How Python resolves imports via sys.path

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The import system checks sys.modules first - if the module is already loaded, it returns that cached object without touching the filesystem. If not cached, Python walks sys.path in order. By default sys.path contains: the directory of the script being run (or '' for the current directory in interactive mode), any paths from the PYTHONPATH environment variable, the standard library directories, and finally site-packages where pip installs third-party libraries. You can inspect this live by printing sys.path. The script-directory-first rule is why a local file named 'json.py' would shadow the standard library json module - a common footgun.