The lookup itself is by design. We should probably try to abort once we notice we're in a loop, though. Would you be interested in submitting a PR for that? This kind of thing probably happens around here: https://github.com/djc/askama/blob/main/askama_derive/src/lib.rs#L50, or inside the Heritage::new() call.

What about the case of the cycle in dependency ?
if a.html --- extends ---> b.html --extends ---> c.html ---extends---> a.html

What about it? Please be more explicit in what you're asking about.

My apologies for that, the last comment suppose to be edited.

As @Rapptz explains there is an issue of self-loop when base.html tries to extend itself and leads to an infinite loop.
My solution is to detect this loop at https://github.com/djc/askama/blob/main/askama_derive/src/lib.rs#L84 by check if the source path and extends are equal then raise CompileError("There is a cyclic dependency between {path} and {extends}").

This solution will solve the self-loop problem, but it will not work for the condition where extends leads to circular dependency that also hangs the build.
if a.html --- extends ---> b.html --extends ---> c.html ---extends---> a.html

So, to solve both of the cases should we use a graph(build based on source paths) to detect the cycle and raise the CompileError if there is any cycle found?

I think the Heritage already contains such a graph. If you can use that to also block indirect cyclic dependencies, that would be great.

This got resolved with #539 .
Please check :)