When writing code that touches network addressing or HTTP headers, follow the exact protocol semantics rather than relying on convenient shortcuts, since subtle deviations break compatibility. - Prefer explicit IP addresses over hostnames when binding sockets or servers. A hostname like `localhost` can resolve to either `127.0.0.1` or `::1` depending on the system's DNS/hosts configuration, so it is not a safe drop-in replacement for an explicit address: ```python # Avoid: relies on OS resolution, may bind to IPv6 instead of IPv4 host = 'localhost' # Prefer: unambiguous host = '127.0.0.1' ``` - When comparing or searching HTTP header tokens (e.g. `Vary`, cookie names, content-disposition directives), always perform case-insensitive comparisons, since HTTP header field values are case-insensitive per spec: ```python # Avoid: case-sensitive check can miss existing entries if 'Cookie' not in vary_headers: ... # Prefer: case-insensitive check if not any(h.lower() == 'cookie' for h in vary_headers): ... ``` - When building multi-part header values (e.g. `Content-Disposition` with both `filename` and `filename*`), preserve the order required by the spec/consuming clients instead of relying on dict iteration order or unpacking, since some clients depend on a specific directive order. Applying these rules prevents hard-to-diagnose networking and HTTP compatibility bugs across different browsers, OS resolvers, and client implementations.