mirror of
https://github.com/pocketpy/pocketpy
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73 lines
1.9 KiB
Markdown
73 lines
1.9 KiB
Markdown
---
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icon: dot
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label: 'Access Attributes'
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order: 80
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---
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### Direct access
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Some python objects have an instance dict, a.k.a, `__dict__` in cpython.
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You can use `obj->attr()` to manipulate the instance dict of an object.
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```cpp
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VM* vm = new VM();
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// get the `builtin` module
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PyVar builtins = vm->builtins;
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// get `dict` type
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PyVar dict = builtins->attr("dict");
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// set `pi = 3.14`
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builtins->attr().set("pi", py_var(vm, 3.14));
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```
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However, you cannot call `attr` on an object which does not have an instance dict.
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For example, the `int` object.
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```cpp
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// create a `int` object
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PyVar obj = py_var(vm, 1);
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// THIS IS WRONG!! WILL LEAD TO A SEGFAULT!!
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PyVar add = obj->attr("__add__");
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```
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To determine whether an object has instance dict or not, you can use this snippet.
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```cpp
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// 1. call `is_tagged` to check the object supports `->` operator
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// 2. call `is_attr_valid` to check the existence of instance dict
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PyVar obj = py_var(vm, 1);
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bool ok = !is_tagged(obj) && obj->is_attr_valid(); // false
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```
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### General access
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As you can see, direct access does not take care of derived attributes or methods.
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In most cases, what you need is `getattr` and `setattr`.
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These two methods handle all possible cases.
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#### `PyVar getattr(PyVar obj, StrName name, bool throw_err=true)`
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This method is equivalent to `getattr` in python.
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If the attribute is not found, it will return `nullptr`
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or throw an `AttributeError` depending on the value of `throw_err`.
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```cpp
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// create a `int` object
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PyVar obj = py_var(vm, 1);
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// get its `__add__` method, which is a `bound_method` object
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PyVar add = vm->getattr(obj, "__add__");
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// call it (equivalent to `1 + 2`)
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PyVar ret = vm->call(add, py_var(vm, 2););
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// get the result
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int result = py_cast<int>(vm, ret);
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std::cout << result << std::endl; // 3
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```
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#### `void setattr(PyVar, StrName, PyVar)`
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This method is equivalent to `setattr` in python.
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It raises `TypeError` if the object does not support attribute assignment.
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