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Golang Error Checkers: os.IsExist vs os.IsNotExist

January 17, 2019

Recently I saw a line of code while working on the exercism cli:

if _, err := os.Lstat(workspaceDir); !os.IsNotExist(err) {
    //...
}

At first glance, it seems someone made a simple logic mistake that makes the code less readable. So I made a pull request suggesting this:

if _, err := os.Lstat(workspaceDir); os.IsExist(err) {
    //...
}

Looks like an easy enough change right? I didn't test it but when the CI job failed, I was shocked.

Error checkers

You may notice that the parameter passed into os.IsExist or os.IsNotExist is err. Normally, you might expect to pass the filepath instead.

Well, golang uses these as error checkers.

So with os.IsExist, it's actually checking for errors that would appear if the file exists.

Let's say you're trying to create a file. The behavior you would expect is for the file to not exist, so if the file does it exist, the corresponding error would be thrown and you could use os.IsExist to check it.

On the contrary, os.IsNotExist checks for errors that would occur if a file doesn't exist.

This is like the above example where we check if workspaceDir exists.

Since os.Lstat expects for it to exist already, it only throws an error if workspaceDir doesn't exist. Hence, the code used !os.IsNotExist(err) to check if an error had been thrown. os.IsExist(err) doesn't work since no error is thrown if the file exists because that is the expected behavior.

!os.IsExist != os.IsNotExist

So, the point of this post was to share my experience learning about these two functions. If you didn't read the whole thing, basically !os.IsExist != os.IsNotExist!

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