python-stdlib/enum/enum.py: Add Enum class.

python-stdlib/enum/README.md

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+# Enum Library
+
+This library provides a lightweight, memory-efficient `Enum` implementation designed for MicroPython environments. It focuses on immutability, reverse lookup capabilities, and serialization support without the complexity of metaclasses.
+
+---
+
+## Core Features
+* **Immutability**: Enum members (`EnumValue`) are protected against modification. Any attempt to change their name or value raises an `AttributeError`.
+* **Static Design**: Once an Enum instance is initialized, it is "frozen." You cannot add new attributes or delete existing members.
+* **Dual Reverse Lookup**:
+    * **Class Constructor**: Retrieve a member by value using the class name (e.g., `Status(1)`).
+    * **Instance Call**: Retrieve a member by value by calling the instance (e.g., `s(1)`).
+* **Serialization Support**: Implements `__repr__` such that `obj == eval(repr(obj))`, allowing easy restoration of Enum states.
+* **Functional API**: Supports dynamic creation of Enums at runtime.
+
+---
+
+## Usage Examples
+
+### 1. Standard Class Definition
+Define your enumeration by inheriting from the `Enum` class. Class-level constants are automatically converted into `EnumValue` objects upon initialization.
+
+```python
+from enum import Enum
+
+class Color(Enum):
+    RED = 'red'
+    GREEN = 'green'
+
+# Initialize the enum to process attributes
+c = Color()
+
+print(c.RED)                  # Output: RED: red
+print(c.RED.name)             # Output: RED
+print(c.RED.value)            # Output: red
+print(c.RED())                # Output: red
+print(c.is_value("RED"))      # Output: true
+print(c.is_value(Color.RED))  # Output: true
+print(c.is_value('red'))      # Output: true
+print(c.list())               # Output: [Color.RED: red, Color.GREEN: green]
+print([m for m in c])         # Output: [Color.RED: red, Color.GREEN: green]
+print([m.name for m in c])    # Output: ['RED', 'GREEN']
+print([m.value for m in c])   # Output: ['red', 'green']
+```
+
+
+### 2. Reverse Lookup
+The library provides two ways to find a member based on its raw value.
+
+```python
+class Status(Enum):
+    IDLE = 0
+    RUNNING = 1
+
+# Method A: Via Class (Simulates interpreting hardware/network bytes)
+# Uses __new__ logic to return the correct EnumValue
+current_status = Status(1)
+print(current_status.name)   # Output: RUNNING
+print(current_status.value)  # Output: 1
+print(current_status)        # Output: Status.RUNNING: 1
+print(current_status())      # Output: 1
+
+# Method B: Via Instance Call
+s = Status()
+print(s(0).name)            # Output: IDLE
+print(s(0).value)           # Output: 0
+print(s(0))                 # Output: Status.IDLE: 0
+print(s(0)())               # Output: 0
+```
+
+
+### 3. Functional API (Dynamic Creation)
+If you need to create an Enum from external data (like a JSON config), use the functional constructor.
+
+```python
+# Create a dynamic Enum instance
+State = Enum(name='State', names={'ON': 1, 'OFF': 2})
+
+print(State)                  # Output: Enum(name='State', names={'ON': 1, 'OFF': 2})
+print(State.ON)               # Output: State.ON: 1
+print(State.ON.name)          # Output: ON
+print(State.ON.value)         # Output: 1
+print(State.ON())             # Output: 1
+assert State.ON == 1          # Comparison
+assert State.ON() == 1        #
+assert State.ON.value == 1    #
+assert State.ON.name == "ON"  #
+```
+
+
+### 4. Serialization (Repr / Eval)
+The library ensures that the string representation can be used to perfectly reconstruct the object.
+
+```python
+from enum import Enum
+
+class Color(Enum):
+    RED = 'red'
+    GREEN = 'green'
+    BLUE = 3
+
+colors = Color()
+# Get serialized string
+serialized = repr(colors)
+# Reconstruct object
+restored_colors = eval(serialized)
+
+print(f"Original: {colors}")           # Output: Original: Enum(name='Color', names={'BLUE': 3, 'RED': 'red', 'GREEN': 'green'})
+print(f"Restored: {restored_colors}")  # Output: Restored: Enum(name='Color', names={'BLUE': 3, 'RED': 'red', 'GREEN': 'green'})
+print(colors == restored_colors)       # Output: True
+```
+
+
+---
+
+## API Reference
+
+### `EnumValue`
+The object representing a specific member of an Enum.
+* `.name`: The string name of the member.
+* `.value`: The raw value associated with the member.
+* `()`: Calling the member object returns its raw value (e.g., `c.RED() -> 'red'`).
+
+### `Enum`
+The base class for all enumerations.
+* `list()`: Returns a list of all defined members.
+* `is_value(value)`: Returns `True` if the provided raw value exists within the Enum.
+* `__len__`: Returns the total number of members.
+* `__iter__`: Allows looping through members (e.g., `[m.name for m in color_inst]`).
+
+---
+
+## Error Handling
+* **`AttributeError`**:
+    * Raised when attempting to modify an `EnumValue`.
+    * Raised when attempting to add new members to an initialized Enum.
+    * Raised when a class-level lookup (`Status(999)`) fails.
+    * Raised when an instance-level lookup (`s(999)`) fails.
+
+## Compare with CPython
+
+```python
+# Run on MicroPython v1.28.0 on 2026-04-06; Generic ESP32 module with ESP32
+# Run on Python 3.12.10
+from enum import Enum
+
+# class syntax
+class Color(Enum):
+    RED = 1
+    GREEN = 2
+    BLUE = 3
+
+# OR
+# functional syntax
+# Color = Enum('Color', {'RED': 1, 'GREEN': 2, 'BLUE': 3})
+
+# List enum members
+try:
+    print(list(Color))
+# [<Color.RED: 1>, <Color.GREEN: 2>, <Color.BLUE: 3>]
+except:
+    print(Color.list())
+# [RED: 1, GREEN: 2, BLUE: 3]
+
+# Accessing enum member by name
+print(Color.GREEN, type(Color.GREEN))
+# Color.GREEN  <enum 'Color'>
+# GREEN: 2 <class 'EnumValue'>
+
+# Accessing enum member by name
+try:
+    print(Color['GREEN'])
+# Color.GREEN
+except:
+    print(Color('GREEN'))
+# GREEN: 2
+
+# Accessing enum member by value
+print(Color(2))
+# Color.GREEN
+
+# Accessing enum member name
+print(Color.GREEN.name, type(Color.GREEN.name))
+#  GREEN <class 'str'>
+
+# Accessing enum member value
+print(Color.GREEN.value, type(Color.GREEN.value))
+# 2 <class 'int'>
+```
+
+### Output is:
+
+| MicroPython v1.28.0  |  Python 3.12.10  |
+|   :---   |   :---  |
+| [Color.RED: 1, Color.GREEN: 2, Color.BLUE: 3] | [<Color.RED: 1>, <Color.GREEN: 2>, <Color.BLUE: 3>] |
+| Color.GREEN: 2 <class 'EnumValue'> | Color.GREEN <enum 'Color'> |
+| Color.GREEN: 2 | Color.GREEN |
+| Color.GREEN: 2 | Color.GREEN |
+| GREEN <class 'str'> | GREEN <class 'str'> |
+| 2 <class 'int'> | 2 <class 'int'> |
+