functional/feature_taproot.py
+++ b/test/functional/feature_taproot.py
@@ -764,7 +764,9 @@ def spenders_taproot_active():
                 add_spender(spenders, "sighash/scriptpath_hashtype_mis_%x" % hashtype, tap=tap, leaf="s0", key=secs[1], annex=annex, standard=no_annex, hashtype_actual=random.choice(VALID_SIGHASHES_TAPROOT_NO_SINGLE), **SINGLE_SIG, failure={"hashtype_actual": hashtype}, **ERR_SCHNORR_SIG_HASHTYPE, need_vin_vout_mismatch=True)
 
         # Test OP_CODESEPARATOR impact on sighashing.
-        hashtype = lambda _: random.choice(VALID_SIGHASHES_TAPROOT)
+        def hashtype(_):
+            return random.choice(VALID_SIGHASHES_TAPROOT)
+
         common = {"annex": annex, "hashtype": hashtype, "standard": no_annex}
         scripts = [
             ("pk_codesep", CScript(random_checksig_style(pubs[1]) + bytes([OP_CODESEPARATOR]))),  # codesep after checksig
@@ -792,7 +794,9 @@ def spenders_taproot_active():
     add_spender(spenders, "sighash/keypath", tap=tap, key=secs[0], **common, failure={"sighash": override(default_sighash, leaf="pk_codesep")}, **ERR_SCHNORR_SIG)
 
     # Test that invalid hashtypes don't work, both in key path and script path spends
-    hashtype = lambda _: random.choice(VALID_SIGHASHES_TAPROOT)
+    def hashtype(_):
+        return random.choice(VALID_SIGHASHES_TAPROOT)
+
     for invalid_hashtype in [x for x in range(0x100) if x not in VALID_SIGHASHES_TAPROOT]:
         add_spender(spenders, "sighash/keypath_unk_hashtype_%x" % invalid_hashtype, tap=tap, key=secs[0], hashtype=hashtype, failure={"hashtype": invalid_hashtype}, **ERR_SCHNORR_SIG_HASHTYPE)
         add_spender(spenders, "sighash/scriptpath_unk_hashtype_%x" % invalid_hashtype, tap=tap, leaf="pk_codesep", key=secs[1], **SINGLE_SIG, hashtype=hashtype, failure={"hashtype": invalid_hashtype}, **ERR_SCHNORR_SIG_HASHTYPE)
@@ -1170,7 +1174,9 @@ def spenders_taproot_active():
         add_spender(spenders, "unkver/1001inputs", standard=False, tap=tap, leaf="bare_unkver", inputs=[b'']*1001, failure={"leaf": "bare_c0"}, **ERR_STACK_SIZE)
 
     # OP_SUCCESSx tests.
-    hashtype = lambda _: random.choice(VALID_SIGHASHES_TAPROOT)
+    def hashtype(_):
+        return random.choice(VALID_SIGHASHES_TAPROOT)
+
     for opval in range(76, 0x100):
         opcode = CScriptOp(opval)
         if not is_op_success(opcode):
@@ -1215,7 +1221,9 @@ def spenders_taproot_active():
 
     # == Test case for https://github.com/bitcoin/bitcoin/issues/24765 ==
 
-    zero_fn = lambda h: bytes([0 for _ in range(32)])
+    def zero_fn(h):
+        return bytes([0] * 32)
+
     tap = taproot_construct(pubs[0], [("leaf", CScript([pubs[1], OP_CHECKSIG, pubs[1], OP_CHECKSIGADD, OP_2, OP_EQUAL])), zero_fn])
     add_spender(spenders, "case24765", tap=tap, leaf="leaf", inputs=[getter("sign"), getter("sign")], key=secs[1], no_fail=True)
 
diff --git a/test/functional/mempool_spend_coinbase.py b/test/functional/mempool_spend_coinbase.py
index b09041c7..3dac5e9d 100755
--- a/test/functional/mempool_spend_coinbase.py
+++ b/test/functional/mempool_spend_coinbase.py
@@ -32,7 +32,8 @@ class MempoolSpendCoinbaseTest(BitcoinTestFramework):
         # Coinbase at height chain_height-100+1 ok in mempool, should
         # get mined. Coinbase at height chain_height-100+2 is
         # too immature to spend.
-        coinbase_txid = lambda h: self.nodes[0].getblock(self.nodes[0].getblockhash(h))['tx'][0]
+        def coinbase_txid(h):
+            return self.nodes[0].getblock(self.nodes[0].getblockhash(h))['tx'][0]
         utxo_mature = wallet.get_utxo(txid=coinbase_txid(chain_height - 100 + 1))
         utxo_immature = wallet.get_utxo(txid=coinbase_txid(chain_height - 100 + 2))
 
diff --git a/test/functional/mining_template_verification.py b/test/functional/mining_template_verification.py
index f00268f6..ad2a42b8 100755
--- a/test/functional/mining_template_verification.py
+++ b/test/functional/mining_template_verification.py
@@ -253,10 +253,11 @@ class MiningTemplateVerificationTest(BitcoinTestFramework):
     def parallel_test(self, node, block_3):
         # Ensure that getblocktemplate can be called concurrently by many threads.
         self.log.info("Check blocks in parallel")
-        check_50_blocks = lambda n: [
-            assert_template(n, block_3, "bad-txns-inputs-missingorspent", submit=False)
-            for _ in range(50)
-        ]
+        def check_50_blocks(n):
+            return [
+                assert_template(n, block_3, "bad-txns-inputs-missingorspent", submit=False)
+                for _ in range(50)
+            ]
         rpcs = [node.cli for _ in range(6)]
         with ThreadPoolExecutor(max_workers=len(rpcs)) as threads:
             list(threads.map(check_50_blocks, rpcs))
diff --git a/test/functional/p2p_sendheaders.py b/test/functional/p2p_sendheaders.py
index cc28fc05..acef0329 100755
--- a/test/functional/p2p_sendheaders.py
+++ b/test/functional/p2p_sendheaders.py
@@ -139,7 +139,9 @@ class BaseNode(P2PInterface):
         self.send_without_ping(getblocks_message)
 
     def wait_for_block_announcement(self, block_hash, timeout=60):
-        test_function = lambda: self.last_blockhash_announced == block_hash
+        def test_function():
+            return self.last_blockhash_announced == block_hash
+
         self.wait_until(test_function, timeout=timeout)
 
     def on_inv(self, message):
@@ -165,7 +167,9 @@ class BaseNode(P2PInterface):
     def check_last_headers_announcement(self, headers):
         """Test whether the last headers announcements received are right.
            Headers may be announced across more than one message."""
-        test_function = lambda: (len(self.recent_headers_announced) >= len(headers))
+        def test_function():
+            return len(self.recent_headers_announced) >= len(headers)
+
         self.wait_until(test_function)
         with p2p_lock:
             assert_equal(self.recent_headers_announced, headers)
@@ -177,7 +181,9 @@ class BaseNode(P2PInterface):
         """Test whether the last announcement received had the right inv.
         inv should be a list of block hashes."""
 
-        test_function = lambda: self.block_announced
+        def test_function():
+            return self.block_announced
+
         self.wait_until(test_function)
 
         with p2p_lock:
diff --git a/test/functional/rpc_generate.py b/test/functional/rpc_generate.py
index a68d9c32..97ebf936 100755
--- a/test/functional/rpc_generate.py
+++ b/test/functional/rpc_generate.py
@@ -87,7 +87,9 @@ class RPCGenerateTest(BitcoinTestFramework):
 
         # Ensure that generateblock can be called concurrently by many threads.
         self.log.info('Generate blocks in parallel')
-        generate_50_blocks = lambda n: [n.generateblock(output=address, transactions=[]) for _ in range(50)]
+        def generate_50_blocks(n):
+            return [n.generateblock(output=address, transactions=[]) for _ in range(50)]
+
         rpcs = [node.cli for _ in range(6)]
         with ThreadPoolExecutor(max_workers=len(rpcs)) as threads:
             list(threads.map(generate_50_blocks, rpcs))
diff --git a/test/functional/rpc_net.py b/test/functional/rpc_net.py
index 405d21fc..81cf4da7 100755
--- a/test/functional/rpc_net.py
+++ b/test/functional/rpc_net.py
@@ -202,7 +202,9 @@ class NetTest(BitcoinTestFramework):
         self.wait_until(lambda: (self.nodes[0].getnettotals()['totalbytesrecv'] >= net_totals_before['totalbytesrecv'] + ping_size * 2), timeout=1)
 
         for peer_before in peer_info_before:
-            peer_after = lambda: next(p for p in self.nodes[0].getpeerinfo() if p['id'] == peer_before['id'])
+            def peer_after():
+                return next(p for p in self.nodes[0].getpeerinfo() if p['id'] == peer_before['id'])
+
             self.wait_until(lambda: peer_after()['bytesrecv_per_msg'].get('pong', 0) >= peer_before['bytesrecv_per_msg'].get('pong', 0) + ping_size, timeout=1)
             self.wait_until(lambda: peer_after()['bytessent_per_msg'].get('ping', 0) >= peer_before['bytessent_per_msg'].get('ping', 0) + ping_size, timeout=1)
 
diff --git a/test/functional/test_framework/p2p.py b/test/functional/test_framework/p2p.py
index 80319479..65260b7d 100755
--- a/test/functional/test_framework/p2p.py
+++ b/test/functional/test_framework/p2p.py
@@ -610,11 +610,15 @@ class P2PInterface(P2PConnection):
         wait_until_helper_internal(test_function, timeout=timeout, lock=p2p_lock, timeout_factor=self.timeout_factor, check_interval=check_interval)
 
     def wait_for_connect(self, *, timeout=60):
-        test_function = lambda: self.is_connected
+        def test_function():
+            return self.is_connected
+
         self.wait_until(test_function, timeout=timeout, check_connected=False)
 
     def wait_for_disconnect(self, *, timeout=60):
-        test_function = lambda: not self.is_connected
+        def test_function():
+            return not self.is_connected
+
         self.wait_until(test_function, timeout=timeout, check_connected=False)
 
     def wait_for_reconnect(self, *, timeout=60):