Calc Interpreter

An interpreter modeled after a calculator implemented in Python 3. The program currently only supports basic mathematical expressions. The package uses Reverse Polish Notation also known as postfix expression internally to easily represent mathematical expressions. This is powerful as it allows operators to be organised in such a way that precedence is absolute meaning an operator that is encountered first will always be executed first.

However, it is obvious that postfix is not normal or rather not usually taught in schools and thus infix expression is still the way for the user to write expressions. The interpreter uses a modified Shunting-Yard algorithm to produce Abstract Syntax Trees. Evaluation of ASTs is trivial, and they are flexible making it easy to extend the grammar and functionality later on.


The package is available in PyPI and can be installed via pip.

pip install expr-calc
python3 -m pip install expr-calc


The best way to run the program currently is to execute the REPL and can be done in a python file or through your terminal.

Assuming your present working directory is inside the cloned repo, you can run the following command without the comment.

# inside /clone_path/expr_calc/
python -m expr_calc

The test suite can also be ran with pytest when inside the cloned repo

pytest  # or python -m pytest


Once inside the REPL, you can start evaluating expressions. Currently, only operators listed in Features are supported.

calc> 1 + 1

calc> 345--500

calc> -2

calc> 123 ^ 4

calc> 32 / 1.5

calc> 123 * 456

calc> 34 % 5

calc> 4 ^ 1/2

calc> 4 ^ (1/2)


  • Infix expressions
  • Basic operators such as +, -, *, /, %, ^
  • Tokens created from an expression can also be fetched to be manipulated if one wanted to do so
  • Expressions are transformed into m-ary Tree objects connected to each other

Features I want to add later

  • variable support
  • custom functions
  • more mathematical functions such as sin, cos, tan, etc
  • and possibly a simple symbolic computation support


GitHub - lestherll/expr_calc: Expression interpreter written in Python
Expression interpreter written in Python. Contribute to lestherll/expr_calc development by creating an account on GitHub.