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Strong versus Weak Typing
A Conversation with Guido van Rossum, Part V
by Bill Venners with Frank Sommers
February 10, 2003

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Weak and Strong Personalities

Bill Venners: To what extent do you think the choice between using a strongly and weakly typed language to build systems has to do with personality? I've met people who just seem to be weak typing kind of people. I know many people who loved Smalltalk, for example. They consider their Smalltalk days a time when they had it best. I heard one ex-Smalltalker complain of "getting spanked by compile-time errors." Such people feel compile-time errors are slowing them down. On the other hand, I've also met many people who like getting compile-time errors. They see the compiler as a friend helping them find errors sooner. To what extent is choosing the right tool a problem of finding the best fit for the programmer's personality?

Guido van Rossum: It could very well be a problem of personality. It's interesting that you mention Smalltalk. I've noticed Python to be a good fit for ex-Smalltalk programmers who are currently forced to write Java code. Many find Python a nice alternative that gets much closer to what they apparently like best. Of course, there are so many different application areas. Writing software for a spacecraft is very different from writing software for telephone exchanges. But you may be right with your personality theory.

Flying with Python

Bill Venners: Speaking of spacecraft, would you be comfortable enough with the robustness of Python systems to fly on an airplane in which all the control software was written in Python?

Guido van Rossum: That depends much more on the attitude of the design team that built it than on the language the team chose. There are situations where doing part of the software in Python makes much more sense than doing it in any other language, even if it must have the reliability requirements of a spacecraft or air traffic control.

Bill Venners: Why?

Guido van Rossum: You'll never get all the bugs out. Making the code easier to read and write, and more transparent to the team of human readers who will review the source code, may be much more valuable than the narrow-focused type checking that some other compiler offers. There have been reported anecdotes about spacecraft or aircraft crashing because of type-related software bugs, where the compilers weren't enough to save you from the problems.

Next Week

Come back Monday, February 17 for the final installment of this conversation with Python creator Guido van Rossum. If you'd like to receive a brief weekly email announcing new articles at, please subscribe to the Artima Newsletter.

Talk Back!

Have an opinion about strong and weak typing, testing, or robustness? Discuss this article in the News & Ideas Forum topic, Strong versus Weak Typing

Resources, the Python Language Website:

James Gosling's comments on weak typing:

Josh Bloch's comments on weak typing:

Introductory Material on Python:

Python Tutorial:

Python FAQ Wizard:

Guido van Rossum's home page:

Other Guido van Rossum Interviews:

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