Catching Errors with try and except

Why this matters

So far an error has meant a crash. Real programs meet bad input, missing files and surprises all the time, and a good program decides what to do about them instead of dying. Python lets you catch an error and carry on.

The idea

text = "abc"
try:
    number = int(text)          # Python tries this...
except ValueError:              # ...and if a ValueError happens, jumps here
    number = 0
print(number)                   # 0

Name the kind of error you expect. Catching too much hides real bugs. A few more parts:

try:
    price = float(text)
except ValueError as err:       # "as err" keeps the error so you can show it
    print("Not a number:", err)
else:                           # runs only if nothing went wrong
    print("Price is", price)
finally:                        # always runs, error or not
    print("done")

One clause can catch several kinds: except (ValueError, TypeError):.

Try it

Run the example with text = "4.5", then "abc", then "". Which of else and finally ran each time? Then try int(None) inside the try: that is a TypeError, so the except ValueError clause does not catch it.

Your turn

Write three functions:

• to_int(text, default=0): return int(text), or default if the text is not a whole number. Anything that is not a ValueError should still crash (for example to_int(None) raises a TypeError).
• safe_divide(a, b): return a / b, or None when b is zero.
• get_age(person): person is a dictionary. Return int(person["age"]). If there is no "age" key return "unknown". If the value cannot be turned into a whole number (text like "abc", or None) return "invalid".

Recap

• try runs code that might fail; except SomeError handles that one kind of failure.
• Catch the specific errors you expect so real bugs still show up.
• else runs when nothing failed; finally always runs.

Common mistakes

• A bare except: that catches everything. It even hides typos (NameError). Name the error.
• Putting too much inside try. Keep it to the lines that can actually fail.
• Forgetting the colon or indentation after try: / except ...:, which gives a SyntaxError or IndentationError.