Conditional block cannot find variable in higher level blocks

[ktlichkid]

Conditional block will automatically try to find the variables in its parent blocks. However, if there are nested blocks, some variables may reside in its parent's parent block. In that case, the conditional block will not be able to find them and cause an error.

Here is a minimal environment to reproduce the issue:

import unittest
import contextlib

import paddle.fluid.core as core
import paddle.fluid.layers as layers
import paddle.fluid.framework as framework
from paddle.fluid.executor import Executor
from paddle.fluid.framework import default_startup_program

class TestHelper(object):
    def __init__(self):
        self._counter = layers.zeros(shape=[1], dtype='int64')
        self._max_len = layers.fill_constant(
            shape=[1], dtype='int64', value=10)
        self._cond = layers.less_than(
            x=self._counter,
            y=layers.fill_constant(
                shape=[1], dtype='int64', value=10))
        self._while_op = layers.While(self._cond)

    @contextlib.contextmanager
    def block(self):
        with self._while_op.block():
            yield
            # A nested block structure to reproduce the issue. 
            # Variable fill_constant cannot be found in parent block, 
            # but could be found in parent's parent block.
            # Conditional block cannot handle this and will raise an error.
            with layers.Switch() as switch:
                with switch.case(self._cond):
                    layers.increment(x=self._counter, value=1.0, in_place=True)
                    layers.less_than(
                        x=self._counter, y=self._max_len, cond=self._cond)


class TestNested(unittest.TestCase):
    def setUp(self):
        helper = TestHelper()
        var = layers.fill_constant(
                shape=[1], dtype='int64', value=7)
        with helper.block():
            layers.Print(var,
                message="The test is to print this var each step")

    def testNested(self):
        cpu = core.CPUPlace()
        exe = Executor(cpu)
        exe.run(default_startup_program())
        exe.run(framework.default_main_program())


if __name__ == '__main__':
    unittest.main()

[pkuyym]

We have figured out where the problem is, in conditional block, currently we only look up vars in one upper level block which means parent block, however in the above example, some vars are defined in two upper level block which means parent block of parent block. Please refer to the following code: https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/fluid/layers/control_flow.py#L1099-L1102

[pkuyym]

A potential fix would be that we can look up the vars in recursive way.