48 lines
1.3 KiB
Python
48 lines
1.3 KiB
Python
"""Evaluator for the arithmetic AST produced by calc.parser.
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Result-type rule: if a computation yields a float that is whole-valued
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(e.g. 4/2 == 2.0), it is coerced to int before returning. Non-whole
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floats (e.g. 7/2 == 3.5) are returned as float.
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"""
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from .parser import BinOp, Num, Unary
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class EvalError(Exception):
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"""Raised on a runtime evaluation error (e.g. division by zero)."""
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def _coerce(value):
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if isinstance(value, float) and value == int(value):
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return int(value)
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return value
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def evaluate(node):
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"""Walk an AST node and return int | float.
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Raises:
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EvalError: on division by zero.
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"""
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if isinstance(node, Num):
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return _coerce(node.value)
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if isinstance(node, Unary):
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return _coerce(-evaluate(node.operand))
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if isinstance(node, BinOp):
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left = evaluate(node.left)
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right = evaluate(node.right)
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op = node.op
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if op == '+':
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return _coerce(left + right)
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if op == '-':
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return _coerce(left - right)
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if op == '*':
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return _coerce(left * right)
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if op == '/':
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if right == 0:
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raise EvalError("division by zero")
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return _coerce(left / right)
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raise EvalError(f"unknown node type: {type(node).__name__}")
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