When decorator code runs

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Accepted Answer

Any code in the decorator body that is outside the wrapper runs at definition time - the moment the @ line is evaluated. Here, print('decorating work') is in trace's body, not in wrapper, so it fires when Python processes @trace. Code inside wrapper only runs when the decorated function is actually called. This distinction bites people in module-level decorators: if your decorator does expensive setup (opens a database connection, imports a heavy library) outside the wrapper, that cost is paid at import time for every decorated function - even if those functions are never called. Move side effects inside wrapper unless you genuinely need them at decoration time.