71 lines
2.2 KiB
Python
71 lines
2.2 KiB
Python
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#
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# Licensed to the Apache Software Foundation (ASF) under one or more
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# contributor license agreements. See the NOTICE file distributed with
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# this work for additional information regarding copyright ownership.
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# The ASF licenses this file to You under the Apache License, Version 2.0
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# (the "License"); you may not use this file except in compliance with
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# the License. You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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#
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"""
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Module defining global singleton classes.
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This module raises a RuntimeError if an attempt to reload it is made. In that
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way the identities of the classes defined here are fixed and will remain so
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even if pyspark itself is reloaded. In particular, a function like the following
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will still work correctly after pyspark is reloaded:
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def foo(arg=pyspark._NoValue):
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if arg is pyspark._NoValue:
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...
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See gh-7844 for a discussion of the reload problem that motivated this module.
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Note that this approach is taken after from NumPy.
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"""
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__ALL__ = ['_NoValue']
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# Disallow reloading this module so as to preserve the identities of the
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# classes defined here.
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if '_is_loaded' in globals():
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raise RuntimeError('Reloading pyspark._globals is not allowed')
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_is_loaded = True
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class _NoValueType(object):
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"""Special keyword value.
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The instance of this class may be used as the default value assigned to a
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deprecated keyword in order to check if it has been given a user defined
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value.
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This class was copied from NumPy.
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"""
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__instance = None
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def __new__(cls):
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# ensure that only one instance exists
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if not cls.__instance:
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cls.__instance = super(_NoValueType, cls).__new__(cls)
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return cls.__instance
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# needed for python 2 to preserve identity through a pickle
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def __reduce__(self):
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return (self.__class__, ())
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def __repr__(self):
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return "<no value>"
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_NoValue = _NoValueType()
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