Currently, the documentation for Popen.returncode states
The child return code, set by poll() and wait() (and indirectly by communicate()). A None value indicates that the process hasn’t terminated yet.
A negative value -N indicates that the child was terminated by signal N (POSIX only).
As far back as https://bugs.python.org/issue1727024 it had been suggested that this fails to emphasize an important piece of information, namely that this attribute can be outdated (it is *only* set if poll() or wait() are called; this is not obvious, e.g. returncode could have beeen implemented as a property that auto-calls poll()). I do realize that a strict reading of the docs does suggest the actual behavior, but a bit of explicitness won't hurt...
The wording suggested in https://bugs.python.org/issue1727024 seems clearer to me ("Note: The value stored in returncode may be out-of-date. Use poll() to reliably find the current return code.").