uneval 0.0.1

Library for generating python-expressions

UnEval

UnEval is a microlibrary for generating python-expressions, which can be converted to runnable pythoncode.

Some use cases of this library are:

• Manipulation of the python-abstract syntax tree.
• Writing domain specific languages.
• Code generation.

I have the intention to build something on top of this later that converts python expressions to linear programming constraints.

Usage

Firstly, the building blocks can be used to generate expressions. Secondly, these expressions can be converted.

Building blocks

Factory	AST-class	Example
quote	ast.Name	quote('a') or quote.a (shortcut)
if_	ast.IfExpr	if_(quote.x >= 0, quote.x, -quote.x)
for_	ast.GenExpr	for_(quote.x * quote.x, (quote.x, quote.range(5)))
lambda_	ast.Lambda	lambda_([quote.x], quote.x * quote.x)

Converters

Converter	Target	Remark
str	String	Convert to readable python
to_expression	Expression	Parse existing string
to_ast	AST-node	Generates AST-node
to_code	Code-object	Compiles the expression

Examples

# Create variables
x, y = quote.x, quote.y  # Shortcut for quote("x"), quote("y")

# Manipulate
z = x*x + y*y
d = z.abs() + x.pow(3) - y.sin(x)

# Convert
print(z)  # x * x + y * y
print(ast.dump(to_ast(z)))  # BinOp(left=BinOp(left=Name(id='x', ctx=Load()), op=Mult(), right=Name(id='x', ctx=Load())), op=Add(), right=BinOp(left=Name(id='y', ctx=Load()), op=Mult(), right=Name(id='y', ctx=Load())))
print(eval(to_code(z), {"x": 3, "y": 4}))  # 25

These ideas can be used to create alternative syntax for lambda-functions:

from uneval import *


def f_x(expr: Expression):
  """Create a lambda with parameter x."""
  return eval(to_code(lambda_([quote.x], expr)))


square = f_x(x * x)  # Same as lambda x: x * x
print(square(5))  # => 25

Similar work

Libraries that implement something similar:

• Macropy has quasiquote.
• Polars - Writing col.x creates something like this Expression-object.
• SymPy - Symbolic manipulation, but its representation is different from Python.
• pulp - Can build objectives and constraints for solving linear programming problems.
• sqlalchemy - Generates properties based on a schema.
• latexify - Converts python to SQL.

Other:

• Fixing lambda - A blog post about alternative lambda syntaxes.
• Mini-lambda - Packages to "fix" lambda.
• Meta - A few utils to work on AST's.
• python macros use cases - Stack-overflow discussion.

Useful references:

• Green tree snakes