bpo-39889: Fix ast.unparse() for subscript. (GH-18824)

Author: serhiy-storchaka

Lib/ast.py

@@ -1261,7 +1261,17 @@ def visit_Subscript(self, node):
  self.set_precedence(_Precedence.ATOM, node.value)
  self.traverse(node.value)
  with self.delimit("[", "]"):
- self.traverse(node.slice)
+ if (isinstance(node.slice, Index)
+ and isinstance(node.slice.value, Tuple)
+ and node.slice.value.elts):
+ if len(node.slice.value.elts) == 1:
+ elt = node.slice.value.elts[0]
+ self.traverse(elt)
+ self.write(",")
+ else:
+ self.interleave(lambda: self.write(", "), self.traverse, node.slice.value.elts)
+ else:
+ self.traverse(node.slice)
 
  def visit_Starred(self, node):
  self.write("*")
@@ -1286,7 +1296,12 @@ def visit_Slice(self, node):
  self.traverse(node.step)
 
  def visit_ExtSlice(self, node):
- self.interleave(lambda: self.write(", "), self.traverse, node.dims)
+ if len(node.dims) == 1:
+ elt = node.dims[0]
+ self.traverse(elt)
+ self.write(",")
+ else:
+ self.interleave(lambda: self.write(", "), self.traverse, node.dims)
 
  def visit_arg(self, node):
  self.write(node.arg)

Lib/test/test_unparse.py

@@ -344,6 +344,20 @@ def test_simple_expressions_parens(self):
  self.check_src_roundtrip("call((yield x))")
  self.check_src_roundtrip("return x + (yield x)")
 
+ def test_subscript(self):
+ self.check_src_roundtrip("a[i]")
+ self.check_src_roundtrip("a[i,]")
+ self.check_src_roundtrip("a[i, j]")
+ self.check_src_roundtrip("a[()]")
+ self.check_src_roundtrip("a[i:j]")
+ self.check_src_roundtrip("a[:j]")
+ self.check_src_roundtrip("a[i:]")
+ self.check_src_roundtrip("a[i:j:k]")
+ self.check_src_roundtrip("a[:j:k]")
+ self.check_src_roundtrip("a[i::k]")
+ self.check_src_roundtrip("a[i:j,]")
+ self.check_src_roundtrip("a[i:j, k]")
+
  def test_docstrings(self):
  docstrings = (
  '"""simple doc string"""',

Misc/NEWS.d/next/Library/2020-03-07-16-44-51.bpo-39889.3RYqeX.rst

@@ -0,0 +1,3 @@
+Fixed :func:`ast.unparse` for extended slices containing a single element
+(e.g. ``a[i:j,]``). Remove redundant tuples when index with a tuple (e.g.
+``a[i, j]``).