Python: lazily evaluate field for structlog

structlog_lazy_field.py

class LazyField:
    """Evaluates __repr__ lazily for heavy calculations.
    Expects callable with args and kwargs of how to calculate the value.
    """

    def __init__(self, func, *args, **kwargs):
        self._func = func
        self._args = args
        self._kwargs = kwargs
        self._value = ...

    def __repr__(self):
        if self._value is ...:
            value = self._func(*self._args, **self._kwargs)
            self._value = value
        return self._value

# Usage:
import structlog

def expensive_computation(base, *, exponent):
  return base**exponent

logger = structlog.get_logger()
logger.debug("Event", lazy_field=LazyField(expensive_computation, 2, exponent=1000))