Is it better to use an exception or a return code in Python?

Question

You may know this recommendation from Microsoft about the use of exceptions in .NET:

Performance Considerations

...

Throw exceptions only for extraordinary conditions, ...

In addition, do not throw an exception when a return code is sufficient...

(See the whole text at http://msdn.microsoft.com/en-us/library/system.exception.aspx.)

As a point of comparison, would you recommend the same for Python code?

Answer 1

The pythonic thing to do is to raise and handle exceptions. The excellent book "Python in a nutshell" discusses this in 'Error-Checking Strategies' in Chapter 6.

The book discusses EAFP ("it's easier to ask forgiveness than permission") vs. LBYL ("look before you leap").

So to answer your question:

No, I would not recommend the same for python code. I suggest you read chapter 6 of Python in a nutshell.

Answer 2

The best way to understand exceptions is "if your method can't do what its name says it does, throw." My personal opinion is that this advice should be applied equally to both .NET and Python.

The key difference is where you have methods that frequently can't do what their name says they should do, for instance, parsing strings as integers or retrieving a record from a database. The C# style is to avoid an exception being thrown in the first place:

int i; if (Int32.TryParse(myString, out i)) {     doWhatever(i); } else {     doWhatever(0); } 

whereas Python is much more at ease with this kind of thing:

try:     i = int(myString) except ValueError:     i = 0 doWhatever(i);