Merge pull request #103 from realpython/python-eval-mathrepl

Python eval mathrepl

python-eval-mathrepl/mathrepl.py

+#!/usr/bin/env python3
+
+"""MathREPL, a math expression evaluator using Python's eval() and math."""
+
+import math
+
+__version__ = "1.0"
+__author__ = "Leodanis Pozo Ramos"
+
+ALLOWED_NAMES = {
+    k: v for k, v in math.__dict__.items() if not k.startswith("__")
+}
+
+PS1 = "mr>>"
+
+WELCOME = f"""
+MathREPL {__version__}, your Python math expressions evaluator!
+Enter a valid math expression after the prompt "{PS1}".
+Type "help" for more information.
+Type "quit" or "exit" to exit.
+"""
+
+USAGE = f"""
+Usage:
+Build math expressions using numeric values and operators.
+Use any of the following functions and constants:
+
+{', '.join(ALLOWED_NAMES.keys())}
+"""
+
+
+def evaluate(expression):
+    """Evaluate a math expression."""
+    # Compile the expression eventually raising a SyntaxError
+    # when the user enters an invalid expression
+    code = compile(expression, "<string>", "eval")
+
+    # Validate allowed names
+    for name in code.co_names:
+        if name not in ALLOWED_NAMES:
+            raise NameError(f"The use of '{name}' is not allowed")
+
+    # Evaluate the expression eventually raising a ValueError
+    # when the user uses a math function with a wrong input value
+    # e.g. math.sqrt(-10)
+    return eval(code, {"__builtins__": {}}, ALLOWED_NAMES)
+
+
+def main():
+    """Main loop: Read and evaluate user's input."""
+    print(WELCOME)
+    while True:
+        # Read user's input
+        try:
+            expression = input(f"{PS1} ")
+        except (KeyboardInterrupt, EOFError):
+            raise SystemExit()
+
+        # Handle special commands
+        if expression.lower() == "help":
+            print(USAGE)
+            continue
+        if expression.lower() in {"quit", "exit"}:
+            raise SystemExit()
+
+        # Evaluate the expression and handle errors
+        try:
+            result = evaluate(expression)
+        except SyntaxError:
+            # If the user enters an invalid expression
+            print("Invalid input expression syntax")
+            continue
+        except (NameError, ValueError) as err:
+            # If the user tries to use a name that isn't allowed
+            # or an invalid value to a given math function
+            print(err)
+            continue
+
+        # Print the result if no error occurs
+        print(f"The result is: {result}")
+
+
+if __name__ == "__main__":
+    main()