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assert

Python: assert statement

assert condition
assert condition, expr

assert …. does nothing if __debug__ is False.

If __debug__ is True, assert condition checks if condition is True and raises an AssertionError exception otherwise.

If the optional expression expr is also present, it passes expr to the constructor of AssertionError.

lst = []

try:
   assert len(lst)

   print('-'.join(lst))

except AssertionError:
   print('assertion failed')

See also