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sys.modules

modules

The dictionary of every loaded module, {name: module}. Python's import cache. A module can be removed from it to force a reload.

Syntax

sys.modules  # dict[str, ModuleType]

This cache is why a module body runs exactly once: a second import in the same session will not pick up edits to the file. Deleting a key from sys.modules does not unload anything — the old classes and objects survive in references already taken, so after a re-import isinstance starts returning False for what looks like the same class. Use importlib.reload() for a reload, and restart the process when you need certainty.

Examples

import sys
import os
print('os' in sys.modules)  # → True
print(type(sys.modules['os']))  # → <class 'module'>
print(len(sys.modules))  # → кол-во загруженных модулей
del sys.modules['os']  # убрали из кэша — сам модуль из памяти не выгружен
import os as os2
print(os2 is os)  # → False — тело модуля выполнено заново, старый объект жив

import sys
import json
first = sys.modules['json']
import json  # повторный import берёт модуль из кэша
print(sys.modules['json'] is first)  # → True — тело модуля не выполнялось заново

See also

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