Terraform: format interpolation function interprets variables as strings

[jeremygaither]

Terraform Version

Terraform v0.11.11

Terraform Configuration Files

name ="foo-${format("%02d", var.some_number)}" # where some_number = 2

Debug Output

Crash Output

Expected Behavior

name is set as foo-02

Actual Behavior

name is set as "foo-%!d(string=2)"

Steps to Reproduce

Additional Context

So far my workaround is to add the variable to zero:

name ="foo-${format("%02d", 0 + var.some_number)}" # where some_number = 2

References

[apparentlymart]

Hi @jeremygaither! Sorry for this odd behavior, and thanks for reporting it.

This strangeness results from the fact that in Terraform 0.11 and earlier the format command is just a wrapper around the Go fmt.Sprintf function, which therefore doesn't do all of the automatic type conversions we normally expect in the Terraform language.

The good news is that there's already a new implementation of format in the master branch ready for inclusion in the forthcoming Terraform v0.12.0 release, and that implementation behaves correctly because it is implemented in terms of the Terraform language's own type system, not the Go type system:

$ terraform console
> "foo-${format("%02d", "2")}"
foo-02

As usual in the Terraform language, Terraform here can see that the %d verb requires a number and so it attempts to convert the argument from string to number.

Naturally this will produce an error if the given value _cannot_ be converted to a number:

> "foo-${format("%02d", "pineapple")}"

Error: Error in function call

Call to function "format" failed: unsupported value for "%02d" at 0: a number
is required.

Since this fix is already in the master branch, I'm going to close this out now. In the mean time, the workaround you found is a reasonable one -- using the Terraform language + operator to force a conversion to number _before_ the function call -- and it'll be harmless once upgraded to Terraform 0.12, so you can remove the workaround at your leisure at some point after upgrading:

> "foo-${format("%02d", 0 + "2")}"
foo-02

Other similar workarounds would include passing the value to the floor function to force conversion to a whole number, and any other function or language operator that implies a conversion to number.

[hashibot[bot]]

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