Merge #19674: refactor: test: use throwaway _ variable for unused loop counters

dac7a111bd refactor: test: use _ variable for unused loop counters (Sebastian Falbesoner)

Pull request description:

  This tiny PR substitutes Python loops in the form of `for x in range(N): ...` by `for _ in range(N): ...` where applicable. The idea is indicating to the reader that a block (or statement, in list comprehensions) is just repeated N times, and that the loop counter is not used in the body, hence using the throwaway variable. This is already done quite often in the current tests (see e.g. `$ git grep "for _ in range("`). Another alternative would be using `itertools.repeat` (according to Python core developer Raymond Hettinger it's [even faster](https://twitter.com/raymondh/status/1144527183341375488)), but that doesn't seem to be widespread in use and I'm not sure about a readability increase.

  The only drawback I see is that whenever one wants to debug loop iterations, one would need to introduce a loop variable again. Reviewing this is basically a no-brainer, since tests would fail immediately if a a substitution has taken place on a loop where the variable is used.

  Instances to replace were found by `$ git grep "for.*in range("` and manually checked.

ACKs for top commit:
  darosior:
    ACK dac7a111bd
  instagibbs:
    manual inspection ACK dac7a111bd
  practicalswift:
    ACK dac7a111bd -- the updated code is easier to reason about since the throwaway nature of a variable is expressed explicitly (using the Pythonic `_` idiom) instead of implicitly. Explicit is better than implicit was we all know by now :)

Tree-SHA512: 5f43ded9ce14e5e00b3876ec445b90acda1842f813149ae7bafa93f3ac3d510bb778e2c701187fd2c73585e6b87797bb2d2987139bd1a9ba7d58775a59392406

test/functional/test_framework/messages.py

@@ -111,7 +111,7 @@ def deser_uint256(f):
 
 def ser_uint256(u):
     rs = b""
-    for i in range(8):
+    for _ in range(8):
         rs += struct.pack("<I", u & 0xFFFFFFFF)
         u >>= 32
     return rs
@@ -134,7 +134,7 @@ def uint256_from_compact(c):
 def deser_vector(f, c):
     nit = deser_compact_size(f)
     r = []
-    for i in range(nit):
+    for _ in range(nit):
         t = c()
         t.deserialize(f)
         r.append(t)
@@ -157,7 +157,7 @@ def ser_vector(l, ser_function_name=None):
 def deser_uint256_vector(f):
     nit = deser_compact_size(f)
     r = []
-    for i in range(nit):
+    for _ in range(nit):
         t = deser_uint256(f)
         r.append(t)
     return r
@@ -173,7 +173,7 @@ def ser_uint256_vector(l):
 def deser_string_vector(f):
     nit = deser_compact_size(f)
     r = []
-    for i in range(nit):
+    for _ in range(nit):
         t = deser_string(f)
         r.append(t)
     return r
@@ -467,7 +467,7 @@ class CTransaction:
         else:
             self.vout = deser_vector(f, CTxOut)
         if flags != 0:
-            self.wit.vtxinwit = [CTxInWitness() for i in range(len(self.vin))]
+            self.wit.vtxinwit = [CTxInWitness() for _ in range(len(self.vin))]
             self.wit.deserialize(f)
         else:
             self.wit = CTxWitness()
@@ -500,7 +500,7 @@ class CTransaction:
             if (len(self.wit.vtxinwit) != len(self.vin)):
                 # vtxinwit must have the same length as vin
                 self.wit.vtxinwit = self.wit.vtxinwit[:len(self.vin)]
-                for i in range(len(self.wit.vtxinwit), len(self.vin)):
+                for _ in range(len(self.wit.vtxinwit), len(self.vin)):
                     self.wit.vtxinwit.append(CTxInWitness())
             r += self.wit.serialize()
         r += struct.pack("<I", self.nLockTime)
@@ -735,7 +735,7 @@ class P2PHeaderAndShortIDs:
         self.header.deserialize(f)
         self.nonce = struct.unpack("<Q", f.read(8))[0]
         self.shortids_length = deser_compact_size(f)
-        for i in range(self.shortids_length):
+        for _ in range(self.shortids_length):
             # shortids are defined to be 6 bytes in the spec, so append
             # two zero bytes and read it in as an 8-byte number
             self.shortids.append(struct.unpack("<Q", f.read(6) + b'\x00\x00')[0])
@@ -852,7 +852,7 @@ class BlockTransactionsRequest:
     def deserialize(self, f):
         self.blockhash = deser_uint256(f)
         indexes_length = deser_compact_size(f)
-        for i in range(indexes_length):
+        for _ in range(indexes_length):
             self.indexes.append(deser_compact_size(f))
 
     def serialize(self):

test/functional/test_framework/script.py

@@ -646,7 +646,7 @@ def LegacySignatureHash(script, txTo, inIdx, hashtype):
 
         tmp = txtmp.vout[outIdx]
         txtmp.vout = []
-        for i in range(outIdx):
+        for _ in range(outIdx):
             txtmp.vout.append(CTxOut(-1))
         txtmp.vout.append(tmp)
 

test/functional/test_framework/util.py

@@ -529,7 +529,7 @@ def create_confirmed_utxos(fee, node, count):
     addr2 = node.getnewaddress()
     if iterations <= 0:
         return utxos
-    for i in range(iterations):
+    for _ in range(iterations):
         t = utxos.pop()
         inputs = []
         inputs.append({"txid": t["txid"], "vout": t["vout"]})
@@ -556,7 +556,7 @@ def gen_return_txouts():
     # So we have big transactions (and therefore can't fit very many into each block)
     # create one script_pubkey
     script_pubkey = "6a4d0200"  # OP_RETURN OP_PUSH2 512 bytes
-    for i in range(512):
+    for _ in range(512):
         script_pubkey = script_pubkey + "01"
     # concatenate 128 txouts of above script_pubkey which we'll insert before the txout for change
     txouts = []
@@ -564,7 +564,7 @@ def gen_return_txouts():
     txout = CTxOut()
     txout.nValue = 0
     txout.scriptPubKey = hex_str_to_bytes(script_pubkey)
-    for k in range(128):
+    for _ in range(128):
         txouts.append(txout)
     return txouts