B028: flag empty skip_file_prefixes tuples

README.rst

@@ -237,7 +237,8 @@ limitations make it difficult.
 **B028**: No explicit stacklevel argument found. The warn method from the warnings module uses a
 stacklevel of 1 by default. This will only show a stack trace for the line on which the warn method is called.
 It is therefore recommended to use a stacklevel of 2 or greater to provide more information to the user.
-The check is skipped when skip_file_prefixes is used.
+The check is skipped when ``skip_file_prefixes`` is used, except for an explicitly empty tuple,
+which does not affect ``stacklevel``.
 
 .. _B029:
 
@@ -500,6 +501,8 @@ Change Log
 UNRELEASED
 ~~~~~~~~~~
 
+* B028: report ``warnings.warn`` calls that pass an explicitly empty
+  ``skip_file_prefixes`` tuple (#510)
 * B019: also flag `async_lru.alru_cache` and check cache decorators on `async def` methods (#488)
 * B023: don't flag a function whose every reference is a direct call inside the loop body:
   such a function cannot outlive the iteration it was defined in (#468, #380)

bugbear.py

@@ -2016,7 +2016,11 @@ def check_for_b028(self, node: ast.Call) -> None:
             and isinstance(node.func.value, ast.Name)
             and node.func.value.id == "warnings"
             and not any(kw.arg == "stacklevel" for kw in node.keywords)
-            and not any(kw.arg == "skip_file_prefixes" for kw in node.keywords)
+            and all(
+                kw.arg != "skip_file_prefixes"
+                or (isinstance(kw.value, ast.Tuple) and not kw.value.elts)
+                for kw in node.keywords
+            )
             and len(node.args) < 3
             and not any(isinstance(a, ast.Starred) for a in node.args)
             and not any(kw.arg is None for kw in node.keywords)

tests/eval_files/b028.py

@@ -2,7 +2,7 @@
 
 """
 Should emit:
-B028 - on lines 8 and 9
+B028 - on lines 8, 9, and 20
 """
 
 warnings.warn("test", DeprecationWarning) # B028: 0
@@ -17,3 +17,6 @@
 warnings.warn(**kwargs)
 warnings.warn(*args, **kwargs)
 warnings.warn("test", DeprecationWarning, skip_file_prefixes=["foo"])
+warnings.warn("test", DeprecationWarning, skip_file_prefixes=()) # B028: 0
+skip_file_prefixes = ()
+warnings.warn("test", DeprecationWarning, skip_file_prefixes=skip_file_prefixes)