assert — Expert Examples

Debugging aid that tests a condition and raises AssertionError if false

EasyIntermediateAdvancedExpert

Assert bytecode and -O flag

How assert compiles and how optimization removes it.

python
import dis

def with_assert():
    x = 42
    assert x > 0, "must be positive"
    return x

print("Assert bytecode:")
dis.dis(with_assert)

# The assert compiles to:
# LOAD x, LOAD 0, COMPARE_OP >, POP_JUMP_IF_TRUE (skip)
# LOAD AssertionError, LOAD "message", CALL, RAISE
#
# With python -O, the entire block is removed

import sys
print(f"\n__debug__ = {__debug__}")
print(f"Optimization level: {sys.flags.optimize}")

assert compiles to a conditional that loads AssertionError and raises it. When Python runs with -O flag, __debug__ is False and all assert statements are stripped from the bytecode entirely.

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