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+# Copyright 2016 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""Lowers list comprehensions into for and if statements.
+
+Example:
+
+ result = [x * x for x in xs]
+
+becomes
+
+ result = []
+ for x in xs:
+ elt = x * x
+ result.append(elt)
+"""
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import gast
+
+from tensorflow.contrib.autograph.core import converter
+from tensorflow.contrib.autograph.pyct import templates
+
+
+# TODO(mdan): This should covert directly to operator calls.
+
+
+class ListCompTransformer(converter.Base):
+ """Lowers list comprehensions into standard control flow."""
+
+ def visit_Assign(self, node):
+ if not isinstance(node.value, gast.ListComp):
+ return self.generic_visit(node)
+ if len(node.targets) > 1:
+ raise NotImplementedError('multiple assignments')
+
+ target, = node.targets
+ list_comp_node = node.value
+
+ template = """
+ target = []
+ """
+ initialization = templates.replace(template, target=target)
+
+ template = """
+ target.append(elt)
+ """
+ body = templates.replace(template, target=target, elt=list_comp_node.elt)
+
+ for gen in reversed(list_comp_node.generators):
+ for gen_if in reversed(gen.ifs):
+ template = """
+ if test:
+ body
+ """
+ body = templates.replace(template, test=gen_if, body=body)
+ template = """
+ for target in iter_:
+ body
+ """
+ body = templates.replace(
+ template, iter_=gen.iter, target=gen.target, body=body)
+
+ return initialization + body
+
+
+def transform(node, ctx):
+ return ListCompTransformer(ctx).visit(node)