Properties with validation in Python

A property turns attribute access into a method call, so invariants hold.

class Run:
    def __init__(self, accuracy):
        self.accuracy = accuracy

    @property
    def accuracy(self):
        return self._accuracy

    @accuracy.setter
    def accuracy(self, value):
        if not 0 <= value <= 100:
            raise ValueError(f"accuracy out of range: {value}")
        self._accuracy = float(value)


run = Run(97.5)
print(run.accuracy)
try:
    run.accuracy = 140
except ValueError as exc:
    print(exc)

How it works

1. The getter is decorated with @property.
2. The setter is @name.setter and validates before storing.
3. Callers still write plain attribute syntax.

Keywords and builtins used here

• Run
• accuracy
• as
• class
• def
• except
• float
• if
• print
• raise
• return
• self
• try

The run, in numbers

Lines

21

Characters to type

370

Tokens

95

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 54 seconds.

Type this snippet

Step 2 of 3 in Attributes & descriptors, step 33 of 53 in Pythonic Python.

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