[lispm]

The difference is that SBCL caught the error directly on entry of MERGE-PATHNAMES. SBCL did on call to MERGE-PATHNAME a runtime type check. It knows the expected types for the arguments.

SBCL told you that the call to MERGE-PATHNAME is already wrong. A backtrace then will only show higher up code from the environment and the call to MERGE-PATHNAME.

Your Python code went into the routine...

Often the Python backtrace will be easier to understand, since it is source/line oriented, since Python code usually does not have extensive code transformations (-> Lisp macros) and using an optimizing compiler like SBCL may make the code less debuggable (for example when using tail call optimization).