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warnings.catch_warnings

filters

A context manager that saves the warning filters and restores them on exit; with record=True it collects warnings into a list instead of printing them.

Syntax

with warnings.catch_warnings(*, record=False, action=None, category=Warning): ...

simplefilter() and filterwarnings() change process-wide settings; inside with catch_warnings(): the changes are rolled back on exit — this is how you silence warnings locally. With record=True warnings are not printed but collected in a list — the way tests check them; but without simplefilter('always') inside the block identical warnings from one line (in a loop, say) are recorded once. In a regular build it mutates global state and is not thread-safe.

Examples

import warnings
def legacy():
    warnings.warn('устарело', UserWarning)
with warnings.catch_warnings(record=True) as caught:
    warnings.simplefilter('always')
    legacy()
    legacy()
print(len(caught), caught[0].category.__name__)  # → 2 UserWarning

# Python 3.11+
import warnings
with warnings.catch_warnings(action='error'):
    try:
        warnings.warn('устарело', UserWarning)
    except UserWarning as e:
        print('стало исключением:', e)  # → стало исключением: устарело

See also

Common errors

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