You might be using assert wrong

Assert is often used in production code as a form of data validation check or sanity testing. You may have seen code bases that contain logic like:

def validate_age(value):
    assert value < 70, "No youngsters allowed!"
response = requests.post(url=url, json={'foo': 'bar'})
assert response.ok, response.text

However, assertions should only be used for testing, development, and debugging purposes. assert is not meant to be used in production code. Don’t take my word for it. The Python docs for assert state:

Assert statements are a convenient way to insert debugging assertions into a program

Optimized out

We can trigger an AssertionError in the shell:

python -c 'assert 80 < 70, "No youngsters allowed!"'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AssertionError: No youngsters allowed!

However, given Python provides assert as a debugging tool, Python also provides a way to remove the assertions from the compiled byte code by specifying -O:

python -O -c 'assert 80 < 70, "No youngsters allowed!"'

No AssertionError is raised! The same behavior can be triggered via PYTHONOPTIMIZE environment variable. Both the PYTHONOPTIMIZE and -O command line switch have the same outcome:

PYTHONOPTIMIZE=1 python -c 'assert 80 < 70, "No youngsters allowed!"'

Again, no AssertionError is raised. As per the docs, both -O flag and PYTHONOPTIMIZE have this effect:

Remove assert statements and any code conditional on the value of __debug__. Augment the filename for compiled (bytecode) files by adding .opt-1 before the .pyc extension (see PEP 488). See also PYTHONOPTIMIZE.

The bytecode “intermediate language” between Python and C: Python source code is compiled into bytecode, the internal representation of a Python program in the CPython interpreter. With this knowledge, we can view the compiled byte code of optimized and non-optimized:

python3 -c 'import dis; dis.dis("assert 80 < 70")'
  1           0 LOAD_CONST               0 (80)
              2 LOAD_CONST               1 (70)
              4 COMPARE_OP               0 (<)
              6 POP_JUMP_IF_TRUE         6 (to 12)
              8 LOAD_ASSERTION_ERROR
             10 RAISE_VARARGS            1
        >>   12 LOAD_CONST               2 (None)
             14 RETURN_VALUE
python3 -O -c 'import dis; dis.dis("assert 80 < 70")'
  1           0 LOAD_CONST               0 (None)
              2 RETURN_VALUE
PYTHONOPTIMIZE=1 python3 -c 'import dis; dis.dis("assert 80 < 70")'
  1           0 LOAD_CONST               0 (None)
              2 RETURN_VALUE

We see in optimized mode Python does not just skip the assertions, they are literally removed. Therefore asserts are not for data validation, and are not for control flow because they are one flag or environment variable away from being automatically removed from the application.

If pytest is ran with PYTHONOPTIMIZE=1 you will get a helpful warning that assertions not in tests or plugins will be ignored. A helpful warning suggesting they too see assertions abused. Instead consider raising exceptions like NotImplimentedError as there is no risk these are optimized out.

Correct usage of assert

Asserts should not be used for things like handling user input or checking for network errors since these can and do occur during normal execution.

Many developers misuse assert statements by using them for general error handling. This is not what assert statements are for. Assert statements are meant to check assumptions that your code makes which needs to be communicated to other developers that are reading the code. For example, you might use an assert statement to check that a parameter is not None before using it. If the parameter is None, the assert statement will throw an error, alerting you that your code has a problem.

Assert statements should only be used for conditions that should NEVER be false. This is because assert statements are used to test for conditions that are expected to be accurate, and if they are wrong, it indicates a bug in the program. By only using assert statements for conditions that should NEVER be false, we can be sure that any failure in the assert statement indicates a real problem with the program.

This is useful for testing as it allows you to check that your code is working as expected.

Debugging and Testing

The assert keyword is used to check for bugs in your code when debugging. Using assert, you can manually test your code to see if it works properly by checking for the AssertionError. If an assert statement fails, it will stop your program from running. This can be very useful for debugging your code.

Unit tests provide a more comprehensive way to test code and libraries like Pytest expect assert to be used in the test.

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