ENH: raise_assert_detail shows the difference between the columns

[MarcoGorelli]

• xref ENH: raise_assert_detail only shows the difference between the columns #48033 (Replace xxxx with the Github issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

This idea came out of #48033 , so I'm opening a PR to see if anyone else would like to see this

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Adding an extra At index {idx}, diff: {left} != {right}, analougously to pytest's output

Demo:

import pandas as pd
import numpy as np

a = np.random.randint(0, 10, size=200)
b = np.random.randint(0, 10, size=200)
df1 = pd.DataFrame({'a': a, 'b': b})
df2 = pd.DataFrame({'a': a, 'b': b})
df2.loc[97, 'b'] = 42
pd.testing.assert_frame_equal(df1, df2)

outputs

AssertionError: DataFrame.iloc[:, 1] (column name="b") are different

DataFrame.iloc[:, 1] (column name="b") values are different (0.5 %)
[index]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, ...]
[left]:  [1, 4, 3, 6, 4, 7, 2, 5, 3, 6, 6, 9, 8, 4, 4, 4, 7, 8, 4, 0, 3, 1, 4, 5, 7, 0, 3, 0, 9, 9, 6, 1, 7, 7, 4, 7, 6, 2, 3, 9, 1, 5, 1, 7, 2, 3, 8, 9, 5, 3, 6, 3, 7, 3, 4, 1, 7, 3, 6, 9, 0, 0, 7, 3, 6, 4, 4, 1, 0, 8, 0, 1, 8, 2, 8, 1, 8, 8, 8, 3, 8, 3, 8, 5, 7, 0, 0, 2, 8, 5, 3, 4, 7, 1, 6, 3, 5, 9, 7, 8, ...]
[right]: [1, 4, 3, 6, 4, 7, 2, 5, 3, 6, 6, 9, 8, 4, 4, 4, 7, 8, 4, 0, 3, 1, 4, 5, 7, 0, 3, 0, 9, 9, 6, 1, 7, 7, 4, 7, 6, 2, 3, 9, 1, 5, 1, 7, 2, 3, 8, 9, 5, 3, 6, 3, 7, 3, 4, 1, 7, 3, 6, 9, 0, 0, 7, 3, 6, 4, 4, 1, 0, 8, 0, 1, 8, 2, 8, 1, 8, 8, 8, 3, 8, 3, 8, 5, 7, 0, 0, 2, 8, 5, 3, 4, 7, 1, 6, 3, 5, 42, 7, 8, ...]
At positional index 97 diff: 9 != 42

This is analogous to how pytest shows diffs between lists:

    def test_me():
        a = [1, 2, 3]
        b = [4, 5, 6]
>       assert a == b
E       assert [1, 2, 3] == [4, 5, 6]
E         At index 0 diff: 1 != 4
E         Use -v to get more diff

[mroeschke]

Suggested change

	first_diff = f"At index {i} diff: {a[i]} != {b[i]}"
	first_diff = f"At index {i}, first diff: {a[i]} != {b[i]}"

What do you think? Was thinking to clarify that this may not be the only diff during the comparison.

[mroeschke]

Thanks @MarcoGorelli