Always test for absence of a value with is None / is not None rather than truthiness or == None/!= None. Truthy checks silently misclassify valid-but-falsy values (0, “”, {}, []) as missing, and == None is non-idiomatic and slower due to implicit bool conversion. This is especially important when building fallback chains with or, since a or b will skip a even when it is a legitimate falsy value (e.g. port 0, an empty JSON body, or an empty string that should stay empty rather than become None). When chaining multiple optional sources, prefer an explicit filter such as next((v for v in (a, b, default) if v is not None), default) instead of a or b or default, and add tests covering the falsy-but-valid case (e.g. port 0).
Also ensure variables that may remain unset are always given an explicit default. For example, don’t rely on an except ... as err: binding surviving past the except block (Python clears it), and don’t assume a helper variable (like sn_host from server_name.partition(':')) is defined when its source is None — initialize defaults before the conditional so every code path has a defined value.
Example:
# Bad: treats {} and 0 as "missing"
if json:
...
if domain != None:
...
subdomain = self.url_map.default_subdomain or None if not self.subdomain_matching else None
# Good: explicit None checks, defaults set up front
if json is not None:
...
if domain is not None:
...
host = host or sn_host or default_host
port = int(next((p for p in (port, sn_port) if p is not None), default_port))