Diff output confusing

[JamesGuthrie]

I find the diff output that pytest-insta produces confusing. On further investigation, it seems as though it's backwards w.r.t. pytest's conventions.

An example:

I setup the following test and get a snapshot:

def test_hello_world(snapshot):
    assert snapshot() == "hello world"

The snapshot file contains:

hello world

Now I change my test:

def test_hello_world(snapshot):
    assert snapshot() == "hello bob"

I expect to see the following diff:

E         - hello world
E         + hello bob

Instead, I see the following:

E         - hello bob
E         + hello world

There was a long discussion in the pytest repo about which direction the +/- symbols should go in a diff. The summary is that the convention is that assertions should be written like:

assert computed == expected

And correspondingly, the diff of assert [1, 4, 3] == [1, 2, 3] should be (from this comment):

# "2" is expected but the result list contains "4" instead.
E           [
E            1,
E         -  2,
E         +  4,
E            3,
E           ]

Similarly, for the following test:

def test_array_eq():
    assert "hello bob" == "hello world"

I get:

>       assert "hello bob" == "hello world"
E       AssertionError: assert 'hello bob' == 'hello world'
E
E         - hello world
E         + hello bob

Because pytest-insta is in control of passing the text to be formatted to pytest, I think that the following diff should be sufficient to fix this:

diff --git a/pytest_insta/review.py b/pytest_insta/review.py
index fdd9804..831dc7c 100644
--- a/pytest_insta/review.py
+++ b/pytest_insta/review.py
@@ -57,10 +57,10 @@ class ReviewTool:
                 yield test, snapshot, original

     def display_assertion(self, old: Any, new: Any):
-        self.tr.write_line("\n>       assert old == new")
+        self.tr.write_line("\n>       assert new == old")

         lines, *_ = self.config.hook.pytest_assertrepr_compare(
-            config=self.config, op="==", left=old, right=new
+            config=self.config, op="==", left=new, right=old
         )

         explanation = "assert " + "\n".join("  " + line for line in lines).strip()