uneval 0.1.2

Library for generating python-expressions

UnEval

UnEval is a microlibrary for generating python-expressions.

If you ever need to use eval, write macros or implement domain specific languages, this library provides a better way to generate python expressions than using strings. Strings can contain syntax errors, make it harder to deal with parentheses and aren't syntax highlighted. Expressions look and act a lot like pythoncode, except that they aren't evaluated immediately.

Installation

Make sure to install pip then run:

pip install uneval

Usage

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

Examples

import ast
from uneval import quote, to_ast, to_bytecode

# Build expressions
x, y = quote.x, quote.y  # Shortcut for quote("x"), quote("y")
z = x * x + y * y

# 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_bytecode(z), {"x": 3, "y": 4}))  # 25

This can be used when working in pandas and you want to use eval or query:

from uneval import quote as q

# No syntax highlighting. Syntax checkers won't catch errors.
df.eval("bmi = mass / height**2")

# With syntax highlighting. Syntax checkers can catch errors here.
df.eval(f"bmi = {q.mass / q.height**2}")

Expressions can also be converted to λ-functions:

from uneval import F, quote

x, y = quote.x, quote.y

hello = F("Hello World!")
hello()  # => "Hello World!"

plus1 = F.x(x + 1)
plus1(4)  # => 5

multiply = F.xy(x * y)
multiply(5, 7)  # => 35

Building blocks

from uneval import quote as q

Factory	Example	Result
quote	q('a') or q.a (shortcut)	a
if_	if_(q.x >= 0, q.x, -q.x)	x if x >= 0 else -x
for_	for_(q.x**2, (q.x, q.range(5)))	(x**2 for x in range(5))
lambda_	lambda_([q.x], q.x * q.x)	lambda x: x * x
and_, or_	and_(q.x >= 10, q.x <= 15)	x >= 10 and x <= 15
not_, in_	not_(in_(q.x, {1, 2, 3}))	not x in {1, 2, 3}
fstr, fmt	fstr("sin(", q.a, ") is ", fmt(q.sin(q.a), ".3"))	f'sin({a}) is {sin(a):.3}'

Converters

Converter	Target	Remark
str	String	Convert to readable python
to_ast	AST-node	Convert to AST-node
to_bytecode	Code-object	Compile the expression
F.parameters	Function	Create a λ-function

Similar work

Libraries that implement something similar:

• Macropy has quasiquote.
• SymPy - Symbolic manipulation, but its representation is different from Python.
• Polars - Writing col.x creates something like this Expression-object.

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.
• latexify and pytexit - Convert python to LaTeX.
• numexpr and aesara / pytensor - Fast evaluation of numerical expressions.

Useful references:

• python macros use cases - Stack-overflow discussion.
• Green tree snakes - Unofficial documentation of AST nodes.