Improve FURB179 error messages:

* Detect `reduce()` when used with `add`/`concat` operator

* Remove duplicated code

* Emit better error message for set conprehensions

* Show more accurate names in error messages

refurb/checks/common.py

@@ -335,4 +335,9 @@ def _stringify(node: Node) -> str:
 
             return f"lambda{args}: {body}"
 
+        case ListExpr(items=items):
+            inner = ", ".join(stringify(x) for x in items)
+
+            return f"[{inner}]"
+
     raise ValueError

refurb/checks/itertools/use_chain_from_iterable.py

@@ -8,8 +8,10 @@
     ListExpr,
     NameExpr,
     RefExpr,
+    SetComprehension,
 )
 
+from refurb.checks.common import stringify
 from refurb.error import Error
 
 
@@ -81,7 +83,7 @@ def is_flatten_generator(node: GeneratorExpr) -> bool:
 
 
 def check(
-    node: ListComprehension | GeneratorExpr | CallExpr,
+    node: ListComprehension | SetComprehension | GeneratorExpr | CallExpr,
     errors: list[Error],
 ) -> None:
     if id(node) in ignore:
@@ -92,41 +94,51 @@ def check(
             old = "[... for ... in x for ... in ...]"
             new = "list(chain.from_iterable(x))"
 
-            msg = f"Replace `{old}` with `{new}`"
+            ignore.add(id(g))
 
-            errors.append(ErrorInfo.from_node(node, msg))
+        case SetComprehension(generator=g) if is_flatten_generator(g):
+            old = "{... for ... in x for ... in ...}"
+            new = "set(chain.from_iterable(x))"
 
             ignore.add(id(g))
 
         case GeneratorExpr() if is_flatten_generator(node):
             old = "... for ... in x for ... in ..."
             new = "chain.from_iterable(x)"
 
-            msg = f"Replace `{old}` with `{new}`"
-
-            errors.append(ErrorInfo.from_node(node, msg))
-
         case CallExpr(
             callee=RefExpr(fullname="builtins.sum"),
-            args=[_, ListExpr(items=[])],
+            args=[arg, ListExpr(items=[])],
         ):
-            old = "sum(x, [])"
-            new = "chain.from_iterable(x)"
+            old = f"sum({stringify(arg)}, [])"
+            new = f"chain.from_iterable({stringify(arg)})"
+
+        case CallExpr(
+            callee=RefExpr(fullname="functools.reduce"),
+            args=[op, arg] | [op, arg, ListExpr(items=[])],
+        ):
+            match op:
+                case RefExpr(fullname="_operator.add" | "_operator.concat"):
+                    pass
 
-            msg = f"Replace `{old}` with `{new}`"
+                case _:
+                    return
 
-            errors.append(ErrorInfo.from_node(node, msg))
+            old = stringify(node)
+            new = f"chain.from_iterable({stringify(arg)})"
 
         case CallExpr(
             callee=RefExpr(fullname="itertools.chain") as callee,
-            args=[_],
+            args=[arg],
             arg_kinds=[ArgKind.ARG_STAR],
         ):
             chain = "chain" if isinstance(callee, NameExpr) else "itertools.chain"
 
-            old = f"{chain}(*x)"
-            new = f"{chain}.from_iterable(x)"
+            old = f"{chain}(*{stringify(arg)})"
+            new = f"{chain}.from_iterable({stringify(arg)})"
 
-            msg = f"Replace `{old}` with `{new}`"
+        case _:
+            return
 
-            errors.append(ErrorInfo.from_node(node, msg))
+    msg = f"Replace `{old}` with `{new}`"
+    errors.append(ErrorInfo.from_node(node, msg))

test/data/err_179.py

@@ -1,5 +1,9 @@
+from functools import reduce
+from operator import add, concat, iadd
 from itertools import chain
+import functools
 import itertools
+import operator
 
 
 rows = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
@@ -30,6 +34,21 @@ def flatten_via_chain_splat(rows):
 def flatten_via_chain_splat_2(rows):
     return itertools.chain(*rows)
 
+def flatten_via_reduce_add(rows):
+    return reduce(add, rows)
+
+def flatten_via_reduce_add_with_default(rows):
+    return reduce(add, rows, [])
+
+def flatten_via_reduce_concat(rows):
+    return reduce(concat, rows)
+
+def flatten_via_reduce_concat_with_default(rows):
+    return reduce(concat, rows, [])
+
+def flatten_via_reduce_full_namespace(rows):
+    return functools.reduce(operator.add, rows)
+
 
 # these should not
 
@@ -68,3 +87,9 @@ def flatten_via_chain_without_splat(rows):
 
 def flatten_via_chain_from_iterable(rows):
     return chain.from_iterable(rows)
+
+def flatten_via_reduce_iadd(rows):
+    return reduce(iadd, rows, [])
+
+def flatten_via_reduce_non_empty_default(rows):
+    return reduce(add, rows, [1, 2, 3])

test/data/err_179.txt

@@ -1,7 +1,12 @@
-test/data/err_179.py:13:12 [FURB179]: Replace `... for ... in x for ... in ...` with `chain.from_iterable(x)`
-test/data/err_179.py:16:12 [FURB179]: Replace `[... for ... in x for ... in ...]` with `list(chain.from_iterable(x))`
-test/data/err_179.py:19:12 [FURB179]: Replace `... for ... in x for ... in ...` with `chain.from_iterable(x)`
-test/data/err_179.py:22:12 [FURB179]: Replace `... for ... in x for ... in ...` with `chain.from_iterable(x)`
-test/data/err_179.py:25:12 [FURB179]: Replace `sum(x, [])` with `chain.from_iterable(x)`
-test/data/err_179.py:28:12 [FURB179]: Replace `chain(*x)` with `chain.from_iterable(x)`
-test/data/err_179.py:31:12 [FURB179]: Replace `itertools.chain(*x)` with `itertools.chain.from_iterable(x)`
+test/data/err_179.py:17:12 [FURB179]: Replace `... for ... in x for ... in ...` with `chain.from_iterable(x)`
+test/data/err_179.py:20:12 [FURB179]: Replace `[... for ... in x for ... in ...]` with `list(chain.from_iterable(x))`
+test/data/err_179.py:23:12 [FURB179]: Replace `{... for ... in x for ... in ...}` with `set(chain.from_iterable(x))`
+test/data/err_179.py:26:12 [FURB179]: Replace `... for ... in x for ... in ...` with `chain.from_iterable(x)`
+test/data/err_179.py:29:12 [FURB179]: Replace `sum(rows, [])` with `chain.from_iterable(rows)`
+test/data/err_179.py:32:12 [FURB179]: Replace `chain(*rows)` with `chain.from_iterable(rows)`
+test/data/err_179.py:35:12 [FURB179]: Replace `itertools.chain(*rows)` with `itertools.chain.from_iterable(rows)`
+test/data/err_179.py:38:12 [FURB179]: Replace `reduce(add, rows)` with `chain.from_iterable(rows)`
+test/data/err_179.py:41:12 [FURB179]: Replace `reduce(add, rows, [])` with `chain.from_iterable(rows)`
+test/data/err_179.py:44:12 [FURB179]: Replace `reduce(concat, rows)` with `chain.from_iterable(rows)`
+test/data/err_179.py:47:12 [FURB179]: Replace `reduce(concat, rows, [])` with `chain.from_iterable(rows)`
+test/data/err_179.py:50:12 [FURB179]: Replace `functools.reduce(operator.add, rows)` with `chain.from_iterable(rows)`