assertion

This small package solves a few issues of the regular assert statements in Python:

• Regular asserts are ignored if the code is run with python -O.

• If an assertion fails, no additional information is given about the parameters which caused the fail.

• Test parameters are fixed (e.g., a variable which is True).

• A timing-critical test might fail now, but would have passed a few seconds later (e.g., after some action has been completed).

• In a set of tests, only tests until the first failed test are executed.

The assertion tests behave differently:

• The tests are always evaluated.

• If a test fails, a string representation of the provided arguments is included in the exception description.

• Test parameters can be

  • constants,
  • functions (sync and async),
  • other mutable variables.

• Optionally, a test can be re-evaluated until a configurable timeout is reached. By default, the timeout is zero and test fail instantly.

• Optionally, raising exceptions of failed tests can be deferred to allow for the completion of a set of tests (works also with pytest).

Quickstart

Here is a simple example:

>>> from assertion import Assertion
>>> assertion = Assertion()
>>> val = 999
>>> assertion.false(val)
Traceback (most recent call last):
    ...
TestError: 999 != False

Here is a more complex example using coroutines:

import asyncio
from assertion import Assertion

assertion = Assertion(msg="D'oh!", timeout=10.0)

class Countdown:
    def __init__(self, start: int):
        self.counter = start

    async def countdown(self) -> bool:
        self.counter -= 1
        return self.counter == 0

async def main():
    print("Test A...")
    await assertion.true(Countdown(10).countdown)  # should be successful
    print("ok")

    print("Test B...")
    await assertion.true(Countdown(50).countdown)  # should fail
    print("ok")

asyncio.run(main())

> ### Note > The test functions act as both regular functions and as awaitables depending > on the test arguments. > > If the test arguments contain coroutines or awaitables, you must await > the test.

There are several pre-defined tests:

In addition to the test parameters, each call accepts optional parameters:

• msg: str, default: ""

  This message is prefixed to the exception text.

• timeout: float, default: 0.0 seconds

  The number of seconds until the test is finally considered as failed.

• raised_exc (Type(Exception), default: TestError):

  Any Exception subclass which is raised if the test fails.

• exc_pass: Type(Exception) or Tuple(Type(Exception)), default: []

  If one of the provided tests raises an exception in exc_pass, the test is considered to be passed, independent of the actual test criteria.

• exc_ignore: Type(Exception) or Tuple(Type(Exception)), default: []

  If one of the provided tests raises an exception in exc_ignore, the exception is ignored, and the test evaluation proceeds.

> ### Note > > If you use a non-zero timeout, the test arguments might be evaluated/called > multiple time. Make sure that any given function call can handle this or set > the timeout to zero.

Detailed Description

Configuration

You can choose different defaults for msg, raised_exc, timeout, exc_pass, and exc_ignore while creating an instance of Assertion.

In addition, you can specify three more parameters:

• msg_length_max: int, default: 100

  This limits the string representation of the provided test parameters to the given number of characters. The optional message is not considered and will always be included in full length).

• delay_init: float, default 0.125 seconds

  Non-zero timeout only: If the initial test fails, the arguments are re-evaluated after this initial delay.

• delay_max: float, default: 5.0 seconds

  Non-zero timeout only: This limits the delay between two test parameter evaluations.

• timeout_fraction_warning: float, default: 0.75 (equals 75%)

  Non-zero timeout only: If more than this fraction of the timeout passed before the test was successful, a warning is logged. If you see this warning, you might want to increase the timeout. Set to None to disable warning.

In the default implementation, the delay between each evaluation is doubled (until it reaches the given maximum). This behavior can be changed by overloading Assert._get_new_delay.

Deferring

If you want to execute all tests in a set of tests even if early tests fail, you can defer the raising of exceptions. You have three options:

• Decorator:

  assertion = Assertion(raised_exc=ZeroDivisionError)
  
  @assertion.deferring_decorator
  def multiple_fails():
      assertion.true(False)
      assertion.true("this will not fail")
      assertion.equal(5, 99, raised_exc=ValueError)
      assertion.false(True, msg="D'oh!")
  

  When calling multiple_fails(), all four tests will be executed (with three failing), but only one exception will be raised when leaving the functions scope. Here is the abbreviated output of the exception:

  ZeroDivisionError: 3 deferred exception(s):
      ZeroDivisionError[File "<input>", line 5, in multiple_fails, "<code>"]: False != True
      ValueError[File "<input>", line 7, in multiple_fails, "<code>"]: 5 != 99
      ZeroDivisionError[File "<input>", line 8, in multiple_fails, "<code>"]: D'oh!: True != False
  

  The decorator works for both synchronous and asynchronous functions.

• Context manager:

  If you want to define a defer scope on a sub-function level, you can use contexts:

  assertion = Assertion(raised_exc=ZeroDivisionError)
  
  with assertion.deferring_context():
      for i in range(3):
          assertion.equal(i, 1)
  

• Explicit start/stop calls (only use as a last resort):

  assertion = Assertion(raised_exc=ZeroDivisionError)
  
  assertion.start_deferring()
  for i in range(3):
      assertion.equal(i, 1)
  assertion.stop_deferring()
  

  When an assertion instance is garbage-collected, it checks if there are open defer scopes (which not always works because of the limitations during the teardown phase). If so, a warning and any remaining fail messages are logged, but no exception is raised to avoid disturbing any ongoing shutdown procedures.

Defer scopes can be nested. The raising of exceptions is deferred until all scopes have been exited.

Exception Handling

If an exception is raised from any of the tests, this exception is not caught by default and therefore terminates the test evaluation. This behavior can be changed with the two parameters exc_pass and exc_ignore:

• If one of the provided tests raises an exception in exc_pass, the test is considered to be passed, independent of the actual test criteria.

• If one of the provided tests raises an exception in exc_ignore, the exception is ignored, and the test evaluation proceeds. This only makes a difference if the timeout parameter is non-zero, and the tests are re-evaluated.

Similar Packages

This package was heavily inspired by the assert magic in pytest _.

assert-info _ has a slightly different focus, but might suit you better.