79 lines
3.2 KiB
C++
79 lines
3.2 KiB
C++
/* Copyright 2020 The TensorFlow Authors. All Rights Reserved.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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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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#ifndef TENSORFLOW_PYTHON_FRAMEWORK_PY_CONTEXT_MANAGER_H_
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#define TENSORFLOW_PYTHON_FRAMEWORK_PY_CONTEXT_MANAGER_H_
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#include <Python.h>
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#include <string>
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#include "tensorflow/python/lib/core/safe_pyobject_ptr.h"
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namespace tensorflow {
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// Class that wraps a Python context manager, and calls the `__enter__` and
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// `__exit__` methods at appropriate times:
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//
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// * When `PyContextManager::Enter(cm)` is called, the context manager `cm`
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// is stored, and `cm.__enter__` is called. The result can be retrieved
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// with `PyContextManager::var()`.
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// * When the `PyContextManager` is destroyed, then `cm.__exit__` is called
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// (with information about any active exception).
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// * `PyContextManager::Enter(cm)` may be called at most once. If
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// `PyContextManager::Enter()` is never called, then the destructor is a
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// no-op (i.e., `__exit__` is not called).
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//
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// PyContextManager places two restrictons on the wrapped context managers:
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//
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// 1. The context manager may not suppress exceptions -- i.e., `__exit__`
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// may not return a True value. If it does, then a new exception will be
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// set, indicating that this is unuspported.
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// 2. The context manager may not raise an exception from `__exit__` if the
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// an exception is not active when it is called. If it does, then an error
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// message will be logged, indicating that this is unsupported, and the
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// exception will be suppressed.
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//
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// These restrictions are both intended to ensure that the state of
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// PyErr_Occured is unchanged by PyContextManager's destructor. This is
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// important, because changing the state of PyErr_Occurred in the destructor
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// would mean that we are returning a nullptr with no exception set, or
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// returning a non-null value with an exception set (both of which are invalid).
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class PyContextManager {
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public:
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// Calls `py_context_manager.__enter__()`, and stores the result in `var`.
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// Return true if `__enter__` succeeds, or false if `__enter__` raises an
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// exception. (Also returns false if `py_context_manager` is nullptr.)
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//
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// Steals a reference to `py_context_manager`. (This reference is deleted
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// when the destructor is called.)
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bool Enter(PyObject* py_context_manager);
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// Calls `py_context_manager.__exit__()`.
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~PyContextManager();
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// Returns the variable returned by `context_manager.__enter__()`.
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// (This is the `var` bound by `with context_manager as var`.)
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// Returns a borrowed reference.
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PyObject* var() { return var_.get(); }
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protected:
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Safe_PyObjectPtr context_manager_;
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Safe_PyObjectPtr var_;
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};
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} // namespace tensorflow
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#endif // TENSORFLOW_PYTHON_FRAMEWORK_PY_CONTEXT_MANAGER_H_
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