Python › Descriptors & Properties

How does @property's read-only behavior (no setter defined) actually raise an AttributeError, and how would you customize that error message?

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Answer

If only a getter is defined (no @x.setter), attempting to ASSIGN to the property raises `AttributeError: can't set attribute` by default; to provide a more descriptive custom message, explicitly define a setter that always raises your own AttributeError with a clearer explanation.

class ReadOnly:
    @property
    def value(self):
        return 42
    @value.setter
    def value(self, new_value):
        raise AttributeError("'value' is read-only and cannot be modified after creation")

r = ReadOnly()
# r.value = 99  # AttributeError: 'value' is read-only and cannot be modified after creation

Real-world example Providing a clearer, more helpful error message when code mistakenly tries to set a computed, read-only property.

Common follow-ups: Is there a performance or design reason to prefer the default terse error over writing this more descriptive custom one?

Exception Handling