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avoid C++ -> Python -> C++ overheads when passing around function objects
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@ -185,21 +185,32 @@ The following interactive session shows how to call them from Python.
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>>> plus_1(number=43)
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>>> plus_1(number=43)
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44L
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44L
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.. note::
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This functionality is very useful when generating bindings for callbacks in
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C++ libraries (e.g. a graphical user interface library).
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The file :file:`example/example5.cpp` contains a complete example that
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demonstrates how to work with callbacks and anonymous functions in more detail.
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.. warning::
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.. warning::
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Keep in mind that passing a function from C++ to Python (or vice versa)
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Keep in mind that passing a function from C++ to Python (or vice versa)
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will instantiate a piece of wrapper code that translates function
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will instantiate a piece of wrapper code that translates function
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invocations between the two languages. Copying the same function back and
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invocations between the two languages. Naturally, this translation
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forth between Python and C++ many times in a row will cause these wrappers
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increases the computational cost of each function call somewhat. A
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to accumulate, which can decrease performance.
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problematic situation can arise when a function is copied back and forth
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between Python and C++ many times in a row, in which case the underlying
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wrappers will accumulate correspondingly. The resulting long sequence of
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C++ -> Python -> C++ -> ... roundtrips can significantly decrease
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performance.
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There is one exception: pybind11 detects case where a stateless function
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(i.e. a function pointer or a lambda function without captured variables)
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is passed as an argument to another C++ function exposed in Python. In this
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case, there is no overhead. Pybind11 will extract the underlying C++
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function pointer from the wrapped function to sidestep a potential C++ ->
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Python -> C++ roundtrip. This is demonstrated in Example 5.
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.. note::
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This functionality is very useful when generating bindings for callbacks in
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C++ libraries (e.g. GUI libraries, asynchronous networking libraries, etc.).
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The file :file:`example/example5.cpp` contains a complete example that
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demonstrates how to work with callbacks and anonymous functions in more detail.
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Overriding virtual functions in Python
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Overriding virtual functions in Python
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======================================
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======================================
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@ -65,6 +65,29 @@ py::cpp_function test_callback5() {
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py::arg("number"));
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py::arg("number"));
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}
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}
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int dummy_function(int i) { return i + 1; }
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int dummy_function2(int i, int j) { return i + j; }
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std::function<int(int)> roundtrip(std::function<int(int)> f) {
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std::cout << "roundtrip.." << std::endl;
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return f;
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}
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void test_dummy_function(const std::function<int(int)> &f) {
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using fn_type = int (*)(int);
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auto result = f.target<fn_type>();
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if (!result) {
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std::cout << "could not convert to a function pointer." << std::endl;
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auto r = f(1);
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std::cout << "eval(1) = " << r << std::endl;
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} else if (*result == dummy_function) {
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std::cout << "argument matches dummy_function" << std::endl;
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auto r = (*result)(1);
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std::cout << "eval(1) = " << r << std::endl;
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} else {
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std::cout << "argument does NOT match dummy_function. This should never happen!" << std::endl;
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}
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}
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void init_ex5(py::module &m) {
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void init_ex5(py::module &m) {
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py::class_<Pet> pet_class(m, "Pet");
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py::class_<Pet> pet_class(m, "Pet");
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pet_class
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pet_class
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@ -113,4 +136,10 @@ void init_ex5(py::module &m) {
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/* p should be cleaned up when the returned function is garbage collected */
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/* p should be cleaned up when the returned function is garbage collected */
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};
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};
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});
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});
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/* Test if passing a function pointer from C++ -> Python -> C++ yields the original pointer */
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m.def("dummy_function", &dummy_function);
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m.def("dummy_function2", &dummy_function2);
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m.def("roundtrip", &roundtrip);
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m.def("test_dummy_function", &test_dummy_function);
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}
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}
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@ -54,3 +54,30 @@ f = test_callback5()
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print("func(number=43) = %i" % f(number=43))
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print("func(number=43) = %i" % f(number=43))
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test_cleanup()
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test_cleanup()
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from example import dummy_function
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from example import dummy_function2
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from example import test_dummy_function
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from example import roundtrip
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test_dummy_function(dummy_function)
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test_dummy_function(roundtrip(dummy_function))
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test_dummy_function(lambda x: x + 2)
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try:
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test_dummy_function(dummy_function2)
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print("Problem!")
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except Exception as e:
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if 'Incompatible function arguments' in str(e):
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print("All OK!")
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else:
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print("Problem!")
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try:
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test_dummy_function(lambda x, y: x + y)
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print("Problem!")
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except Exception as e:
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if 'missing 1 required positional argument' in str(e):
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print("All OK!")
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else:
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print("Problem!")
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@ -1,20 +1,13 @@
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Rabbit is a parrot
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Rabbit is a parrot
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Rabbit is a parrot
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Polly is a parrot
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Polly is a parrot
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Polly is a parrot
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Molly is a dog
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Molly is a dog
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Molly is a dog
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Woof!
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Woof!
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func(43) = 44
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Payload constructor
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Payload copy constructor
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Payload move constructor
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Payload destructor
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Payload destructor
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Payload destructor
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Rabbit is a parrot
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Polly is a parrot
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Molly is a dog
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The following error is expected: Incompatible function arguments. The following argument types are supported:
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The following error is expected: Incompatible function arguments. The following argument types are supported:
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1. (example.Dog) -> NoneType
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1. (example.Dog) -> NoneType
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Invoked with: <Pet object at 0>
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Invoked with: <example.Pet object at 0>
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Callback function 1 called!
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Callback function 1 called!
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False
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False
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Callback function 2 called : Hello, x, True, 5
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Callback function 2 called : Hello, x, True, 5
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@ -24,4 +17,22 @@ False
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Callback function 3 called : Partial object with one argument
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Callback function 3 called : Partial object with one argument
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False
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False
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func(43) = 44
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func(43) = 44
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func(43) = 44
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func(number=43) = 44
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func(number=43) = 44
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Payload constructor
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Payload copy constructor
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Payload move constructor
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Payload destructor
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Payload destructor
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Payload destructor
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argument matches dummy_function
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eval(1) = 2
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roundtrip..
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argument matches dummy_function
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eval(1) = 2
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could not convert to a function pointer.
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eval(1) = 3
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could not convert to a function pointer.
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All OK!
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could not convert to a function pointer.
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All OK!
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@ -113,6 +113,9 @@ struct function_record {
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/// True if name == '__init__'
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/// True if name == '__init__'
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bool is_constructor : 1;
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bool is_constructor : 1;
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/// True if this is a stateless function pointer
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bool is_stateless : 1;
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/// True if the function has a '*args' argument
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/// True if the function has a '*args' argument
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bool has_args : 1;
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bool has_args : 1;
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@ -23,6 +23,29 @@ public:
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src_ = detail::get_function(src_);
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src_ = detail::get_function(src_);
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if (!src_ || !PyCallable_Check(src_.ptr()))
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if (!src_ || !PyCallable_Check(src_.ptr()))
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return false;
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return false;
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{
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/*
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When passing a C++ function as an argument to another C++
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function via Python, every function call would normally involve
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a full C++ -> Python -> C++ roundtrip, which can be prohibitive.
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Here, we try to at least detect the case where the function is
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stateless (i.e. function pointer or lambda function without
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captured variables), in which case the roundtrip can be avoided.
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*/
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if (PyCFunction_Check(src_.ptr())) {
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capsule c(PyCFunction_GetSelf(src_.ptr()), true);
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auto rec = (function_record *) c;
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using FunctionType = Return (*) (Args...);
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if (rec && rec->is_stateless && rec->data[1] == &typeid(FunctionType)) {
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struct capture { FunctionType f; };
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value = ((capture *) &rec->data)->f;
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return true;
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}
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}
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}
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object src(src_, true);
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object src(src_, true);
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value = [src](Args... args) -> Return {
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value = [src](Args... args) -> Return {
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gil_scoped_acquire acq;
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gil_scoped_acquire acq;
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@ -35,7 +58,11 @@ public:
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template <typename Func>
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template <typename Func>
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static handle cast(Func &&f_, return_value_policy policy, handle /* parent */) {
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static handle cast(Func &&f_, return_value_policy policy, handle /* parent */) {
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return cpp_function(std::forward<Func>(f_), policy).release();
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auto result = f_.template target<Return (*)(Args...)>();
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if (result)
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return cpp_function(*result, policy).release();
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else
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return cpp_function(std::forward<Func>(f_), policy).release();
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}
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}
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PYBIND11_TYPE_CASTER(type, _("function<") +
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PYBIND11_TYPE_CASTER(type, _("function<") +
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@ -82,6 +82,9 @@ protected:
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/* Store the capture object directly in the function record if there is enough space */
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/* Store the capture object directly in the function record if there is enough space */
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if (sizeof(capture) <= sizeof(rec->data)) {
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if (sizeof(capture) <= sizeof(rec->data)) {
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/* Without these pragmas, GCC warns that there might not be
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enough space to use the placement new operator. However, the
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'if' statement above ensures that this is the case. */
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#if defined(__GNUG__) && !defined(__clang__) && __GNUC__ >= 6
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#if defined(__GNUG__) && !defined(__clang__) && __GNUC__ >= 6
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# pragma GCC diagnostic push
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# pragma GCC diagnostic push
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# pragma GCC diagnostic ignored "-Wplacement-new"
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# pragma GCC diagnostic ignored "-Wplacement-new"
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@ -118,7 +121,7 @@ protected:
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capture *cap = (capture *) (sizeof(capture) <= sizeof(rec->data)
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capture *cap = (capture *) (sizeof(capture) <= sizeof(rec->data)
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? &rec->data : rec->data[0]);
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? &rec->data : rec->data[0]);
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/* Perform the functioncall */
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/* Perform the function call */
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handle result = cast_out::cast(args_converter.template call<Return>(cap->f),
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handle result = cast_out::cast(args_converter.template call<Return>(cap->f),
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rec->policy, parent);
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rec->policy, parent);
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@ -140,6 +143,16 @@ protected:
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if (cast_in::has_args) rec->has_args = true;
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if (cast_in::has_args) rec->has_args = true;
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if (cast_in::has_kwargs) rec->has_kwargs = true;
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if (cast_in::has_kwargs) rec->has_kwargs = true;
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/* Stash some additional information used by an important optimization in 'functional.h' */
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using FunctionType = Return (*)(Args...);
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constexpr bool is_function_ptr =
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std::is_convertible<Func, FunctionType>::value &&
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sizeof(capture) == sizeof(void *);
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if (is_function_ptr) {
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rec->is_stateless = true;
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rec->data[1] = (void *) &typeid(FunctionType);
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}
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}
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}
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/// Register a function call with Python (generic non-templated code goes here)
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/// Register a function call with Python (generic non-templated code goes here)
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else if (a.value)
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else if (a.value)
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a.descr = strdup(((std::string) ((object) handle(a.value).attr("__repr__"))().str()).c_str());
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a.descr = strdup(((std::string) ((object) handle(a.value).attr("__repr__"))().str()).c_str());
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}
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}
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auto const ®istered_types = detail::get_internals().registered_types_cpp;
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auto const ®istered_types = detail::get_internals().registered_types_cpp;
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/* Generate a proper function signature */
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/* Generate a proper function signature */
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rec->name = strdup("__nonzero__");
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rec->name = strdup("__nonzero__");
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}
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}
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#endif
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#endif
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rec->signature = strdup(signature.c_str());
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rec->signature = strdup(signature.c_str());
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rec->args.shrink_to_fit();
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rec->args.shrink_to_fit();
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rec->is_constructor = !strcmp(rec->name, "__init__") || !strcmp(rec->name, "__setstate__");
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rec->is_constructor = !strcmp(rec->name, "__init__") || !strcmp(rec->name, "__setstate__");
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rec->is_stateless = false;
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rec->has_args = false;
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rec->has_args = false;
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rec->has_kwargs = false;
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rec->has_kwargs = false;
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rec->nargs = (uint16_t) args;
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rec->nargs = (uint16_t) args;
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