Fix error on attributes-of-attributes in except (...): clauses (#109)

* Fix #108, error on nested Attributes
  * Add fuzz-tests to check for crashes
  * Require attrs>=19.2.0 (this is only for hypothesmith - so could be moved to .travis.yml)

Author: Zac-HD

.travis.yml

@@ -14,7 +14,7 @@ matrix:
     - python: nightly
 
 install:
-  - pip install --upgrade pip setuptools coverage black
+  - pip install --upgrade pip setuptools coverage black hypothesmith
   - pip install -e .
 
 script:

bugbear.py

@@ -110,6 +110,15 @@ def should_warn(self, code):
         return False
 
 
+def _to_name_str(node):
+    # Turn Name and Attribute nodes to strings, e.g "ValueError" or
+    # "pkg.mod.error", handling any depth of attribute accesses.
+    if isinstance(node, ast.Name):
+        return node.id
+    assert isinstance(node, ast.Attribute)
+    return _to_name_str(node.value) + "." + node.attr
+
+
 @attr.s
 class BugBearVisitor(ast.NodeVisitor):
     filename = attr.ib()
@@ -141,13 +150,7 @@ def visit_ExceptHandler(self, node):
                 B001(node.lineno, node.col_offset, vars=("bare `except:`",))
             )
         elif isinstance(node.type, ast.Tuple):
-            names = []
-            for e in node.type.elts:
-                if isinstance(e, ast.Name):
-                    names.append(e.id)
-                else:
-                    assert isinstance(e, ast.Attribute)
-                    names.append("{}.{}".format(e.value.id, e.attr))
+            names = [_to_name_str(e) for e in node.type.elts]
             as_ = " as " + node.name if node.name is not None else ""
             if len(names) == 0:
                 vs = ("`except (){}:`".format(as_),)

setup.py

@@ -38,7 +38,7 @@
     py_modules=["bugbear"],
     zip_safe=False,
     python_requires=">=3.6",
-    install_requires=["flake8 >= 3.0.0", "attrs"],
+    install_requires=["flake8 >= 3.0.0", "attrs>=19.2.0"],
     test_suite="tests.test_bugbear",
     classifiers=[
         "Development Status :: 5 - Production/Stable",

tests/b013.py

@@ -28,3 +28,9 @@
 except (re.error,):
     # pointless use of tuple with dotted attribute
     pass
+
+try:
+    pass
+except (a.b.c.d, b.c.d):
+    # attribute of attribute, etc.
+    pass

tests/test_bugbear.py

@@ -1,8 +1,14 @@
+import ast
+import os
 from pathlib import Path
+import site
 import subprocess
 import unittest
 
-from bugbear import BugBearChecker
+from hypothesis import given
+from hypothesmith import from_grammar
+
+from bugbear import BugBearChecker, BugBearVisitor
 from bugbear import (
     B001,
     B002,
@@ -262,5 +268,24 @@ def test_selfclean_test_bugbear(self):
         self.assertEqual(proc.stderr, b"")
 
 
+class TestFuzz(unittest.TestCase):
+    @given(from_grammar().map(ast.parse))
+    def test_does_not_crash_on_any_valid_code(self, syntax_tree):
+        # Given any syntatically-valid source code, flake8-bugbear should
+        # not crash.  This tests doesn't check that we do the *right* thing,
+        # just that we don't crash on valid-if-poorly-styled code!
+        BugBearVisitor(filename="<string>", lines=[]).visit(syntax_tree)
+
+    def test_does_not_crash_on_site_code(self):
+        # Because the generator isn't perfect, we'll also test on all the code
+        # we can easily find in our current Python environment - this includes
+        # the standard library, and all installed packages.
+        for base in sorted(set(site.PREFIXES)):
+            for dirname, _, files in os.walk(base):
+                for f in files:
+                    if f.endswith(".py"):
+                        BugBearChecker(filename=str(Path(dirname) / f))
+
+
 if __name__ == "__main__":
     unittest.main()