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|
Read-only Tree
==============
**Application**: A read-only tree data structure, which denies modifications.
The `Node._pre_attach` and `Node._pre_detach` hookups can be used
for blocking tree modifications.
If they raise an `Exception`, the tree is not modified.
>>> from anytree import NodeMixin, RenderTree
The exception:
>>> class ReadOnlyError(RuntimeError):
... pass
Permanent
---------
The read-only attribute needs to be set after attaching to parent:
>>> class ReadOnlyNode(NodeMixin):
...
... def __init__(self, foo, parent=None):
... super(ReadOnlyNode, self).__init__()
... self.foo = foo
... self.__readonly = False
... self.parent = parent
... self.__readonly = True
...
... def _pre_attach(self, parent):
... if self.__readonly:
... raise ReadOnlyError()
...
... def _pre_detach(self, parent):
... raise ReadOnlyError()
An example tree:
>>> a = ReadOnlyNode("a")
>>> a0 = ReadOnlyNode("a0", parent=a)
>>> a1 = ReadOnlyNode("a1", parent=a)
>>> a1a = ReadOnlyNode("a1a", parent=a1)
>>> a2 = ReadOnlyNode("a2", parent=a)
>>> print(RenderTree(a).by_attr("foo"))
a
├── a0
├── a1
│ └── a1a
└── a2
Modifications raise an `ReadOnlyError`
>>> a0.parent = a2
Traceback (most recent call last):
...
ReadOnlyError
>>> a.children = [a1]
Traceback (most recent call last):
...
ReadOnlyError
The tree structure is untouched:
>>> print(RenderTree(a).by_attr("foo"))
a
├── a0
├── a1
│ └── a1a
└── a2
.. note::
It is important to use the ``_pre_*`` and **not** the ``_post_*`` methods.
An exception raised by `_pre_detach(parent)` and `_pre_attach(parent)` will **prevent** the tree structure to be updated.
The node keeps the old state.
An exception raised by `_post_detach(parent)` and `_post_attach(parent)` does **not rollback** the tree structure modification.
Temporary
---------
To select the read-only mode temporarily, the root node should provide
an attribute for all child nodes, set *after* construction.
>>> class ReadOnlyNode(NodeMixin):
... def __init__(self, foo, parent=None):
... super(ReadOnlyNode, self).__init__()
... self.readonly = False
... self.foo = foo
... self.parent = parent
... def _pre_attach(self, parent):
... if self.root.readonly:
... raise ReadOnlyError()
... def _pre_detach(self, parent):
... if self.root.readonly:
... raise ReadOnlyError()
An example tree:
>>> a = ReadOnlyNode("a")
>>> a0 = ReadOnlyNode("a0", parent=a)
>>> a1 = ReadOnlyNode("a1", parent=a)
>>> a1a = ReadOnlyNode("a1a", parent=a1)
>>> a2 = ReadOnlyNode("a2", parent=a)
>>> print(RenderTree(a).by_attr("foo"))
a
├── a0
├── a1
│ └── a1a
└── a2
Switch to read-only mode:
>>> a.readonly = True
>>> a0.parent = a2
Traceback (most recent call last):
...
ReadOnlyError
>>> a.children = [a1]
Traceback (most recent call last):
...
ReadOnlyError
Disable read-only mode:
>>> a.readonly = False
Modifications are allowed now:
>>> a0.parent = a2
>>> print(RenderTree(a).by_attr("foo"))
a
├── a1
│ └── a1a
└── a2
└── a0
|
