Remove deprecated aliases to Abstract Base Classes from thecollections module.

for compatibility with Python 3.10.The package `nose` is not compatible with Python 3.10,because it imports abstract base classes from themodule `collections`,[`nose == 1.3.7`](https://pypi.org/project/nose/1.3.7/#history)imports in Python 3.10 without error, but `nosetests` fails,due to imports from the module `collections` of classes thathave moved to the module `collections.abc`. The error raised contains:```  File ".../.virtualenvs/.../lib/python3.10/site-packages/nose/suite.py", line 106, in _set_tests    if isinstance(tests, collections.Callable) and not is_suite:AttributeError: module 'collections' has no attribute 'Callable'```Relevant information about CPython changes in `collections` (removal ofABCs):-python/cpython#23754-https://bugs.python.org/issue37324-https://docs.python.org/3.10/library/collections.abc.html#collections-abstract-base-classes- Deprecation since Python 3.3, present until Python 3.9:https://docs.python.org/3.9/library/collections.html#module-collections- Removed in Python 3.10:https://docs.python.org/3.10/whatsnew/3.10.html#removedThe script `run_tests.py` is rewritten, based on `pytest` functionality.The file `test/transys_mathset_test.py` has been changed to mode 100644in order to not need an equivalent `pytest` option for option `--exe`of `nose`.In the test script `tests/mathfunc_test.py`, the function`_get_state_input_output_pair_test` has been renamed to`_get_state_input_pair` to avoid its name being matched by thepattern `python_functions = test_* *_test` defined in theconfiguration file `tests/pytest.ini`.

for compatibility with Python 3.10.The package `nose` is not compatible with Python 3.10,because it imports abstract base classes from themodule `collections`,[`nose == 1.3.7`](https://pypi.org/project/nose/1.3.7/#history)imports in Python 3.10 without error, but `nosetests` fails,due to imports from the module `collections` of classes thathave moved to the module `collections.abc`. The error raised contains:```  File ".../.virtualenvs/.../lib/python3.10/site-packages/nose/suite.py", line 106, in _set_tests    if isinstance(tests, collections.Callable) and not is_suite:AttributeError: module 'collections' has no attribute 'Callable'```Relevant information about CPython changes in `collections` (removal ofABCs):-python/cpython#23754-https://bugs.python.org/issue37324-https://docs.python.org/3.10/library/collections.abc.html#collections-abstract-base-classes- Deprecation since Python 3.3, present until Python 3.9:https://docs.python.org/3.9/library/collections.html#module-collections- Removed in Python 3.10:https://docs.python.org/3.10/whatsnew/3.10.html#removedThe script `run_tests.py` is rewritten, based on `pytest` functionality.The file `test/transys_mathset_test.py` has been changed to mode 100644in order to not need an equivalent `pytest` option for option `--exe`of `nose`.In the test script `tests/mathfunc_test.py`, the function`_get_state_input_output_pair_test` has been renamed to`_get_state_input_pair` to avoid its name being matched by thepattern `python_functions = test_* *_test` defined in theconfiguration file `tests/pytest.ini`.

for compatibility with Python 3.10.- add `pytest >= 4.6.11` to argument `tests_require` of  the function `setuptools.setup` in script `setup.py`  This lower bound has been selected to ensure compatibility  with both Python 3 and Python 2.7. See below for details.- add `pytest >= 4.6.11` to file `requirements.txt`- remove `nose` from argument `tests_require` of  the function `setuptools.setup` in script `setup.py`- remove `nose` from file `requirements.txt`- update file `.travis.yml`- remove the collection of coverage measurements on Travis CI,  because coverage measurement is incorrect (lower than the  real coverage, and even lower after switching to `pytest`)  due to Cython coverage not being correctly collected with  the build configuration currently used for testing.  Also, negligible changes in coverage measurements affected  the status of commits on GitHub, turning it to a red "X",  which can give the false impression that the tests failed  when they passed, and requires clicking on the "X" in order  to see more information that clarifies that the tests  actually passed.- remove `coveralls` from file `requirements.txt`,  because `coveralls` was used only on Travis CI.- add a configuration file `tests/pytest.ini`  and include it in `MANIFEST.in`- ignore `.pytest_cache/` in `.gitignore`Motivation==========The change from [`nose == 1.3.7`](https://pypi.org/project/nose/1.3.7/#history)to [`pytest`](https://pypi.org/project/pytest)is motivated by compatibility with Python 3.10.`nose` is incompatible with Python 3.10=======================================The package `nose`, which was used to run the tests of `dd`,is not compatible with Python 3.10 (for details,read the last section below). And `nose` is unmaintained.(Also, `nose` uses the `imp` module from Python's standard library,which [is deprecated](https://docs.python.org/3.10/library/imp.html),so may be removed in some future Python version.)Summary of transition to `pytest`=================================In summary, using `pytest` with the existing tests requires addinga [configuration file `tests/pytest.ini`](https://docs.pytest.org/en/latest/reference/customize.html#configuration-file-formats)to tell `pytest` which functions, classes, and methods to collect testsfrom (called "discovery" of tests).The [parameter `--continue-on-collection-errors`](https://docs.pytest.org/en/latest/reference/reference.html#command-line-flags)tells `pytest` to not stop in case any test module fails to import,and continue with running the tests. The ability to run the tests whensome `dd` C extension modules are not installed is necessary.After transitioning the tests to `pytest`, the tests have been confirmedto run successfully:- on Python 2.7 with `pytest == 4.6.11`, and- on Python 3.9 with `pytest == 6.2.4`.Failed attempts to use `unittest`=================================First, I tried to use [`unittest`](https://docs.python.org/3/library/unittest.html)(part of CPython's standard library).For writing `dd` tests, `unittest` suffices.For *discovering* the tests, `unittest` seems to require that tests bemethods of subclasses of the class `unittest.TestCase`.This is not the case in the tests of `dd` tests.Using `pytest` allows changing the test runner from `nosetests`with minimal changes to the tests themselves.Test discovery using `unittest` could possibly be implemented by addinga file `tests/__init__.py`, and defining in that file a function`load_tests`, following the [documentation of `unittest`](https://docs.python.org/3/library/unittest.html#unittest.TestLoader.discover).In any case, it is simpler to use `pytest`, which requires onlya configuration file.If `unittest` encounters an `ImportError` during collection of the tests(i.e., when it tries to import test modules), then it stops. There doesnot appear to be any way to tell `unittest` to continue and run the rest ofthe test modules (those that *could* be imported).Usage of `nose`===============The dependence on `nose` is minimal.Only one function is used from `nose`:the function `nose.tools.assert_raises`.The function `assert_raises` is dynamically defined[in the module `nose.tools.trivial`](https://github.com/nose-devs/nose/blob/release_1.3.7/nose/tools/trivial.py#L32-L54)by instantiating the class `unittest.TestCase`,and setting `assert_raises` to equal the[bound method `assertRaises`](https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertRaises)of the instance of `TestCase`.So the function `assert_raises` from `nose` is just a PEP8-compliantbinding for the method `unittest.TestCase.assertRaises`.Reading the code of `unittest`:-https://github.com/python/cpython/blob/6fdc4d37f3fdbc1bd51f841be6e5e4708a3b8798/Lib/unittest/case.py#L156-L243-https://github.com/python/cpython/blob/6fdc4d37f3fdbc1bd51f841be6e5e4708a3b8798/Lib/unittest/case.py#L704-L735it follows that the existing usage:```pythonwith nose.tools.assert_raises(AssertionError):    foo(1, 2)```is equivalent to the following code(the use of `AssertionError` here is just as an example):```pythonwith unittest.TestCase().assertRaises(AssertionError):    foo(1, 2)```Replacing usage of `nose` with `pytest` in test code====================================================The [context manager `pytest.raises`](https://docs.pytest.org/en/latest/reference/reference.html#pytest-raises)is a [drop-in replacement](https://en.wikipedia.org/wiki/Drop-in_replacement)for the function `nose.tools.assert_raises`:```pythonwith pytest.raises(AssertionError):    foo(1, 2)```Also, the tests can still be run with `nosetests` on Python versionswhere `nose` is still available.Replacing the test runner `nosetests` with `pytest`===================================================- `pytest` correctly recognized the test files by default- `pytest` does not recognize by default methods of classes that do not  start with "Test" as test methods, even if the methods start with `test_`.  The configuration file is necessary to change this behavior of `pytest`  (in particular the command-line parameter `-k` did not seem to work  for classes). Relevant documentation:  -https://docs.pytest.org/en/latest/explanation/goodpractices.html#conventions-for-python-test-discovery  -https://docs.pytest.org/en/latest/example/pythoncollection.html#changing-naming-conventions  -https://docs.pytest.org/en/latest/how-to/nose.html  -https://docs.pytest.org/en/latest/reference/reference.html#confval-python_classes  -https://docs.pytest.org/en/latest/reference/reference.html#confval-python_functions  -https://docs.pytest.org/en/latest/reference/reference.html#confval-python_files- The call `pytest tests/foo_test.py` imports the package `dd` from  `site-packages` (assuming that the module `foo_test.py` contains the  statement `import dd`). So the default behavior of `pytest` is asdesired.  In contrast, `nosetests tests/foo_test.py` imports the package `dd`from  the local directory `dd/`, even though `dd` *is* installed under  `site-packages`.  In any case, `pytest` is called from within the directory `tests/`,  as was done for `nosetests`.  `python -m pytest tests/foo_test.py` and  `PYTHONPATH=. pytest tests/foo_test.py` both result in importing `dd`  from the local directory `dd/`. Relevant documentation:  -https://docs.pytest.org/en/latest/explanation/pythonpath.html#invoking-pytest-versus-python-m-pytest  -https://docs.pytest.org/en/latest/how-to/usage.html#invoke-pythonAs remarked above, the `pytest` [parameter `--continue-on-collection-errors`](https://docs.pytest.org/en/latest/reference/reference.html#command-line-flags)needs to be used for running the tests when some of the C extension modules arenot installed, for example:```cd tests/pytest -v --continue-on-collection-errors .```or, to also activate [Python's development mode](https://docs.python.org/3/library/devmode.html):```cd tests/python -X dev -m pytest -vvv --continue-on-collection-errors .```Colored output of test results from `pytest`============================================With `nose`, I used to use [`rednose`](https://pypi.org/project/rednose/)for coloring test results, which was convenient.`pytest` colors its output by default, no plugin is required.This capability is an optional way of viewing test results,so the coloring comparison is mentioned only for completeness.Observations about `pytest`:- shows colored source code that includes more source lines- detects assertions that failed, and marks their source lines- avoids the deprecated `imp` module (standard library) that `nose` uses  (and thus the associated `DeprecationWarning`)https://docs.python.org/3.10/library/imp.html- running the tests of `dd` with `pytest` revealed several  `DeprecationWarnings` that were previously hidden by `nose`  (these warnings were about invalid escape sequences due to   backslashes appearing in non-raw strings).Further remarks===============[`pytest == 6.2.4`](https://pypi.org/project/pytest/6.2.4) is notcompatible with Python 2.7.[`pytest == 4.6.11`](https://pypi.org/project/pytest/4.6.11/)is the latest version of `pytest` that is compatible with Python 2.7(released on June 5, 2020).`pytest` specifies `python_requires` [PEP 345](https://www.python.org/dev/peps/pep-0345/#requires-python),[PEP 503](https://www.python.org/dev/peps/pep-0503/):-https://github.com/pytest-dev/pytest/blob/4.6.11/setup.cfg#L48-https://github.com/pytest-dev/pytest/blob/5.0.0/setup.cfg#L43So including `pytest>=4.6.11` in the file `requirements.txt` suffices toinstall, on each Python version, the latest version of `pytest` that iscompatible with that Python version.This simplifies testing on CI, and packaging.In other words, conditional installations in the file `.travis.yml` arenot needed for `pytest`, neither conditional definition of `tests_require`in the script `setup.py`.This approach leaves implicit the upper bound on `pytest` in`tests_require`. This upper bound is specified explicitly by`pytest` itself, depending on the Python version of the interpreter.It appears that `pip == 9.0.0` and `setuptools == 24.2.0` are requiredto correctly implement `python_requires`:-https://pip.pypa.io/en/stable/news/#v9-0-0-https://setuptools.readthedocs.io/en/latest/history.html#v24-2-1How replacing usage of `nose` with `unittest` would have looked like====================================================================A way to replace `nose` could have been to add a module `tests/utils.py`containing:```python"""Common functionality for tests."""import unittest_test_case = unittest.TestCase()assert_raises = _test_case.assertRaises```which is close to what `nose` does. The function `assert_raises` couldthen be imported from the module `utils` in test modules, and used.Using `pytest` avoids the need for this workaround.Details about the incompatibility of `nose` with Python 3.10============================================================[`nose == 1.3.7`](https://pypi.org/project/nose/1.3.7/#history)imports in Python 3.10 fine, but `nosetests` fails, due to importsfrom the module `collections` of classes that have moved to themodule `collections.abc`.Relevant information about CPython changes in `collections`(removal of ABCs):-python/cpython#23754-https://bugs.python.org/issue37324-https://docs.python.org/3.10/library/collections.abc.html#collections-abstract-base-classes- Deprecation since Python 3.3, present until Python 3.9:https://docs.python.org/3.9/library/collections.html#module-collections- Removed in Python 3.10:https://docs.python.org/3.10/whatsnew/3.10.html#removedAbout skipping tests====================The decorator `unittest.skipIf` is recognized by `pytest`, and skippedtests are correctly recorded and reported. In any case, note alsothe `pytest` test-skipping facilities:-https://docs.pytest.org/en/latest/how-to/skipping.html-https://docs.pytest.org/en/latest/how-to/unittest.html-https://docs.pytest.org/en/latest/example/simple.html#control-skipping-of-tests-according-to-command-line-optionAbout passing `-X dev` to `python` in the `Makefile`====================================================The argument `dev` is available for the `python` option[`-X` only on Python 3.7 and higher](https://docs.python.org/3/library/devmode.html#devmode).So the `Makefile` rules where `-X dev` appears arenot compatible with ealier Python versions supported by `dd`.This is not an issue: the development environment isintended to be Python 3.9 or higher, so there is no issuewith using `-X dev`.Avoiding interaction between tests via class attributes=======================================================Avoid class attributes in test classes. Use [data attributes][1]instead. Initialize the data attributes in setup methods of thetest classes, as is common practice. This approach avoidsinteraction (via class attributes) between test scripts thatimport the same modules of common tests.With `nose`, this kind of interaction apparently did notoccur, as observed by test failures that were expected to happen.However, `pytest` apparently runs tests in a way thatchanges to imported modules (e.g., class attributes)persist between different test scripts.This `pytest` behavior was observed by the disappearance oftest failures when running with `pytest` (the test failureswere observable with `pytest` only when telling `pytest` torun individual test scripts, instead of collecting tests fromall test scripts.The cause of the issue with `pytest` was the modification ofclass attributes (not [data attributes][1]) from the importingmodule, of classes in the imported module. The modificationswere done by setting the class attribute `DD` that defines theBDD or ZDD manager class. Both the scripts `cudd_test.py` and`autoref_test.py` made modifications. The end result was`autoref` tests being run using the class `dd.cudd.BDD`.Using data attributes instead of class attributes,and subclassing, avoids this kind of erroneous testing.This approach is explicit [PEP 20][4].Also, note that the `pytest` extension packages[`pytest-xdist`][2] and [`pytest-forked`][3]do not avoid this issue.[1]:https://docs.python.org/3/tutorial/classes.html#instance-objects[2]:https://github.com/pytest-dev/pytest-xdist[3]:https://github.com/pytest-dev/pytest-forked[4]:https://www.python.org/dev/peps/pep-0020/

for compatibility with Python 3.10.The package `nose` is not compatible with Python 3.10,because it imports abstract base classes from themodule `collections`,[`nose == 1.3.7`](https://pypi.org/project/nose/1.3.7/#history)imports in Python 3.10 without error, but `nosetests` fails,due to imports from the module `collections` of classes thathave moved to the module `collections.abc`. The error raised contains:```  File ".../.virtualenvs/.../lib/python3.10/site-packages/nose/suite.py", line 106, in _set_tests    if isinstance(tests, collections.Callable) and not is_suite:AttributeError: module 'collections' has no attribute 'Callable'```Relevant information about CPython changes in `collections` (removal ofABCs):-python/cpython#23754-https://bugs.python.org/issue37324-https://docs.python.org/3.10/library/collections.abc.html#collections-abstract-base-classes- Deprecation since Python 3.3, present until Python 3.9:https://docs.python.org/3.9/library/collections.html#module-collections- Removed in Python 3.10:https://docs.python.org/3.10/whatsnew/3.10.html#removedThe script `run_tests.py` is rewritten, based on `pytest` functionality.The file `test/transys_mathset_test.py` has been changed to mode 100644in order to not need an equivalent `pytest` option for option `--exe`of `nose`.In the test script `tests/mathfunc_test.py`, the function`_get_state_input_output_pair_test` has been renamed to`_get_state_input_output_pair` to avoid its name being matched by thepattern `python_functions = test_* *_test` defined in theconfiguration file `tests/pytest.ini`.

for compatibility with Python 3.10.The package `nose` is not compatible with Python 3.10,because it imports abstract base classes from themodule `collections`,[`nose == 1.3.7`](https://pypi.org/project/nose/1.3.7/#history)imports in Python 3.10 without error, but `nosetests` fails,due to imports from the module `collections` of classes thathave moved to the module `collections.abc`. The error raised contains:```  File ".../.virtualenvs/.../lib/python3.10/site-packages/nose/suite.py", line 106, in _set_tests    if isinstance(tests, collections.Callable) and not is_suite:AttributeError: module 'collections' has no attribute 'Callable'```Relevant information about CPython changes in `collections` (removal ofABCs):-python/cpython#23754-https://bugs.python.org/issue37324-https://docs.python.org/3.10/library/collections.abc.html#collections-abstract-base-classes- Deprecation since Python 3.3, present until Python 3.9:https://docs.python.org/3.9/library/collections.html#module-collections- Removed in Python 3.10:https://docs.python.org/3.10/whatsnew/3.10.html#removedThe script `run_tests.py` is rewritten, based on `pytest` functionality.The file `test/transys_mathset_test.py` has been changed to mode 100644in order to not need an equivalent `pytest` option for option `--exe`of `nose`.In the test script `tests/mathfunc_test.py`, the function`_get_state_input_output_pair_test` has been renamed to`_get_state_input_output_pair` to avoid its name being matched by thepattern `python_functions = test_* *_test` defined in theconfiguration file `tests/pytest.ini`.

for compatibility with Python 3.10.- REL: add `pytest >= 4.6.11` to the argument  `tests_require` of the function `setuptools.setup`  in the script `setup.py`.  This lower bound has been selected to  ensure compatibility with both Python 3 and  Python 2.7. See below for details.- REL: add `pytest >= 4.6.11` to file `requirements.txt`- REL: remove `nose` from argument `tests_require` of  the function `setuptools.setup` in script `setup.py`- REL: remove `nose` from file `requirements.txt`- CI: update file `.travis.yml`- CI: remove the collection of  coverage measurements on Travis CI,  because these measurements do not collect  Cython coverage with the build configuration  currently used for CI testing.- DEV: remove `coveralls` from the  file `requirements.txt`, because the  package `coveralls` was used only on Travis CI.- TST: add a configuration file `tests/pytest.ini`  and include it in `MANIFEST.in`- GIT: ignore `.pytest_cache/` in `.gitignore`## MotivationThe change from [`nose == 1.3.7`](https://pypi.org/project/nose/1.3.7/#history)to [`pytest`](https://pypi.org/project/pytest)is motivated by compatibility with Python 3.10.## `nose` is incompatible with Python 3.10The package `nose`, which was used to runthe tests of `dd`, is not compatible with Python 3.10(for details, read the last section below).Also, `nose` uses the `imp` module fromCPython's standard library.The module `imp` [is deprecated](https://docs.python.org/3.10/library/imp.html),so it may be removed in some future Python version.## Summary of transition to `pytest`In summary, using `pytest` with theexisting tests requires addinga [configuration file `tests/pytest.ini`](https://docs.pytest.org/en/latest/reference/customize.html#configuration-file-formats)to tell `pytest` which functions, classes, andmethods to collect tests from(called "discovery" of tests).The [parameter `--continue-on-collection-errors`](https://docs.pytest.org/en/latest/reference/reference.html#command-line-flags)tells `pytest` to not stop in case anytest module fails to import,and to continue with running the tests.The ability to run the tests when some`dd` C extension modules are not installed is necessary.After transitioning the tests to `pytest`,the tests have been confirmedto run successfully:- on Python 2.7 with `pytest == 4.6.11`, and- on Python 3.9 with `pytest == 6.2.4`.## Considering using `unittest`For writing `dd` tests, the module [`unittest`](https://docs.python.org/3/library/unittest.html)(part of CPython's standard library) suffices.For *discovering* the tests, `unittest` seems torequire that tests be methods of subclasses ofthe class `unittest.TestCase`.This is not the case in the tests of `dd` tests.Using `pytest` allows changing the test runner from `nosetests`with minimal changes to the tests themselves.Test discovery using `unittest` could possibly beimplemented by adding a file `tests/__init__.py`, anddefining in that file a function `load_tests`,following the [documentation of `unittest`](https://docs.python.org/3/library/unittest.html#unittest.TestLoader.discover).In any case, it is simpler to use `pytest`,which requires only a configuration file.If `unittest` encounters an `ImportError` duringcollection of the tests (i.e., when `unittest` tries toimport test modules), then it stops.There does not appear to be any way to tell `unittest`to continue and run the rest of the test modules(those that *could* be imported).## Usage of `nose`The dependence on `nose` was minimal.Only one function was used from `nose`:the function `nose.tools.assert_raises`.The function `assert_raises` is dynamically defined[in the module `nose.tools.trivial`](https://github.com/nose-devs/nose/blob/release_1.3.7/nose/tools/trivial.py#L32-L54)by instantiating the class `unittest.TestCase`,and setting `assert_raises` to equal the[bound method `assertRaises`](https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertRaises)of the instance of `TestCase`.So the function `assert_raises` from `nose` isa PEP8-compliant binding for the method`unittest.TestCase.assertRaises`.Reading the code of `unittest`:- <https://github.com/python/cpython/blob/6fdc4d37f3fdbc1bd51f841be6e5e4708a3b8798/Lib/unittest/case.py#L156-L243>- <https://github.com/python/cpython/blob/6fdc4d37f3fdbc1bd51f841be6e5e4708a3b8798/Lib/unittest/case.py#L704-L735>it follows that the usage:```pythonwith nose.tools.assert_raises(AssertionError):    foo(1, 2)```is equivalent to the following code(`AssertionError` here is used as an example):```pythonwith unittest.TestCase().assertRaises(AssertionError):    foo(1, 2)```## Replacing usage of `nose` with `pytest` in test codeThe [context manager `pytest.raises`](https://docs.pytest.org/en/latest/reference/reference.html#pytest-raises)is a [drop-in replacement](https://en.wikipedia.org/wiki/Drop-in_replacement)for the function `nose.tools.assert_raises`:```pythonwith pytest.raises(AssertionError):    foo(1, 2)```Also, the tests can still be run with `nosetests`on Python versions where `nose` is still available.## Replacing the test runner `nosetests` with `pytest`- `pytest` correctly recognized the  test files by default- `pytest` does not recognize by default methods of  classes that do not start with "Test" as test methods.  The configuration file is necessary for  changing this behavior of `pytest`  (in particular, the command-line parameter `-k`   did not seem to work for classes).  Relevant documentation:  - <https://docs.pytest.org/en/latest/explanation/goodpractices.html#conventions-for-python-test-discovery>  - <https://docs.pytest.org/en/latest/example/pythoncollection.html#changing-naming-conventions>  - <https://docs.pytest.org/en/latest/how-to/nose.html>  - <https://docs.pytest.org/en/latest/reference/reference.html#confval-python_classes>  - <https://docs.pytest.org/en/latest/reference/reference.html#confval-python_functions>  - <https://docs.pytest.org/en/latest/reference/reference.html#confval-python_files>- The call `pytest tests/foo_test.py` imports  the package `dd` from `site-packages`  (assuming that the module `foo_test.py` contains   the statement `import dd`).  So the default behavior of `pytest` is  as intended.  In contrast, `nosetests tests/foo_test.py` imports  the package `dd` from the local directory `dd/`,  even though `dd` *is* installed under `site-packages`.  In any case, `pytest` is called from within  the directory `tests/`, as was done for `nosetests`.  Both:  - `python -m pytest tests/foo_test.py` and  - `PYTHONPATH=. pytest tests/foo_test.py`  result in importing `dd` from the local directory `dd/`.  Relevant documentation:  - <https://docs.pytest.org/en/latest/explanation/pythonpath.html#invoking-pytest-versus-python-m-pytest>  - <https://docs.pytest.org/en/latest/how-to/usage.html#invoke-python>As remarked above, the parameter[`--continue-on-collection-errors`](https://docs.pytest.org/en/latest/reference/reference.html#command-line-flags)of `pytest` needs to be used for running the testswhen some of the C extension modules are not installed.For example:```cd tests/pytest -v --continue-on-collection-errors .```To activate [Python's development mode](https://docs.python.org/3/library/devmode.html):```cd tests/python -X dev -m pytest -vvv --continue-on-collection-errors .```## Further remarksObservations about `pytest`:- detects assertions that failed, and marks  their source lines- avoids the deprecated `imp` module (of the  standard library) that `nose` uses  (and thus the associated `DeprecationWarning`)   <https://docs.python.org/3.10/library/imp.html>- running the tests of `dd` with `pytest` revealed  several `DeprecationWarning`s that were previously  not shown by `nose` (these warnings were about   invalid escape sequences due to   backslashes appearing in non-raw strings).[`pytest == 6.2.4`](https://pypi.org/project/pytest/6.2.4) is notcompatible with Python 2.7.[`pytest == 4.6.11`](https://pypi.org/project/pytest/4.6.11/)is the latest version of `pytest` that is compatible with Python 2.7(released on June 5, 2020).`pytest` specifies `python_requires` [PEP 345](https://www.python.org/dev/peps/pep-0345/#requires-python),[PEP 503](https://www.python.org/dev/peps/pep-0503/):- <https://github.com/pytest-dev/pytest/blob/4.6.11/setup.cfg#L48>- <https://github.com/pytest-dev/pytest/blob/5.0.0/setup.cfg#L43>So including `pytest>=4.6.11` in the file`requirements.txt` suffices to install,on each Python version, the latest version of`pytest` that is compatible with that Python version.This simplifies testing on CI, and packaging.In other words, conditional installations inthe file `.travis.yml` are not needed for `pytest`,neither conditional definition of `tests_require`in the script `setup.py`.This approach leaves implicit the upper bound on`pytest` in `tests_require`.This upper bound is specified explicitly by`pytest` itself, depending on the Python version ofthe interpreter.It appears that `pip == 9.0.0` and`setuptools == 24.2.0` are required,to correctly implement `python_requires`:- <https://pip.pypa.io/en/stable/news/#v9-0-0>- <https://setuptools.readthedocs.io/en/latest/history.html#v24-2-1>## How replacing usage of `nose` with `unittest` would have looked likeA way to replace `nose` could have been toadd a module `tests/utils.py` containing:```python"""Common functionality for tests."""import unittest_test_case = unittest.TestCase()assert_raises = _test_case.assertRaises```which is close to what `nose` does.The function `assert_raises` could thenbe imported from the module `utils` intest modules, and used.Using `pytest` avoids the need forthis workaround.## Details about the incompatibility of `nose` with Python 3.10[`nose == 1.3.7`](https://pypi.org/project/nose/1.3.7/#history)imports in Python 3.10. Running `nosetests` fails,due to imports from the module `collections` ofclasses that have moved to the module `collections.abc`.Relevant information about CPython changesin `collections` (removal of ABCs):- <python/cpython#23754>- <https://bugs.python.org/issue37324>- <https://docs.python.org/3.10/library/collections.abc.html#collections-abstract-base-classes>- Deprecation since Python 3.3,  present until Python 3.9:      <https://docs.python.org/3.9/library/collections.html#module-collections>- Removed in Python 3.10:      <https://docs.python.org/3.10/whatsnew/3.10.html#removed>## About skipping testsThe decorator `unittest.skipIf` is recognizedby `pytest`, and skipped tests are correctlyrecorded and reported. In any case, note alsothe `pytest` test-skipping facilities:- <https://docs.pytest.org/en/latest/how-to/skipping.html>- <https://docs.pytest.org/en/latest/how-to/unittest.html>- <https://docs.pytest.org/en/latest/example/simple.html#control-skipping-of-tests-according-to-command-line-option>## About passing `-X dev` to `python` in the `Makefile`The argument `dev` is available for the `python` option[`-X` only on Python 3.7 and higher](https://docs.python.org/3/library/devmode.html#devmode).So the `Makefile` rules where `-X dev` appears arenot compatible with ealier Python versions supported by `dd`.This is not an issue: the development environment isintended to be Python 3.9 or higher, so there is no issuewith using `-X dev`.## Avoiding interaction between tests via class attributesAvoid class attributes in test classes.Use [data attributes][1] instead.Initialize the data attributes in setup methodsof the test classes, as is common practice.This approach avoids interaction (via classattributes) between test scripts thatimport the same modules of common tests.With `nose`, this kind of interactionapparently did not occur, as observed bytest failures that were expected to happen.However, `pytest` apparently runs tests ina way that changes to imported modules(e.g., class attributes)persist between different test scripts.This `pytest` behavior was observed bythe disappearance of test failures whenrunning with `pytest` (the test failureswere observable with `pytest` only whentelling `pytest` to run individual testscripts, instead of collecting tests fromall test scripts.The cause of the issue with `pytest` wasthe modification of class attributes(not [data attributes][1]) from theimporting module, of classes in theimported module.The modifications were done by setting theclass attribute `DD` that defines the BDD orZDD manager class. Both the scripts`cudd_test.py` and `autoref_test.py` mademodifications. The end result was`autoref` tests being run using theclass `dd.cudd.BDD`.Using data attributes, instead of classattributes, and subclassing, avoids thiskind of erroneous testing.This approach is explicit [PEP 20][4].[1]:https://docs.python.org/3/tutorial/classes.html#instance-objects[2]:https://github.com/pytest-dev/pytest-xdist[3]:https://github.com/pytest-dev/pytest-forked[4]:https://www.python.org/dev/peps/pep-0020/

for compatibility with Python 3.10.- REL: add `pytest >= 4.6.11` to the argument  `tests_require` of the function `setuptools.setup`  in the script `setup.py`.  This lower bound has been selected to  ensure compatibility with both Python 3 and  Python 2.7. See below for details.- REL: add `pytest >= 4.6.11` to file `requirements.txt`- REL: remove `nose` from argument `tests_require` of  the function `setuptools.setup` in script `setup.py`- REL: remove `nose` from file `requirements.txt`- CI: update file `.travis.yml`- CI: remove the collection of  coverage measurements on Travis CI,  because these measurements do not collect  Cython coverage with the build configuration  currently used for CI testing.- DEV: remove `coveralls` from the  file `requirements.txt`, because the  package `coveralls` was used only on Travis CI.- TST: add a configuration file `tests/pytest.ini`  and include it in `MANIFEST.in`- GIT: ignore `.pytest_cache/` in `.gitignore`## MotivationThe change from [`nose == 1.3.7`](https://pypi.org/project/nose/1.3.7/#history)to [`pytest`](https://pypi.org/project/pytest)is motivated by compatibility with Python 3.10.## `nose` is incompatible with Python 3.10The package `nose`, which was used to runthe tests of `dd`, is not compatible with Python 3.10(for details, read the last section below).Also, `nose` uses the `imp` module fromCPython's standard library.The module `imp` [is deprecated](https://docs.python.org/3.10/library/imp.html),so it may be removed in some future Python version.## Summary of transition to `pytest`In summary, using `pytest` with theexisting tests requires addinga [configuration file `tests/pytest.ini`](https://docs.pytest.org/en/latest/reference/customize.html#configuration-file-formats)to tell `pytest` which functions, classes, andmethods to collect tests from(called "discovery" of tests).The [parameter `--continue-on-collection-errors`](https://docs.pytest.org/en/latest/reference/reference.html#command-line-flags)tells `pytest` to not stop in case anytest module fails to import,and to continue with running the tests.The ability to run the tests when some`dd` C extension modules are not installed is necessary.After transitioning the tests to `pytest`,the tests have been confirmedto run successfully:- on Python 2.7 with `pytest == 4.6.11`, and- on Python 3.9 with `pytest == 6.2.4`.## Considering using `unittest`For writing `dd` tests, the module [`unittest`](https://docs.python.org/3/library/unittest.html)(part of CPython's standard library) suffices.For *discovering* the tests, `unittest` seems torequire that tests be methods of subclasses ofthe class `unittest.TestCase`.This is not the case in the tests of `dd` tests.Using `pytest` allows changing the test runner from `nosetests`with minimal changes to the tests themselves.Test discovery using `unittest` could possibly beimplemented by adding a file `tests/__init__.py`, anddefining in that file a function `load_tests`,following the [documentation of `unittest`](https://docs.python.org/3/library/unittest.html#unittest.TestLoader.discover).In any case, it is simpler to use `pytest`,which requires only a configuration file.If `unittest` encounters an `ImportError` duringcollection of the tests (i.e., when `unittest` tries toimport test modules), then it stops.There does not appear to be any way to tell `unittest`to continue and run the rest of the test modules(those that *could* be imported).## Usage of `nose`The dependence on `nose` was minimal.Only one function was used from `nose`:the function `nose.tools.assert_raises`.The function `assert_raises` is dynamically defined[in the module `nose.tools.trivial`](https://github.com/nose-devs/nose/blob/release_1.3.7/nose/tools/trivial.py#L32-L54)by instantiating the class `unittest.TestCase`,and setting `assert_raises` to equal the[bound method `assertRaises`](https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertRaises)of the instance of `TestCase`.So the function `assert_raises` from `nose` isa PEP8-compliant binding for the method`unittest.TestCase.assertRaises`.Reading the code of `unittest`:- <https://github.com/python/cpython/blob/6fdc4d37f3fdbc1bd51f841be6e5e4708a3b8798/Lib/unittest/case.py#L156-L243>- <https://github.com/python/cpython/blob/6fdc4d37f3fdbc1bd51f841be6e5e4708a3b8798/Lib/unittest/case.py#L704-L735>it follows that the usage:```pythonwith nose.tools.assert_raises(AssertionError):    foo(1, 2)```is equivalent to the following code(`AssertionError` here is used as an example):```pythonwith unittest.TestCase().assertRaises(AssertionError):    foo(1, 2)```## Replacing usage of `nose` with `pytest` in test codeThe [context manager `pytest.raises`](https://docs.pytest.org/en/latest/reference/reference.html#pytest-raises)is a [drop-in replacement](https://en.wikipedia.org/wiki/Drop-in_replacement)for the function `nose.tools.assert_raises`:```pythonwith pytest.raises(AssertionError):    foo(1, 2)```Also, the tests can still be run with `nosetests`on Python versions where `nose` is still available.## Replacing the test runner `nosetests` with `pytest`- `pytest` correctly recognized the  test files by default- `pytest` does not recognize by default methods of  classes that do not start with "Test" as test methods.  The configuration file is necessary for  changing this behavior of `pytest`  (in particular, the command-line parameter `-k`   did not seem to work for classes).  Relevant documentation:  - <https://docs.pytest.org/en/latest/explanation/goodpractices.html#conventions-for-python-test-discovery>  - <https://docs.pytest.org/en/latest/example/pythoncollection.html#changing-naming-conventions>  - <https://docs.pytest.org/en/latest/how-to/nose.html>  - <https://docs.pytest.org/en/latest/reference/reference.html#confval-python_classes>  - <https://docs.pytest.org/en/latest/reference/reference.html#confval-python_functions>  - <https://docs.pytest.org/en/latest/reference/reference.html#confval-python_files>- The call `pytest tests/foo_test.py` imports  the package `dd` from `site-packages`  (assuming that the module `foo_test.py` contains   the statement `import dd`).  So the default behavior of `pytest` is  as intended.  In contrast, `nosetests tests/foo_test.py` imports  the package `dd` from the local directory `dd/`,  even though `dd` *is* installed under `site-packages`.  In any case, `pytest` is called from within  the directory `tests/`, as was done for `nosetests`.  Both:  - `python -m pytest tests/foo_test.py` and  - `PYTHONPATH=. pytest tests/foo_test.py`  result in importing `dd` from the local directory `dd/`.  Relevant documentation:  - <https://docs.pytest.org/en/latest/explanation/pythonpath.html#invoking-pytest-versus-python-m-pytest>  - <https://docs.pytest.org/en/latest/how-to/usage.html#invoke-python>As remarked above, the parameter[`--continue-on-collection-errors`](https://docs.pytest.org/en/latest/reference/reference.html#command-line-flags)of `pytest` needs to be used for running the testswhen some of the C extension modules are not installed.For example:```cd tests/pytest -v --continue-on-collection-errors .```To activate [Python's development mode](https://docs.python.org/3/library/devmode.html):```cd tests/python -X dev -m pytest -vvv --continue-on-collection-errors .```## Further remarksObservations about `pytest`:- detects assertions that failed, and marks  their source lines- avoids the deprecated `imp` module (of the  standard library) that `nose` uses  (and thus the associated `DeprecationWarning`)   <https://docs.python.org/3.10/library/imp.html>- running the tests of `dd` with `pytest` revealed  several `DeprecationWarning`s that were previously  not shown by `nose` (these warnings were about   invalid escape sequences due to   backslashes appearing in non-raw strings).[`pytest == 6.2.4`](https://pypi.org/project/pytest/6.2.4) is notcompatible with Python 2.7.[`pytest == 4.6.11`](https://pypi.org/project/pytest/4.6.11/)is the latest version of `pytest` that is compatible with Python 2.7(released on June 5, 2020).`pytest` specifies `python_requires` [PEP 345](https://www.python.org/dev/peps/pep-0345/#requires-python),[PEP 503](https://www.python.org/dev/peps/pep-0503/):- <https://github.com/pytest-dev/pytest/blob/4.6.11/setup.cfg#L48>- <https://github.com/pytest-dev/pytest/blob/5.0.0/setup.cfg#L43>So including `pytest>=4.6.11` in the file`requirements.txt` suffices to install,on each Python version, the latest version of`pytest` that is compatible with that Python version.This simplifies testing on CI, and packaging.In other words, conditional installations inthe file `.travis.yml` are not needed for `pytest`,neither conditional definition of `tests_require`in the script `setup.py`.This approach leaves implicit the upper bound on`pytest` in `tests_require`.This upper bound is specified explicitly by`pytest` itself, depending on the Python version ofthe interpreter.It appears that `pip == 9.0.0` and`setuptools == 24.2.0` are required,to correctly implement `python_requires`:- <https://pip.pypa.io/en/stable/news/#v9-0-0>- <https://setuptools.readthedocs.io/en/latest/history.html#v24-2-1>## How replacing usage of `nose` with `unittest` would have looked likeA way to replace `nose` could have been toadd a module `tests/utils.py` containing:```python"""Common functionality for tests."""import unittest_test_case = unittest.TestCase()assert_raises = _test_case.assertRaises```which is close to what `nose` does.The function `assert_raises` could thenbe imported from the module `utils` intest modules, and used.Using `pytest` avoids the need forthis workaround.## Details about the incompatibility of `nose` with Python 3.10[`nose == 1.3.7`](https://pypi.org/project/nose/1.3.7/#history)imports in Python 3.10. Running `nosetests` fails,due to imports from the module `collections` ofclasses that have moved to the module `collections.abc`.Relevant information about CPython changesin `collections` (removal of ABCs):- <python/cpython#23754>- <https://bugs.python.org/issue37324>- <https://docs.python.org/3.10/library/collections.abc.html#collections-abstract-base-classes>- Deprecation since Python 3.3,  present until Python 3.9:      <https://docs.python.org/3.9/library/collections.html#module-collections>- Removed in Python 3.10:      <https://docs.python.org/3.10/whatsnew/3.10.html#removed>## About skipping testsThe decorator `unittest.skipIf` is recognizedby `pytest`, and skipped tests are correctlyrecorded and reported. In any case, note alsothe `pytest` test-skipping facilities:- <https://docs.pytest.org/en/latest/how-to/skipping.html>- <https://docs.pytest.org/en/latest/how-to/unittest.html>- <https://docs.pytest.org/en/latest/example/simple.html#control-skipping-of-tests-according-to-command-line-option>## About passing `-X dev` to `python` in the `Makefile`The argument `dev` is available for the `python` option[`-X` only on Python 3.7 and higher](https://docs.python.org/3/library/devmode.html#devmode).So the `Makefile` rules where `-X dev` appears arenot compatible with ealier Python versions supported by `dd`.This is not an issue: the development environment isintended to be Python 3.9 or higher, so there is no issuewith using `-X dev`.## Avoiding interaction between tests via class attributesAvoid class attributes in test classes.Use [data attributes][1] instead.Initialize the data attributes in setup methodsof the test classes, as is common practice.This approach avoids interaction (via classattributes) between test scripts thatimport the same modules of common tests.With `nose`, this kind of interactionapparently did not occur, as observed bytest failures that were expected to happen.However, `pytest` apparently runs tests ina way that changes to imported modules(e.g., class attributes)persist between different test scripts.This `pytest` behavior was observed bythe disappearance of test failures whenrunning with `pytest` (the test failureswere observable with `pytest` only whentelling `pytest` to run individual testscripts, instead of collecting tests fromall test scripts.The cause of the issue with `pytest` wasthe modification of class attributes(not [data attributes][1]) from theimporting module, of classes in theimported module.The modifications were done by setting theclass attribute `DD` that defines the BDD orZDD manager class. Both the scripts`cudd_test.py` and `autoref_test.py` mademodifications. The end result was`autoref` tests being run using theclass `dd.cudd.BDD`.Using data attributes, instead of classattributes, and subclassing, avoids thiskind of erroneous testing.This approach is explicit [PEP 20][4].[1]:https://docs.python.org/3/tutorial/classes.html#instance-objects[2]:https://github.com/pytest-dev/pytest-xdist[3]:https://github.com/pytest-dev/pytest-forked[4]:https://www.python.org/dev/peps/pep-0020/