pytest-accept

pytest-accept is a pytest plugin for automatically updating doctest outputs. It runs doctests, observes the generated outputs, and writes them to the doctests' documented outputs.

It's designed for a couple of use cases:

• People who work with doctests and don't enjoy manually copying generated outputs from the pytest error log and pasting them into their doctests' documented outputs. pytest-accept does the copying & pasting for you.
• People who generally find writing tests a bit annoying, and prefer to develop by "running the code and seeing whether it works". This library aims to make testing a joyful part of that development loop.

pytest-accept is decoupled from the doctests it works with — it can be used with existing doctests, and the doctests it edits are no different from normal doctests.

Jesse, what the?

Here's an example of pytest-accept does: given a file like add.py containing an incorrect documented output:

def add(x, y):
    """
    Adds two values.

    >>> add(1, 1)
    3

    >>> add("ab", "c")
    'bac'
    """

    return x + y

...running doctests using pytest and passing --accept replaces the existing incorrect values with correct values:

pytest --doctest-modules examples/add.py --accept

diff --git a/examples/add.py b/examples/add.py
index 10a71fd..c2c945f 100644
--- a/examples/add.py
+++ b/examples/add.py
@@ -3,10 +3,10 @@ def add(x, y):
     Adds two values.
 
     >>> add(1, 1)
-    3
+    2
 
     >>> add("ab", "c")
-    'bac'
+    'abc'
     """
 
     return x + y

This style of testing is fairly well-developed in some languages, although still doesn't receive the attention I think it deserves, and historically hasn't had good support in python.

Confusingly, it's referred to "snapshot testing" or "regression testing" or "expect testing" or "literate testing" or "acceptance testing". The best explanation I've seen on this testing style is from Ron Minsky in a Jane Street Blogpost. @matklad also has an excellent summary in his blog post How to Test.

Installation

pip install pytest-accept

What about normal tests?

A previous effort in assert_plugin.py attempted to do this for assert statements, and the file contains some notes on the effort. The biggest problem is pytest stops on the first assert failure in each test, which is very limiting. (Whereas pytest can be configured to continue on doctest failures, which this library takes advantage of.)

It's probably possible to change pytest's behavior here, but it's a significant effort on the pytest codebase.

Some alternatives:

• Use an existing library like pytest-regtest, which offers file snapshot testing (i.e. not inline).
• We could write a specific function / fixture, like accept(result, "abc"), similar to frameworks like rust's excellent insta (which I developed some features for), or ocaml's ppx_expect.
  • But this has the disadvantage of coupling the test to the plugin: it's not possible to run tests independently of the plugin, or use the plugin on general assert tests. And one of the great elegances of pytest is its deferral to a normal assert statement.
• Some of this testing feels like writing a notebook and testing that. pytest-notebook fully implements this.

Anything else?

Not really! Some things to watch out for:

• It'll replace the file at the end of a test. So — to the extent there are useful changes to the file between the start and and the end of a test — it'll overwrite them. Passing --accept-copy will cause the plugin to instead create a file named {file}.py.new.
  • TODO: Should we disable the plugin on --pdb as one way of long-running tests?
  • It will overwrite the existing values, though these aren't generally useful — they're designed to match the results of the code.
• This is early, and there are probably some small bugs. Let me know and I'll attempt to fix them.
• It currently doesn't affect the printing of test results; the doctests will still print as failures.
  • TODO: A future version could print something about them being fixed.
• Python's doctest library is imperfect:
  • It can't handle indents, and probably other things. (We do handle blank lines though, and TODO: check whether the output we paste is valid doctest output).
  • The syntax for .* is an ellipsis ..., which is also the syntax for continuing a code line, so it can't be at the start of a line.
  • The syntax for all the directives is arguably less than aesthetically pleasing.
  • It reports line numbers incorrectly in some cases — two docstring lines separated with continuation character \ is counted as one, meaning this library will not have access to the correct line number for doctest inputs and outputs.