Tinypost

Tinypost is a tiny infix-to-postfix expression parser and interpreter for math expressions. Quickly evaluate math expressions quickly with just a few lines of code. Tinypost supports many of the functions in the python math module.

Features

• No external dependencies
• Fast performance
• Abides by standard operator precedence
• Implements many mathematical functions in the math module
• Easily bind custom variables and functions
• MIT License
• Local stacks
• No recursion

Install

Install easily using the following command.

pip install tinypost

Simple Example

The following code evaluates an expression at runtime.

import tinypost as tp
print(tp.eval_expr("3 + 12 / 4")) # Prints 6.0

Binding Variables

Setting and updating your own custom variables is simple.

import tinypost as tp

x = 5.5
tp.add_var("x", x)
print(tp.eval_expr("(x + 4) / 2"))) # Prints 4.75

x = 3.7
tp.update_var("x", x)
print(tp.eval_expr("(x + 4) / 2"))) # Prints 3.85

long_var_name = 5.5
tp.add_var("long_var_name", long_var_name)
print(tp.eval_expr("long_var_name * 3 / 6.23"))) # Prints 2.648...

Variable names must start with letter and can only contain letters, underscores, and numbers. You can also use multiple variables in a single expression.

Optimizing Variable Usage

If you find yourself evalulating the same expression over and over again but just updating the value of the variable, you can try the following:

import tinypost as tp

x = 3
tp.update_var("x", x)

expr = tp.compile_expr("x^2 + 4")

for i in range(3):
    x = i
    tp.update_var("x", i)
    print(tp.eval_compiled(expr), end=" ") # Prints 4 5 8

This will run faster than if you were to call tp.eval_expr() for each iteration of the loop.

Mathematical Functions

Tinypost support many mathematical functions, including abs, sqrt, sin, cos, tan, ln, acos, asin, atan, atan2, exp, log, sinh, cosh, tanh, max, and pow. When calling a function, make sure to put a pair of parentheses around its arguments, and if it has multiple arguments, separate each argument with a comma.

import tinypost as tp
expr = "max(sin(3 + 4 / 3.5), 0.6)"
print(tp.eval_expr(expr)) # Prints 0.6

If a function you would like to use is not supported by Tinypost or you would like to bind your own functions, you can do the following.

import tinypost as tp

def my_weird_sum(a, b):
    weird_num = 2.3551
    return (a / weird_num) ** 2 + b

# Functions cannot have optional parameters, they
# must have a constant number of arguments.
# Functions must return a singular int or float.

tp.add_func("my_weird_sum", my_weird_sum)
expr = "my_weird_sum(3, 9.2) + 3"
print(tp.eval_expr(expr)) # Prints 13.822...

Also, the constants pi and e are available for your use.

How This Works

Tinypost uses Dijkstra's Shunting Yard Algorithm to convert infix expressions to postfix expressions (Tinypost uses right postfix notation). Afterwards, Tinypost uses a postfix expressions evaluation algorithm to evaluate the compiled expression. Both algorithms make extensive use of stacks, meaning that popping and pushing to the stack is an O(1) process. The entire algorithm runs in about O(n) time because it has to loop through every token in the expression.

Notes

Tinypost raises python SyntaxErrors if it finds an error in an expresion you passed in. Variables, functions, and modules prefixed by an _ are meant for internal use only. Using these incorrectly could result in unpredictable behavior.