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1# Copyright 2016 The TensorFlow Authors. All Rights Reserved.
2#
3# Licensed under the Apache License, Version 2.0 (the "License");
4# you may not use this file except in compliance with the License.
5# You may obtain a copy of the License at
6#
7#     http://www.apache.org/licenses/LICENSE-2.0
8#
9# Unless required by applicable law or agreed to in writing, software
10# distributed under the License is distributed on an "AS IS" BASIS,
11# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12# See the License for the specific language governing permissions and
13# limitations under the License.
14# ==============================================================================
15"""Lowers list comprehensions into for and if statements.
16
17Example:
18
19  result = [x * x for x in xs]
20
21becomes
22
23  result = []
24  for x in xs:
25    elt = x * x
26    result.append(elt)
27"""
28
29import gast
30
31from tensorflow.python.autograph.core import converter
32from tensorflow.python.autograph.pyct import templates
33
34
35# TODO(mdan): This should covert directly to operator calls.
36
37
38class ListCompTransformer(converter.Base):
39  """Lowers list comprehensions into standard control flow."""
40
41  def visit_Assign(self, node):
42    if not isinstance(node.value, gast.ListComp):
43      return self.generic_visit(node)
44    if len(node.targets) > 1:
45      raise NotImplementedError('multiple assignments')
46
47    target, = node.targets
48    list_comp_node = node.value
49
50    template = """
51      target = []
52    """
53    initialization = templates.replace(template, target=target)
54
55    template = """
56      target.append(elt)
57    """
58    body = templates.replace(template, target=target, elt=list_comp_node.elt)
59
60    for gen in reversed(list_comp_node.generators):
61      for gen_if in reversed(gen.ifs):
62        template = """
63          if test:
64            body
65        """
66        body = templates.replace(template, test=gen_if, body=body)
67      template = """
68        for target in iter_:
69          body
70      """
71      body = templates.replace(
72          template, iter_=gen.iter, target=gen.target, body=body)
73
74    return initialization + body
75
76
77def transform(node, ctx):
78  return ListCompTransformer(ctx).visit(node)
79