bpo-29762: More use "raise from None". (#569)

This hides unwanted implementation details from tracebacks.

3 files changed, 4 insertions, 4 deletions

diff --git a/Lib/lib2to3/patcomp.py b/Lib/lib2to3/patcomp.py
index 06a4b9dd23..0fefa9a3a4 100644
--- a/Lib/lib2to3/patcomp.py
+++ b/Lib/lib2to3/patcomp.py
@@ -59,7 +59,7 @@ class PatternCompiler(object):
         try:
             root = self.driver.parse_tokens(tokens, debug=debug)
         except parse.ParseError as e:
-            raise PatternSyntaxError(str(e))
+            raise PatternSyntaxError(str(e)) from None
         if with_tree:
             return self.compile_node(root), root
         else:
diff --git a/Lib/lib2to3/pgen2/literals.py b/Lib/lib2to3/pgen2/literals.py
index 4f50d31e6b..b9b63e6e55 100644
--- a/Lib/lib2to3/pgen2/literals.py
+++ b/Lib/lib2to3/pgen2/literals.py
@@ -29,12 +29,12 @@ def escape(m):
         try:
             i = int(hexes, 16)
         except ValueError:
-            raise ValueError("invalid hex string escape ('\\%s')" % tail)
+            raise ValueError("invalid hex string escape ('\\%s')" % tail) from None
     else:
         try:
             i = int(tail, 8)
         except ValueError:
-            raise ValueError("invalid octal string escape ('\\%s')" % tail)
+            raise ValueError("invalid octal string escape ('\\%s')" % tail) from None
     return chr(i)
 
 def evalString(s):
diff --git a/Lib/lib2to3/refactor.py b/Lib/lib2to3/refactor.py
index c5a1aa2d0c..70b2a002fb 100644
--- a/Lib/lib2to3/refactor.py
+++ b/Lib/lib2to3/refactor.py
@@ -248,7 +248,7 @@ class RefactoringTool(object):
             try:
                 fix_class = getattr(mod, class_name)
             except AttributeError:
-                raise FixerError("Can't find %s.%s" % (fix_name, class_name))
+                raise FixerError("Can't find %s.%s" % (fix_name, class_name)) from None
             fixer = fix_class(self.options, self.fixer_log)
             if fixer.explicit and self.explicit is not True and \
                     fix_mod_path not in self.explicit: