validate if is valite for several output

validate if is valite for several output

https://github.com/apache/incubator-wayang/blob/4cc0bfdca06dda171b661822daae5fa438d9d475/python/src/pywy/platforms/python/execution.py#L81

```python

#
# Licensed to the Apache Software Foundation (ASF) under one or more
# contributor license agreements. See the NOTICE file distributed with
# this work for additional information regarding copyright ownership.
# The ASF licenses this file to You 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.
#

from pywy.graph.types import WGraphOfOperator, NodeOperator
from pywy.core import ChannelDescriptor
from pywy.core import Executor
from pywy.core import PywyPlan
from pywy.operators import TextFileSource
from pywy.platforms.python.channels import PY_ITERATOR_CHANNEL_DESCRIPTOR
from pywy.platforms.python.operator.py_execution_operator import PyExecutionOperator

class PyExecutor(Executor):

def __init__(self):
super(PyExecutor, self).__init__()

def execute(self, plan):
pywyPlan: PywyPlan = plan
graph = WGraphOfOperator(pywyPlan.sinks)

# TODO get this information by a configuration and ideally by the context
descriptor_default: ChannelDescriptor = PY_ITERATOR_CHANNEL_DESCRIPTOR
files_pool = []

def execute(op_current: NodeOperator, op_next: NodeOperator):
if op_current is None:
return

py_current: PyExecutionOperator = op_current.current
if py_current.outputs == 0:
py_current.execute(py_current.inputChannel, [])
return

if op_next is None:
return
py_next: PyExecutionOperator = op_next.current
outputs = py_current.get_output_channeldescriptors()
inputs = py_next.get_input_channeldescriptors()

intersect = outputs.intersection(inputs)
if len(intersect) == 0:
raise Exception(
"The operator(A) {} can't connect with (B) {}, "
"because the output of (A) is {} and the input of (B) is {} ".format(
py_current,
py_next,
outputs,
inputs
)
)

if len(intersect) > 1:
if descriptor_default is None:
raise Exception(
"The interaction between the operator (A) {} and (B) {}, "
"can't be decided because are several channel availables {}".format(
py_current,
py_next,
intersect
)
)
descriptor = descriptor_default
else:
descriptor = intersect.pop()

# TODO validate if is valite for several output
py_current.outputChannel[0] = descriptor.create_instance()

py_current.execute(py_current.inputChannel, py_current.outputChannel)

py_next.inputChannel = py_current.outputChannel

if isinstance(py_current, TextFileSource):
files_pool.append(py_current.outputChannel[0].provide_iterable())

graph.traversal(graph.starting_nodes, execute)
# close the files used during the execution
for f in files_pool:
f.close()

```

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