Error Handling

Error handling: try/except instead of try/catch

Python’s error handling maps directly onto TypeScript’s try/catch/finally — with one powerful addition: an else clause that runs only when NO exception was raised. The keywords are try/except/else/finally, and you raise exceptions instead of throw.

Basic try / except

// TypeScript
function divide(a: number, b: number): number {
  if (b === 0) throw new Error("Cannot divide by zero");
  return a / b;
}

try {
  const result = divide(10, 2);
  console.log(`Result: ${result}`);
} catch (err) {
  if (err instanceof Error) {
    console.error(`Error: ${err.message}`);
  }
} finally {
  console.log("Always runs");
}

# Python
def divide(a: float, b: float) -> float:
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

try:
    result = divide(10, 2)
    print(f"Result: {result}")
except ValueError as e:
    print(f"Error: {e}")
finally:
    print("Always runs")

Key differences:

except instead of catch.
raise instead of throw.
You catch a specific exception type directly (e.g. except ValueError), not a generic value.
The as e syntax binds the exception object.

Multiple exception types

// TypeScript
try {
  const data = JSON.parse("{ bad json }");
  const num = parseInt("abc", 10);
  if (isNaN(num)) throw new RangeError("NaN");
} catch (err) {
  if (err instanceof SyntaxError) {
    console.error("JSON error:", err.message);
  } else if (err instanceof RangeError) {
    console.error("Range error:", err.message);
  } else {
    throw err;  // re-raise unknown errors
  }
}

# Python — multiple except clauses
import json

try:
    data = json.loads("{ bad json }")
    value = int("abc")
except json.JSONDecodeError as e:
    print(f"JSON error: {e}")
except ValueError as e:
    print(f"Value error: {e}")
except Exception as e:
    raise  # re-raise any other exception

# You can also catch multiple types in one clause:
# except (ValueError, TypeError) as e:
#     print(f"Either error: {e}")

Custom exception classes

// TypeScript
class AppError extends Error {
  constructor(
    message: string,
    public readonly code: number
  ) {
    super(message);
    this.name = "AppError";
  }
}

try {
  throw new AppError("Not found", 404);
} catch (err) {
  if (err instanceof AppError) {
    console.log(`[${err.code}] ${err.message}`);
  }
}

# Python
class AppError(Exception):
    def __init__(self, message: str, code: int) -> None:
        super().__init__(message)
        self.code = code

try:
    raise AppError("Not found", 404)
except AppError as e:
    print(f"[{e.code}] {e}")

Try it

Open in an online REPL

def divide(a: float, b: float) -> float:
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

# Successful case
try:
    result = divide(10, 2)
    print(f"10 / 2 = {result}")
except ValueError as e:
    print(f"Error: {e}")
else:
    print("No error occurred (else clause)")
finally:
    print("Always runs")

print()

# Error case
try:
    result = divide(10, 0)
except ValueError as e:
    print(f"Caught: {e}")
else:
    print("This will NOT print")
finally:
    print("Finally again")

🟦 The else clause on try — no TypeScript equivalent

TypeScript’s try/catch has no else clause. Python’s else runs only if the try block completed without raising any exception — after the try block, before finally. This is a clean way to separate “what to do on success” from “what to do on error” without nesting:

import json

def parse_config(raw: str) -> dict:
    try:
        data = json.loads(raw)
    except json.JSONDecodeError as e:
        print(f"Invalid JSON: {e}")
        return {}
    else:
        # Runs only if json.loads succeeded
        print(f"Parsed {len(data)} keys")
        return data
    finally:
        # Always runs — good for cleanup
        print("parse_config done")

The else clause avoids a common bug: if you put the success-path code inside the try block, any exception it raises is caught by the except, which may be unintended.