What is the use of "assert" in Python?

python assert assertions
Hossein picture Hossein · Feb 28, 2011 · Viewed 792.5k times · Source

I have been reading some source code and in several places I have seen the usage of assert.

What does it mean exactly? What is its usage?

Answer

slezica picture slezica · Feb 28, 2011

The assert statement exists in almost every programming language. It helps detect problems early in your program, where the cause is clear, rather than later as a side-effect of some other operation.

When you do...

assert condition

... you're telling the program to test that condition, and immediately trigger an error if the condition is false.

In Python, it's roughly equivalent to this:

if not condition:
    raise AssertionError()

Try it in the Python shell:

>>> assert True # nothing happens
>>> assert False
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

Assertions can include an optional message, and you can disable them when running the interpreter.

To print a message if the assertion fails:

assert False, "Oh no! This assertion failed!"

Do not use parenthesis to call assert like a function. It is a statement. If you do assert(condition, message) you'll be running the assert with a (condition, message) tuple as first parameter.

As for disabling them, when running python in optimized mode, where __debug__ is False, assert statements will be ignored. Just pass the -O flag:

python -O script.py

See here for the relevant documentation.

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