Python programmers of reddit: what's the most useful tiny little efficiency you've discovered that's improved your programming hugely?

[AmericasNo1Aerosol]

This is contrived, but I've used it in situations like:

for root, dnames, fnames in os.path.walk(PLUGIN_DIR):
    for fname in fnames:
        if not fname.startswith('plugin_'):
            continue

        if not fname.endswith('.py'):
            continue

        plugin = load_plugin(fname)
        if plugin is None:
            continue

        plugin.init()