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.. SPDX-FileCopyrightText: 2017 Ole Martin Bjorndalen <ombdalen@gmail.com>
..
.. SPDX-License-Identifier: CC-BY-4.0
Frozen Messages
---------------
.. versionadded:: 1.2
Since Mido messages are *mutable* (can change) they can not be hashed or
put in dictionaries. This makes it hard to use them for things like
Markov chains.
In these situations you can use *frozen messages*:
.. code-block:: python
from mido.frozen import FrozenMessage
msg = FrozenMessage('note_on')
d = {msg: 'interesting'}
*Frozen messages* are used and behave in exactly the same way as normal
messages with one exception: **attributes are not settable**.
There are also variants for meta messages (``FrozenMetaMessage`` and
``FrozenUnknownMetaMessage``).
You can *freeze* and *thaw* messages with:
.. code-block:: python
from mido.frozen import freeze_message, thaw_message
frozen = freeze_message(msg)
thawed = thaw_message(frozen)
``thaw_message()`` will always return a *copy*. Passing a *frozen message*
to ``freeze_message()`` will return the original message.
Both functions return ``None`` if you pass ``None`` which is handy for
things like:
.. code-block:: python
msg = freeze_message(port.receive())
for msg in map(freeze_message, port):
...
To check if a message is *frozen*:
.. code-block:: python
from mido.frozen import is_frozen
if is_frozen(msg):
...
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