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Contracts in Python
A Conversation with Guido van Rossum, Part IV
by Bill Venners
February 3, 2003

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Python Has Implicit Contracts

Bill Venners: I have a theory that 99 percent of the time when I invoke a method in Python it will do what I expect, because in practice it usually does. When I invoke a method in a strongly typed language, 1 percent of the time a bug or misunderstanding may cause the method to do something unexpected. Because even though the contract is clear from a variable's type in a strong typing system, it doesn't mean people don't break the contract. People do break contracts, because of bugs and misunderstandings. I suspect that in both strongly and weakly typed languages, situations in which invoked methods mean something unexpected are equally rare, and for the most part discovered and dealt with during testing.

Guido van Rossum: In Python, you have an argument passed to a method. You don't know what your argument is. You're assuming that it supports the readline method, so you call readline. Now, it could be that the object doesn't support the readline method.

Bill Venners: And then I'll get an exception.

Guido van Rossum: You'll get an exception, which is probably OK. If this is a mainline piece of code and something could possibly be passed to you that doesn't have a readline method, you'll discover that early on during testing. Just as much as in a typed language when you have an interface and you know you're getting something that has the right interface but doesn't implement the right thing, or it throws an unexpected exception. You'll hopefully find that during testing.

In addition in Python, because there aren't fixed protocols, something else can be passed that also supports readline and doesn't happen to be a file, but does exactly what you need. All you need at that point is something that returns lines.

Bill Venners: But it's also possible to pass something that supports readline that does something I don't expect.

Guido van Rossum: In general in Python, there is a contract, but the contract is implicit. The contract isn't specified by an interface. There's nothing in what the parser sees at least that says x has to be an object that supports readline that you can call with no arguments and it returns a string that means a certain thing. But that contract is certainly in the documentation or specification.

In Java, if you say this is something that has a readline method that returns a string, what does it mean? Do you expect it to always return the same string? Does it ever return an empty string? There are all sorts of things that aren't expressed by that interface that you still have to specify in documentation. That's where the interesting competition between the different languages exists.

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