Comment by acqq | Hacker News Reader

Comment by acqq | Hacker News Reader

Parent Full thread

Exactly. I am certainly in the "exceptions should be really exceptional" camp and therefore I didn't "agree" with Python's open function:

https://docs.python.org/3/library/functions.html#open

"Open file and return a corresponding file object. If the file cannot be opened, an OSError is raised."

in the sense that in a program that is not only a few lines, I would prefer such kinds of library functions not to produce exceptions, thank you very much, so I had to wrap all such. But it is not optimal not being able to directly use language primitives as much as possible.

On another side, writing really short programs gets to be faster: if I write only a few lines, having an "implicit" error handling shortens the code.

And that's what I think is the main reason for many ills and misunderstanding in many popular languages: the practices that look good for small examples (or in the books) aren't the same as those which work well for large projects. Python's not detecting the typos in the variable names in compile time is also a typical bias towards the short programs.

For large projects detecting as much at compile time and having only very exceptional exceptions and unsurprising control flow (visible reading ifs, and with a guarantee that there can't be "landings from anywhere" like in exceptions) is of huge advantage.

Python is far more promiscuous with Exceptions than languages like C#; they're not just used for "truly exceptional" conditions (see things like StopIteration or GeneratorExit).

Different languages have different attitudes to this and that's reflected in their APIs. This will naturally affect your choice of language when designing a system, depending on your tolerance for failures, your demands for robustness, or your definition of "exceptional".

Exceptions when opening a file has been chosen because they have the nice propery of encouraging a race condition free EAFP rather than a hard to get right LBYL.

It also fits nicely into the context manager pattern.

Besides, remember exceptions are cheap in python. So cheap that for loops are basically syntaxic sugar for a try/catch.

For those that don't know (I didn't):

    EAFP - Easier to ask for forgiveness than permission
    LBYL - Look before you leap

They are opposite strategies, Python's preferred strategy is to do the first, just try the operation and throw an exception if it doesn't work. The second is to check a pre-condition before attempting an operation.

I've always known the second as exhibiting TOCTOU, otherwise known as a time-of-check vs time-of-use race condition.

But that is a false claim. It's not about being EAFP or LBYL: if the open API returns a handle to file or null, the open operation can still be atomic and therefore race-free.

Which is exactly how it's in all the programing languages and operating system calls which don't use exceptions for that.

It's just the question if one has to write try-catch for something as common as file not existing.

LBYL can easily be enforced by Maybe types, leaving exceptions for really exceptional things (which a nonexisting file definitely isn't). So we see how not having a real type system hurts and deforms a language in ways unrelated to types.

That's still EAFP (you didn't check the precondition before trying), just through a different mechanism.

Reply on news.ycombinator.com