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main.py

import dataclasses
from typing import List

@dataclasses.dataclass
class AverageableQueue:
    '''
    A queue of numbers with a O(1) average() operation.
    inv: self._total == sum(self._values)
    '''
    _values: List[float]
    _total: float

    def push(self, val: float):
        '''
        pre: not math.isnan(val)
        '''
        self._values.append(val)
        self._total += val

    def pop(self) -> float:
        ''' pre: len(self._values) > 0 '''
        val = self._values.pop(0)
        # Oops. We are forgetting to do something here.
        self._total -= val
        return val

    def average(self) -> float:
        ''' pre: self._values '''
        return self._total / len(self._values)