一个用 python 实现的简单python解释器
分版本逐步实现完整功能，适合初学者学习解释器的工作原理
only support single-digit integers +
支持整数的加法运算
support multi-digit integers +/-, support process whitespace
支持加法减法，支持处理表达式中的空格
support to parse (recognize) and interpret arithmetic expressions that have any number of plus or minus operators in it, for example “7 - 3 + 2 - 1”.
支持包含多个数字的加减表达式
support to parse and interpret arithmetic expressions with any number of multiplication and division operators in them, for example “7 * 4 / 2 * 3”
支持包含多个数字的乘除表达式
support to handle valid arithmetic expressions containing integers and any number of addition, subtraction, multiplication, and division operators.
支持包含多个数字的加减乘除混合表达式
support to evaluates arithmetic expressions that have different operators and parentheses.
支持包含括号的混合表达式处理
using ASTs represent the operator-operand model of arithmetic expressions. 支持使用 AST (abstract syntax tree 抽象语法树）来表示算术表达式
python genastdot.py "7 + 3 * (10 / (12 / (3 + 1) - 1))" > ast.dot && dot -Tpng -o ast.png ast.dot
执行之前需要先按照dot, 参考：https://graphviz.org/
support unary operators (+, -)
python genastdot.py "5---2" > ast.dot && dot -Tpng -o ast_v8.png ast.dot 
support to handle python assignment statements.
python interpreter.py assignments.txtpython genastdot.py assignments.txt > ast.dot && dot -Tpng -o ast_v9.png ast.dot