The closest thing Python has to a specification is the Language Reference. It shies well away from performance details and aims to describe "syntax and core semantics" while acknowledging that some parts of what it specifies may just be implementation details of CPython [0].
To draw the line on what you can assume re performance, you need a combination of background knowledge, folk wisdom and common sense. (You could say "or read the code" but that could change in the next minor version: you still need common sense to know if that's likely). This is unlike, e.g. C++ where there is a specification and standards-compliant compilers must implement certain functions with the specified runtime complexity.
To draw the line on what you can assume re performance, you need a combination of background knowledge, folk wisdom and common sense. (You could say "or read the code" but that could change in the next minor version: you still need common sense to know if that's likely). This is unlike, e.g. C++ where there is a specification and standards-compliant compilers must implement certain functions with the specified runtime complexity.
[0] https://docs.python.org/3/reference/introduction.html#altern...