$7\mathrm{.}26\mathrm{.}2$: Write a function that computes the value of an arithmetic expression.

$599086\mathrm{.}4644418.$qx$3$zqy$7$

Write a function that computes the value of an arithmetic expression. The operator string should be one of $("+","-","*",$ or $"/").$ Otherwise, the function should raise a ValueError exception with the message invalid operator. Also raise a ValueError exception with the message value$2$ cannot be $0$ if the operator is $"/"$ and the second argument is zero.