Update DeprecatedABCImport Rule (#474)

* Updated rule to catch case where import is wrapped in try block

* Satisfied typechecker

* Added comments

* Updated to use ParentNodeProvider

* Updated to walk up the node instead of checking a deterministic level

* Fixed typo

src/fixit/rules/deprecated_abc_import.py

@@ -6,9 +6,10 @@
 from typing import List, Optional, Union
 
 import libcst as cst
-
 import libcst.matchers as m
 
+from libcst.metadata import ParentNodeProvider
+
 from fixit import Invalid, LintRule, Valid
 
 
@@ -54,6 +55,7 @@ class DeprecatedABCImport(LintRule):
 
     MESSAGE = "ABCs must be imported from collections.abc"
     PYTHON_VERSION = ">= 3.3"
+    METADATA_DEPENDENCIES = (ParentNodeProvider,)
 
     VALID = [
         Valid("from collections.abc import Container"),
@@ -71,6 +73,47 @@ def test(self):
                     pass
             """
         ),
+        Valid(
+            """
+            try:
+                from collections.abc import Mapping
+            except ImportError:
+                from collections import Mapping
+            """
+        ),
+        Valid(
+            """
+            try:
+                from collections.abc import Mapping, Container
+            except ImportError:
+                from collections import Mapping, Container
+            """
+        ),
+        Valid(
+            """
+            try:
+                from collections.abc import Mapping, Container
+            except ImportError:
+                def fallback_import():
+                    from collections import Mapping, Container
+            """
+        ),
+        Valid(
+            """
+            try:
+                from collections.abc import Mapping, Container
+            except Exception:
+                exit()
+            """
+        ),
+        Valid(
+            """
+            try:
+                from collections import defaultdict
+            except Exception:
+                exit()
+            """
+        ),
     ]
     INVALID = [
         Invalid(
@@ -122,10 +165,27 @@ def __init__(self) -> None:
         # The original imports
         self.imports_names: List[str] = []
 
+    def is_except_block(self, node: cst.CSTNode) -> bool:
+        """
+        Check if the node is in an except block - if it is, we know to ignore it, as it
+        may be a fallback import
+        """
+        parent = self.get_metadata(ParentNodeProvider, node)
+        while not isinstance(parent, cst.Module):
+            if isinstance(parent, cst.ExceptHandler):
+                return True
+
+            parent = self.get_metadata(ParentNodeProvider, parent)
+
+        return False
+
     def visit_ImportFrom(self, node: cst.ImportFrom) -> None:
         """
         This catches the `from collections import <ABC>` cases
         """
+        if self.is_except_block(node):
+            return
+
         # Get imports in this statement
         import_names = (
             [name.name.value for name in node.names]