[fix] calculator: call python using subprocess.Popen

call Python directly with -S and -I parameters to skip loading
of the standard library and the SearXNG module.

The actual calculator is moved to a standalone script: calculator_process.py

searx/plugins/calculator/calculator_process.py

@@ -0,0 +1,58 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Standalone script to actually calculate mathematical expressions using ast
+
+This is not a module, the SearXNG modules are not available here
+"""
+
+import ast
+import sys
+import operator
+from typing import Callable
+
+
+operators: dict[type, Callable] = {
+    ast.Add: operator.add,
+    ast.Sub: operator.sub,
+    ast.Mult: operator.mul,
+    ast.Div: operator.truediv,
+    ast.Pow: operator.pow,
+    ast.BitXor: operator.xor,
+    ast.USub: operator.neg,
+}
+
+
+def _eval_expr(expr):
+    """
+    >>> _eval_expr('2^6')
+    4
+    >>> _eval_expr('2**6')
+    64
+    >>> _eval_expr('1 + 2*3**(4^5) / (6 + -7)')
+    -5.0
+    """
+    try:
+        return _eval(ast.parse(expr, mode='eval').body)
+    except ZeroDivisionError:
+        # This is undefined
+        return ""
+
+
+def _eval(node):
+    if isinstance(node, ast.Constant) and isinstance(node.value, (int, float)):
+        return node.value
+
+    if isinstance(node, ast.BinOp):
+        return operators[type(node.op)](_eval(node.left), _eval(node.right))
+
+    if isinstance(node, ast.UnaryOp):
+        return operators[type(node.op)](_eval(node.operand))
+
+    raise TypeError(node)
+
+
+def main():
+    print(_eval_expr(sys.argv[1]), end="")
+
+
+if __name__ == "__main__":
+    main()