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Taking snapshots of polars data frames

Write a snapshot test for a data frame matrix

# test_dataframe.py

import polars as pl
import numpy as np

def test_dataframe(snapshot):
    df = pl.DataFrame(np.eye(3), schema=["a", "b", "c"], orient="col")
    snapshot.check(df, atol=1e-2, rtol=1e-2)

Run the test

The first execution of this test fails:

$ pytest -v test_dataframe.py
============================= test session starts ==============================
platform linux -- Python 3.12.12, pytest-9.0.3, pluggy-1.6.0 -- /home/docs/checkouts/readthedocs.org/user_builds/pytest-regtest/checkouts/latest/.venv/bin/python
cachedir: .pytest_cache
rootdir: /tmp/tmpa7e0icoa
plugins: regtest-2.5.0, cov-7.1.0
collecting ... collected 1 item

test_dataframe.py::test_dataframe FAILED                                 [100%]

=================================== FAILURES ===================================
________________________________ test_dataframe ________________________________

snapshot error(s) for test_dataframe.py::test_dataframe:

snapshot not recorded yet:
    > test_dataframe.py +6
    > snapshot.check(df, atol=1e-2, rtol=1e-2)
    shape: (3, 3)
    ┌─────┬─────┬─────┐
    │ a   ┆ b   ┆ c   │
    │ --- ┆ --- ┆ --- │
    │ f64 ┆ f64 ┆ f64 │
    ╞═════╪═════╪═════╡
    │ 1.0 ┆ 0.0 ┆ 0.0 │
    │ 0.0 ┆ 1.0 ┆ 0.0 │
    │ 0.0 ┆ 0.0 ┆ 1.0 │
    └─────┴─────┴─────┘
---------------------------- pytest-regtest report -----------------------------
total number of failed regression tests: 0
total number of failed snapshot tests  : 1
=========================== short test summary info ============================
FAILED test_dataframe.py::test_dataframe
============================== 1 failed in 0.12s ===============================

Reset the snapshot

Let us record the snapshot:

$ pytest -v --regtest-reset test_dataframe.py
============================= test session starts ==============================
platform linux -- Python 3.12.12, pytest-9.0.3, pluggy-1.6.0 -- /home/docs/checkouts/readthedocs.org/user_builds/pytest-regtest/checkouts/latest/.venv/bin/python
cachedir: .pytest_cache
rootdir: /tmp/tmpa7e0icoa
plugins: regtest-2.5.0, cov-7.1.0
collecting ... collected 1 item

test_dataframe.py::test_dataframe RESET                                  [100%]

---------------------------- pytest-regtest report -----------------------------
total number of failed regression tests: 0
total number of failed snapshot tests  : 0
the following output files have been reset:
  _regtest_outputs/test_dataframe.test_dataframe__0
============================== 1 passed in 0.01s ===============================

Now test test passes:

$ pytest -v test_dataframe.py
============================= test session starts ==============================
platform linux -- Python 3.12.12, pytest-9.0.3, pluggy-1.6.0 -- /home/docs/checkouts/readthedocs.org/user_builds/pytest-regtest/checkouts/latest/.venv/bin/python
cachedir: .pytest_cache
rootdir: /tmp/tmpa7e0icoa
plugins: regtest-2.5.0, cov-7.1.0
collecting ... collected 1 item

test_dataframe.py::test_dataframe PASSED                                 [100%]

=============================== warnings summary ===============================
test_dataframe.py::test_dataframe
  /home/docs/checkouts/readthedocs.org/user_builds/pytest-regtest/checkouts/latest/src/pytest_regtest/polars_handler.py:42: DeprecationWarning: the argument `rtol` for `assert_frame_equal` is deprecated. It was renamed to `rel_tol` in version 1.32.3.
    assert_frame_equal(

test_dataframe.py::test_dataframe
  /home/docs/checkouts/readthedocs.org/user_builds/pytest-regtest/checkouts/latest/src/pytest_regtest/polars_handler.py:42: DeprecationWarning: the argument `atol` for `assert_frame_equal` is deprecated. It was renamed to `abs_tol` in version 1.32.3.
    assert_frame_equal(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
---------------------------- pytest-regtest report -----------------------------
total number of failed regression tests: 0
total number of failed snapshot tests  : 0
======================== 1 passed, 2 warnings in 0.01s =========================

Break the test

# test_dataframe.py

import polars as pl
import numpy as np

def test_dataframe(snapshot):
    df = pl.DataFrame(np.eye(3), schema=["a", "b", "d"], orient="col")
    snapshot.check(df, atol=1e-2, rtol=1e-2)

$ pytest -v test_dataframe.py
============================= test session starts ==============================
platform linux -- Python 3.12.12, pytest-9.0.3, pluggy-1.6.0 -- /home/docs/checkouts/readthedocs.org/user_builds/pytest-regtest/checkouts/latest/.venv/bin/python
cachedir: .pytest_cache
rootdir: /tmp/tmpa7e0icoa
plugins: regtest-2.5.0, cov-7.1.0
collecting ... collected 1 item

test_dataframe.py::test_dataframe FAILED                                 [100%]

=================================== FAILURES ===================================
________________________________ test_dataframe ________________________________

snapshot error(s) for test_dataframe.py::test_dataframe:

snapshot mismatch:
    > test_dataframe.py +6:
    > snapshot.check(df, atol=1e-2, rtol=1e-2)
    --- current
    +++ expected
    @@ -3,4 +3,4 @@
     ---  ------  --------------  -----  
      0   a       3 non-null      Float64
      1   b       3 non-null      Float64
    - 2   d       3 non-null      Float64
    + 2   c       3 non-null      Float64

    --- current
    +++ expected
    @@ -1,6 +1,6 @@
     shape: (3, 3)
     ┌─────┬─────┬─────┐
    -│ a   ┆ b   ┆ d   │
    +│ a   ┆ b   ┆ c   │
     │ --- ┆ --- ┆ --- │
     │ f64 ┆ f64 ┆ f64 │
     ╞═════╪═════╪═════╡
=============================== warnings summary ===============================
test_dataframe.py::test_dataframe
  /home/docs/checkouts/readthedocs.org/user_builds/pytest-regtest/checkouts/latest/src/pytest_regtest/polars_handler.py:42: DeprecationWarning: the argument `rtol` for `assert_frame_equal` is deprecated. It was renamed to `rel_tol` in version 1.32.3.
    assert_frame_equal(

test_dataframe.py::test_dataframe
  /home/docs/checkouts/readthedocs.org/user_builds/pytest-regtest/checkouts/latest/src/pytest_regtest/polars_handler.py:42: DeprecationWarning: the argument `atol` for `assert_frame_equal` is deprecated. It was renamed to `abs_tol` in version 1.32.3.
    assert_frame_equal(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
---------------------------- pytest-regtest report -----------------------------
total number of failed regression tests: 0
total number of failed snapshot tests  : 1
=========================== short test summary info ============================
FAILED test_dataframe.py::test_dataframe
======================== 1 failed, 2 warnings in 0.12s =========================