Exceptions and try/except

Catching what goes wrong

When Python cannot do what you asked, it raises an exception. Uncaught, it stops the program. try/except lets you decide instead:

try:
    value = int(text)
except ValueError:
    value = 0

Python runs the try block; if the named exception occurs it jumps to except.

The ones you will meet most:

Name the exception you expect. A bare except: swallows everything, including the typo in your own code, and turns a five-second bug into an afternoon.

Your turn: write to_int(text, fallback=0) returning the text as an integer, or the fallback when it will not convert.

else and finally

Two more clauses complete the picture:

try:
    result = risky()
except ValueError:
    print("that failed")
else:
    print("worked:", result)     # only when nothing was raised
finally:
    cleanup()                    # always, raised or not

else keeps the try block down to the single line that might fail, which makes it obvious what you are actually guarding. finally runs whatever happens — including when an exception is on its way up — so it is where cleanup belongs.

Your turn: write divide(a, b) returning a / b, or the string "cannot divide by zero" when b is zero.