在内部类变量分配内访问类变量

Question

I would like to use a class variable as part of the definition of another one within the same class as follows:

import enum, struct

class Message:
    """ExtendedIO related message IDs and `Struct.struct`s."""

    # protocol
    BYTE_ORDER = "<"

    # Message ID's
    class ID(enum.IntEnum):
        """Message ID's definitions."""
        EX_IO = 0x01
        ALIVE = 0x02
        RESETCOMPONENT = 0x03

    STRUCTS = {
        ID[message_id]: struct.Struct(
            ''.join((BYTE_ORDER, format)))
        for message_id, format in {
            'EX_IO': "B",          # command
            'ALIVE': "HH",         # version, version
            'RESETCOMPONENT': ""}  #
        .items()}

This gives a NameError: name 'BYTE_ORDER' is not defined .

I can do:

import enum, struct

class Message:
    """ExtendedIO related message IDs and `Struct.struct`s."""

    # protocol
    BYTE_ORDER = "<"

    # Message ID's
    class ID(enum.IntEnum):
        """Message ID's definitions."""
        EX_IO = 0x01
        ALIVE = 0x02
        RESETCOMPONENT = 0x03

class Message(Message):

    STRUCTS = {
        Message.ID[message_id]: struct.Struct(
            ''.join((Message.BYTE_ORDER, format)))
        for message_id, format in {
            'EX_IO': "B",          # command, *
            'ALIVE': "HH",         # version, version
            'RESETCOMPONENT': ""}  #
        .items()}

And it works, but the extra class Message(Message): looks ugly and pylint complains with an E0102 .

The benefit of the way Message.STRUCT is assigned is that it uses a very readable table. (The actual code I am working on is a bit more complex.)

Based on the comments, I tried the following code, which works:

import enum, struct

class Message:
    """ExtendedIO related LWF message IDs and `Struct.struct`s."""

    # protocol
    BYTE_ORDER = "<"

    # Message ID's
    class ID(enum.IntEnum):
        """Message ID's definitions."""
        EX_IO = 0x01
        ALIVE = 0x02
        RESETCOMPONENT = 0x03

    EXAMPLE = ''.join((BYTE_ORDER, '1'))

    STRUCTS = lambda ID, BYTE_ORDER: {
        ID[message_id]: struct.Struct(
            ''.join((BYTE_ORDER, format)))
        for message_id, format in {
            'EX_IO': "B",          # command
            'ALIVE': "HH",         # version, version
            'RESETCOMPONENT': ""}  #
        .items()}(ID, BYTE_ORDER)

So:

• Directly using a class variable works.
• Inserting the variables into the comprehension using a lambda also works.
• Using the extra class Message(Message): works. (and importlib.reload() works on the first code example, but the result is wrong...)

Is there a sane way of using a class variable within another class variable in the same class in Python?

Answer 1

Depending on the way you make use of the variables being class instead of instance variables, you could also change an existing class variable in your constructor. This way, your variable is still a class variable shared by all instances of the class.

class Message:
    BYTE_ORDER = "<"
    STRUCTS = {}

    def __init__(self):
        STRUCTS = self.SomethingDependantOn(self.BYTE_ORDER)    

You would not be able to access the variable without instantiating an object first though.

Answer 2

You can access this variable using self

class Message:
    """ExtendedIO related message IDs and `Struct.struct`s."""

    # protocol
    BYTE_ORDER = "<"

    # Message ID's
    class ID(self, enum.IntEnum):
        """Message ID's definitions."""
        EX_IO = 0x01
        ALIVE = 0x02
        RESETCOMPONENT = 0x03

    def something(self):
        STRUCTS = {
            ID[message_id]: struct.Struct(
                ''.join((self.BYTE_ORDER, format)))
            for message_id, format in {
                'EX_IO': "B",          # command
                'ALIVE': "HH",         # version, version
                'RESETCOMPONENT': ""}  #
            .items()}

or just set your varible global and access it by your classes

# protocol
BYTE_ORDER = "<"

class foo():
    def something():
        print(BYTE_ORDER)

class bar():
     def something():
        print(BYTE_ORDER)