try except else finally in Python

A try statement can have four parts. Each one runs at a different time:

• try: the code that might fail.
• except: runs only if that code raised a matching exception.
• else: runs only if the try block finished with no exception.
• finally: always runs, error or not. It is the place to clean up, for example to close a file.

The program

# create two sample files for this example
with open("good.txt", "w") as f:
    f.write("42\n")
with open("bad.txt", "w") as f:
    f.write("forty-two\n")


def read_number(filename):
    print(f"[{filename}]")
    file = None
    try:
        file = open(filename)
        number = int(file.read())
    except FileNotFoundError:
        print("  except: the file does not exist")
    except ValueError:
        print("  except: the file does not contain a whole number")
    else:
        print(f"  else: the number is {number}")
    finally:
        if file is not None:
            file.close()
            print("  finally: file closed")
        else:
            print("  finally: no file to close")


read_number("good.txt")
read_number("bad.txt")
read_number("missing.txt")

[good.txt]
  else: the number is 42
  finally: file closed
[bad.txt]
  except: the file does not contain a whole number
  finally: file closed
[missing.txt]
  except: the file does not exist
  finally: no file to close

How it works

• For good.txt nothing fails, so both except blocks are skipped, else runs, and then finally runs.
• For bad.txt the file opens, but int("forty-two\n") raises a ValueError. The ValueError block runs, else is skipped, and finally still closes the file.
• For missing.txt, open() itself fails, so file is still None and there is nothing to close. That is why file = None is set before the try.
• Why use else instead of putting the print inside try? Code in else is not protected by the except blocks. Keep the try block short, so that you only catch errors from the lines you expect to fail.

For files, a with block closes the file for you, so in real code you rarely need finally for this. The context manager program shows how with works. finally is still useful for other clean-up, such as closing a network connection or a database.

finally runs even after return

finally runs even when the try block leaves the function with return:

def check():
    try:
        print("try: about to return")
        return "the return value"
    finally:
        print("finally: runs before the function returns")


print(check())

try: about to return
finally: runs before the function returns
the return value

Related programs: try except example.