Python Tip: try...except

Keep one unexpected bad value from stopping the script

Learn how to use try and except to skip an unexpected bad value without stopping the rest of a loop.

Python Tip: try...except

Does one unexpected bad value cause your entire script to crash?

Use try...except to skip the bad value and keep going!

One Bad Value Stops Everything

Imagine we read a list of prices from a file. Each price is text, so the loop uses float() to convert it to a number. One unexpected entry is "missing".

prices = ["12.50", "missing", "8.25"]

for price_text in prices:
    price = float(price_text)  # This will crash!
    print("Price:", price)

The value "missing" is not a valid number, so float() crashes. The crash stops the loop before it reaches the last value.

Use a try Block

If we want to run code but protect the script when something goes wrong, we can use a try block. A try block looks like this:

price_text = "missing"

try:
    price = float(price_text)
    print("Price:", price)
except:
    print("Could not convert:", price_text)

Could not convert: missing

Python runs the code under try first. float() cannot convert "missing", so the first print() does not run. Python then runs the print() under except: instead.

Put It Inside the Loop

We can put the try block inside the loop so that one bad value does not crash the script. If a price cannot be converted, continue moves the loop to the next value.

prices = ["12.50", "missing", "8.25"]

for price_text in prices:
    try:
        price = float(price_text)
    except:
        print("Skipped:", price_text)
        continue

    print("Price:", price)

Price: 12.5
Skipped: missing
Price: 8.25

The loop skips "missing" and still prints the final price.

Catch Only Certain Errors

So far, except: has caught every error. Python gives each type of error a name. Bad number text causes ValueError, while division by zero causes ZeroDivisionError.

We can put an error name after except to catch only that type. Other errors can still crash the script, which helps us fail fast when an unexpected bug happens.

try:
    price = float("missing")
except ValueError:
    print("Bad price")  # This runs

try:
    result = 10 / 0  # Raises ZeroDivisionError
except ValueError:
    print("Bad price")  # This does not run

# ZeroDivisionError still crashes the script!

except ValueError: catches the bad price text but not division by zero. The unrelated division error passes through instead of being hidden.

Wrap-Up

Use try...except when one unexpected bad value should not stop all the remaining work.

Put the code that may crash under try. Name the error you want to catch after except.

Only a matching error is caught. Other errors still crash.

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- Shep4