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html/template: default missingkey behavior does not match documentation #54053

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rittneje opened this issue Jul 25, 2022 · 3 comments
Open

html/template: default missingkey behavior does not match documentation #54053

rittneje opened this issue Jul 25, 2022 · 3 comments
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Documentation help wanted NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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@rittneje
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What version of Go are you using (go version)?

$ go version
go version go1.17.12 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What did you do?

https://go.dev/play/p/i3N0a5PATuI

What did you expect to see?

<b><no value></b>

This would align with the documented behavior of Template.Option.

"missingkey=default" or "missingkey=invalid"
The default behavior: Do nothing and continue execution.
If printed, the result of the index operation is the string
"".

This is what text/template outputs.

What did you see instead?

<b></b>
@seankhliao
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Looks like the behavior is expected (see tests).
Though not very consistent, using {{ printf "%v" .Foo }} we can observe a value of &lt;nil&gt; or <nil>

cc @empijei

@seankhliao seankhliao added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jul 25, 2022
@ianlancetaylor
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I think this is related to #18716 and #25875. When using html/template the result of .Foo is passed through the HTML escaper function. The value passed to the HTML escaper function is nil. This winds up getting printed as nothing.

I don't think we can change this behavior now. It would be nice to document it better. The current documentation says

For these internal escaping functions, if an action pipeline evaluates to a nil interface value, it is treated as though it were an empty string.

but it's not clear that a missing map key is a nil interface value.

@ianlancetaylor ianlancetaylor added this to the Backlog milestone Jul 25, 2022
@gopherbot
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Change https://go.dev/cl/419694 mentions this issue: html/template: update document about the missing map key

@seankhliao seankhliao mentioned this issue Jan 26, 2023
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@ianlancetaylor @gopherbot @seankhliao @rittneje