bug: generic function expression fails to parse

[MoAly98]

Hi,

I am using pyhs3==0.3.0 and trying to write a Generic function which is very simple:

import pyhs3
from pyhs3.functions import GenericFunction

print(f"pyhs3 version: {pyhs3.__version__}")

# This fails with: unsupported operand type(s) for +: 'One' and 'FunctionClass'
config = {
    "type": "generic_function",
    "name": "beta_factor",
    "expression": "1 + beta"
}
func = GenericFunction(**config)

Running this, I get the following parsing failure:

/Users/moaly/Work/iris-hep/everwillow/.venv/lib/python3.12/site-packages/pyhs3/core.py:15: FutureWarning: `pytensor.graph.basic.applys_between` was moved to `pytensor.graph.traversal.applys_between`. Calling it from the old location will fail in a future release.
  from pytensor.graph.basic import applys_between, graph_inputs
/Users/moaly/Work/iris-hep/everwillow/.venv/lib/python3.12/site-packages/pyhs3/core.py:15: FutureWarning: `pytensor.graph.basic.graph_inputs` was moved to `pytensor.graph.traversal.graph_inputs`. Calling it from the old location will fail in a future release.
  from pytensor.graph.basic import applys_between, graph_inputs
pyhs3 version: 0.3.0
ValueError: Error from parse_expr with transformed code: 'Integer (1 )+beta '

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/moaly/Work/iris-hep/everwillow/.venv/lib/python3.12/site-packages/pyhs3/generic_parse.py", line 78, in parse_expression
    return sympy_parser.parse_expr(expr_str, transformations=transformations)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/moaly/Work/iris-hep/everwillow/.venv/lib/python3.12/site-packages/sympy/parsing/sympy_parser.py", line 1090, in parse_expr
    raise e from ValueError(f"Error from parse_expr with transformed code: {code!r}")
  File "/Users/moaly/Work/iris-hep/everwillow/.venv/lib/python3.12/site-packages/sympy/parsing/sympy_parser.py", line 1081, in parse_expr
    rv = eval_expr(code, local_dict, global_dict)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/moaly/Work/iris-hep/everwillow/.venv/lib/python3.12/site-packages/sympy/parsing/sympy_parser.py", line 905, in eval_expr
    expr = eval(
           ^^^^^
  File "<string>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'One' and 'FunctionClass'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/moaly/Work/iris-hep/everwillow/test_genericfunction_bug.py", line 14, in <module>
    func = GenericFunction(**config)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/moaly/Work/iris-hep/everwillow/.venv/lib/python3.12/site-packages/pydantic/main.py", line 250, in __init__
    validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/moaly/Work/iris-hep/everwillow/.venv/lib/python3.12/site-packages/pyhs3/functions.py", line 158, in setup_expression
    self._sympy_expr = parse_expression(self.expression_str)
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/moaly/Work/iris-hep/everwillow/.venv/lib/python3.12/site-packages/pyhs3/generic_parse.py", line 81, in parse_expression
    raise ExpressionParseError(msg) from exc
pyhs3.exceptions.ExpressionParseError: Failed to parse expression '1 + beta': unsupported operand type(s) for +: 'One' and 'FunctionClass'