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# Contributing to the code base

## Code standards

Writing good code is not just about what you write. It is also about how you write it. During Continuous Integration testing, several tools will be run to check your code for stylistic errors. Generating any warnings will cause the test to fail. Thus, good style is a requirement for submitting code to pandas.

There are a couple of tools in pandas to help contributors verify their changes before contributing to the project

- `./ci/code_checks.sh`: a script validates the doctests, formatting in docstrings, and imported modules. It is possible to run the checks independently by using the parameters `docstrings`, `code`, and `doctests` (e.g. `./ci/code_checks.sh doctests`);

- `pre-commit`, which we go into detail on in the next section.

In addition, because a lot of people use our library, it is important that we do not make sudden changes to the code that could have the potential to break a lot of user code as a result, that is, we need it to be as backwards compatible as possible to avoid mass breakages.

## Pre-commit

Additionally, Continuous Integration will run code formatting checks like `ruff`, `isort`, and `clang-format` and more using pre-commit hooks. Any warnings from these checks will cause the Continuous Integration to fail; therefore, it is helpful to run the check yourself before submitting code. This can be done by installing `pre-commit` (which should already have happened if you followed the instructions in Setting up your development environment) and then running:

    pre-commit install

from the root of the pandas repository. Now all of the styling checks will be run each time you commit changes without your needing to run each one manually. In addition, using `pre-commit` will also allow you to more easily remain up-to-date with our code checks as they change.

Note that if needed, you can skip these checks with `git commit --no-verify`.

If you don’t want to use `pre-commit` as part of your workflow, you can still use it to run its checks with one of the following:

    pre-commit run --files <files you have modified>
    pre-commit run --from-ref=upstream/main --to-ref=HEAD --all-files

without needing to have done `pre-commit install` beforehand.

Finally, we also have some slow pre-commit checks, which don’t run on each commit but which do run during continuous integration. You can trigger them manually with:

    pre-commit run --hook-stage manual --all-files

Note

You may want to periodically run `pre-commit gc`, to clean up repos which are no longer used.

Note

If you have conflicting installations of `virtualenv`, then you may get an error - see here.

Also, due to a bug in virtualenv, you may run into issues if you’re using conda. To solve this, you can downgrade `virtualenv` to version `20.0.33`.

Note

If you have recently merged in main from the upstream branch, some of the dependencies used by `pre-commit` may have changed. Make sure to update your development environment.

## Optional dependencies

Optional dependencies (e.g. matplotlib) should be imported with the private helper `pandas.compat._optional.import_optional_dependency`. This ensures a consistent error message when the dependency is not met.

All methods using an optional dependency should include a test asserting that an `ImportError` is raised when the optional dependency is not found. This test should be skipped if the library is present.

All optional dependencies should be documented in Optional dependencies and the minimum required version should be set in the `pandas.compat._optional.VERSIONS` dict.

## Backwards compatibility

Please try to maintain backward compatibility. pandas has lots of users with lots of existing code, so don’t break it if at all possible. If you think breakage is required, clearly state why as part of the pull request. Also, be careful when changing method signatures and add deprecation warnings where needed. Also, add the deprecated sphinx directive to the deprecated functions or methods.

If a function with the same arguments as the one being deprecated exist, you can use the `pandas.util._decorators.deprecate`:

    from pandas.util._decorators import deprecate

    deprecate('old_func', 'new_func', '1.1.0')

Otherwise, you need to do it manually:

    import warnings
    from pandas.util._exceptions import find_stack_level

    def old_func():
        """Summary of the function.

        .. deprecated:: 1.1.0
           Use new_func instead.
        """
        warnings.warn(
            'Use new_func instead.',
            FutureWarning,
            stacklevel=find_stack_level(),
        )
        new_func()

    def new_func():
        pass

You’ll also need to

1. Write a new test that asserts a warning is issued when calling with the deprecated argument

2. Update all of pandas existing tests and code to use the new argument

See Testing a warning for more.

## Type hints

pandas strongly encourages the use of PEP 484 style type hints. New development should contain type hints and pull requests to annotate existing code are accepted as well!

### Style guidelines

Type imports should follow the `from typing import ...` convention. Your code may be automatically re-written to use some modern constructs (e.g. using the built-in `list` instead of `typing.List`) by the pre-commit checks.

In some cases in the code base classes may define class variables that shadow builtins. This causes an issue as described in Mypy 1775. The defensive solution here is to create an unambiguous alias of the builtin and use that without your annotation. For example, if you come across a definition like

    class SomeClass1:
        str = None

The appropriate way to annotate this would be as follows

    str_type = str

    class SomeClass2:
        str: str_type = None

In some cases you may be tempted to use `cast` from the typing module when you know better than the analyzer. This occurs particularly when using custom inference functions. For example

    from typing import cast

    from pandas.core.dtypes.common import is_number

    def cannot_infer_bad(obj: Union[str, int, float]):

        if is_number(obj):
            ...
        else:  # Reasonably only str objects would reach this but...
            obj = cast(str, obj)  # Mypy complains without this!
            return obj.upper()

The limitation here is that while a human can reasonably understand that `is_number` would catch the `int` and `float` types mypy cannot make that same inference just yet (see mypy #5206). While the above works, the use of `cast` is strongly discouraged. Where applicable a refactor of the code to appease static analysis is preferable

    def cannot_infer_good(obj: Union[str, int, float]):

        if isinstance(obj, str):
            return obj.upper()
        else:
            ...

With custom types and inference this is not always possible so exceptions are made, but every effort should be exhausted to avoid `cast` before going down such paths.

### pandas-specific types

Commonly used types specific to pandas will appear in pandas._typing and you should use these where applicable. This module is private and is meant for pandas development. Types that are meant for user consumption should be exposed in pandas.api.typing.aliases and ideally added to the pandas-stubs project.

For example, quite a few functions in pandas accept a `dtype` argument. This can be expressed as a string like `"object"`, a `numpy.dtype` like `np.int64` or even a pandas `ExtensionDtype` like `pd.CategoricalDtype`. Rather than burden the user with having to constantly annotate all of those options, this can simply be imported and reused from the pandas._typing module

    from pandas._typing import Dtype

    def as_type(dtype: Dtype) -> ...:
        ...

This module will ultimately house types for repeatedly used concepts like “path-like”, “array-like”, “numeric”, etc… and can also hold aliases for commonly appearing parameters like `axis`. Development of this module is active so be sure to refer to the source for the most up to date list of available types.

### Validating type hints

pandas uses mypy and pyright to statically analyze the code base and type hints. After making any change you can ensure your type hints are consistent by running

    pre-commit run --hook-stage manual --all-files mypy
    pre-commit run --hook-stage manual --all-files pyright
    pre-commit run --hook-stage manual --all-files pyright_reportGeneralTypeIssues
    # the following might fail if the installed pandas version does not correspond to your local git version
    pre-commit run --hook-stage manual --all-files stubtest

in your python environment.

Warning

- Please be aware that the above commands will use the current python environment. If your python packages are older/newer than those installed by the pandas CI, the above commands might fail. This is often the case when the `mypy` or `numpy` versions do not match. Please see how to setup the python environment or select a recently succeeded workflow, select the “Docstring validation, typing, and other manual pre-commit hooks” job, then click on “Set up Conda” and “Environment info” to see which versions the pandas CI installs.

### Testing type hints in code using pandas

Warning

- pandas is not yet a py.typed library (PEP 561)! The primary purpose of locally declaring pandas as a py.typed library is to test and improve the pandas-builtin type annotations.

Until pandas becomes a py.typed library, it is possible to easily experiment with the type annotations shipped with pandas by creating an empty file named “py.typed” in the pandas installation folder:

    python -c "import pandas; import pathlib; (pathlib.Path(pandas.__path__[0]) / 'py.typed').touch()"

The existence of the py.typed file signals to type checkers that pandas is already a py.typed library. This makes type checkers aware of the type annotations shipped with pandas.

## Testing with continuous integration

The pandas test suite will run automatically on GitHub Actions continuous integration services, once your pull request is submitted. However, if you wish to run the test suite on a branch prior to submitting the pull request, then the continuous integration services need to be hooked to your GitHub repository. Instructions are here for GitHub Actions.

A pull-request will be considered for merging when you have an all ‘green’ build. If any tests are failing, then you will get a red ‘X’, where you can click through to see the individual failed tests. This is an example of a green build.

## Test-driven development

pandas is serious about testing and strongly encourages contributors to embrace test-driven development (TDD). This development process “relies on the repetition of a very short development cycle: first the developer writes an (initially failing) automated test case that defines a desired improvement or new function, then produces the minimum amount of code to pass that test.” So, before actually writing any code, you should write your tests. Often the test can be taken from the original GitHub issue. However, it is always worth considering additional use cases and writing corresponding tests.

We use code coverage to help understand the amount of code which is covered by a test. We recommend striving to ensure code you add or change within Pandas is covered by a test. Please see our code coverage dashboard through Codecov for more information.

Adding tests is one of the most common requests after code is pushed to pandas. Therefore, it is worth getting in the habit of writing tests ahead of time so this is never an issue.

### Writing tests

All tests should go into the `tests` subdirectory of the specific package. This folder contains many current examples of tests, and we suggest looking to these for inspiration.

As a general tip, you can use the search functionality in your integrated development environment (IDE) or the git grep command in a terminal to find test files in which the method is called. If you are unsure of the best location to put your test, take your best guess, but note that reviewers may request that you move the test to a different location.

To use git grep, you can run the following command in a terminal:

`git grep "function_name("`

This will search through all files in your repository for the text `function_name(`. This can be a useful way to quickly locate the function in the codebase and determine the best location to add a test for it.

Ideally, there should be one, and only one, obvious place for a test to reside. Until we reach that ideal, these are some rules of thumb for where a test should be located.

1. Does your test depend only on code in `pd._libs.tslibs`? This test likely belongs in one of:

   - tests.tslibs

     Note

     No file in `tests.tslibs` should import from any pandas modules outside of `pd._libs.tslibs`

   - tests.scalar

   - tests.tseries.offsets

2. Does your test depend only on code in `pd._libs`? This test likely belongs in one of:

   - tests.libs

   - tests.groupby.test_libgroupby

3. Is your test for an arithmetic or comparison method? This test likely belongs in one of:

   - tests.arithmetic

     Note

     These are intended for tests that can be shared to test the behavior of DataFrame/Series/Index/ExtensionArray using the `box_with_array` fixture.

   - tests.frame.test_arithmetic

   - tests.series.test_arithmetic

4. Is your test for a reduction method (min, max, sum, prod, …)? This test likely belongs in one of:

   - tests.reductions

     Note

     These are intended for tests that can be shared to test the behavior of DataFrame/Series/Index/ExtensionArray.

   - tests.frame.test_reductions

   - tests.series.test_reductions

   - tests.test_nanops

5. Is your test for an indexing method? This is the most difficult case for deciding where a test belongs, because there are many of these tests, and many of them test more than one method (e.g. both `Series.__getitem__` and `Series.loc.__getitem__`)

   1. Is the test specifically testing an Index method (e.g. `Index.get_loc`, `Index.get_indexer`)? This test likely belongs in one of:

      - tests.indexes.test_indexing

      - tests.indexes.fooindex.test_indexing

      Within that files there should be a method-specific test class e.g. `TestGetLoc`.

      In most cases, neither `Series` nor `DataFrame` objects should be needed in these tests.

   2. Is the test for a Series or DataFrame indexing method other than `__getitem__` or `__setitem__`, e.g. `xs`, `where`, `take`, `mask`, `lookup`, or `insert`? This test likely belongs in one of:

      - tests.frame.indexing.test_methodname

      - tests.series.indexing.test_methodname

   3. Is the test for any of `loc`, `iloc`, `at`, or `iat`? This test likely belongs in one of:

      - tests.indexing.test_loc

      - tests.indexing.test_iloc

      - tests.indexing.test_at

      - tests.indexing.test_iat

      Within the appropriate file, test classes correspond to either types of indexers (e.g. `TestLocBooleanMask`) or major use cases (e.g. `TestLocSetitemWithExpansion`).

      See the note in section D) about tests that test multiple indexing methods.

   4. Is the test for `Series.__getitem__`, `Series.__setitem__`, `DataFrame.__getitem__`, or `DataFrame.__setitem__`? This test likely belongs in one of:

      - tests.series.test_getitem

      - tests.series.test_setitem

      - tests.frame.test_getitem

      - tests.frame.test_setitem

      If many cases such a test may test multiple similar methods, e.g.

          import pandas as pd
          import pandas._testing as tm

          def test_getitem_listlike_of_ints():
              ser = pd.Series(range(5))

              result = ser[[3, 4]]
              expected = pd.Series([2, 3])
              tm.assert_series_equal(result, expected)

              result = ser.loc[[3, 4]]
              tm.assert_series_equal(result, expected)

      In cases like this, the test location should be based on the underlying method being tested. Or in the case of a test for a bugfix, the location of the actual bug. So in this example, we know that `Series.__getitem__` calls `Series.loc.__getitem__`, so this is really a test for `loc.__getitem__`. So this test belongs in `tests.indexing.test_loc`.

6. Is your test for a DataFrame or Series method?

   1. Is the method a plotting method? This test likely belongs in one of:

      - tests.plotting

   2. Is the method an IO method? This test likely belongs in one of:

      - tests.io

        Note

        This includes `to_string` but excludes `__repr__`, which is tested in `tests.frame.test_repr` and `tests.series.test_repr`. Other classes often have a `test_formats` file.

   3. Otherwise This test likely belongs in one of:

      - tests.series.methods.test_mymethod

      - tests.frame.methods.test_mymethod

        Note

        If a test can be shared between DataFrame/Series using the `frame_or_series` fixture, by convention it goes in the `tests.frame` file.

7. Is your test for an Index method, not depending on Series/DataFrame? This test likely belongs in one of:

   - tests.indexes

8. Is your test for one of the pandas-provided ExtensionArrays (`Categorical`, `DatetimeArray`, `TimedeltaArray`, `PeriodArray`, `IntervalArray`, `NumpyExtensionArray`, `FloatArray`, `BoolArray`, `StringArray`)? This test likely belongs in one of:

   - tests.arrays

9. Is your test for all ExtensionArray subclasses (the “EA Interface”)? This test likely belongs in one of:

   - tests.extension

### Using `pytest`

#### Test structure

pandas existing test structure is mostly class-based, meaning that you will typically find tests wrapped in a class.

    class TestReallyCoolFeature:
        def test_cool_feature_aspect(self):
            pass

We prefer a more functional style using the pytest framework, which offers a richer testing framework that will facilitate testing and developing. Thus, instead of writing test classes, we will write test functions like this:

    def test_really_cool_feature():
        pass

#### Preferred `pytest` idioms

- Functional tests named `def test_*` and only take arguments that are either fixtures or parameters.

- Use a bare `assert` for testing scalars and truth-testing

- Use `tm.assert_series_equal(result, expected)` and `tm.assert_frame_equal(result, expected)` for comparing `Series` and `DataFrame` results respectively.

- Use @pytest.mark.parameterize when testing multiple cases.

- Use pytest.mark.xfail when a test case is expected to fail.

- Use pytest.mark.skip when a test case is never expected to pass.

- Use pytest.param when a test case needs a particular mark.

- Use @pytest.fixture if multiple tests can share a setup object.

Warning

Do not use `pytest.xfail` (which is different than `pytest.mark.xfail`) since it immediately stops the test and does not check if the test will fail. If this is the behavior you desire, use `pytest.skip` instead.

If a test is known to fail but the manner in which it fails is not meant to be captured, use `pytest.mark.xfail`. It is common to use this method for a test that exhibits buggy behavior or a non-implemented feature. If the failing test has flaky behavior, use the argument `strict=False`. This will make it so pytest does not fail if the test happens to pass. Using `strict=False` is highly undesirable, please use it only as a last resort.

Prefer the decorator `@pytest.mark.xfail` and the argument `pytest.param` over usage within a test so that the test is appropriately marked during the collection phase of pytest. For xfailing a test that involves multiple parameters, a fixture, or a combination of these, it is only possible to xfail during the testing phase. To do so, use the `request` fixture:

    def test_xfail(request):
        mark = pytest.mark.xfail(raises=TypeError, reason="Indicate why here")
        request.applymarker(mark)

xfail is not to be used for tests involving failure due to invalid user arguments. For these tests, we need to verify the correct exception type and error message is being raised, using `pytest.raises` instead.

#### Testing a warning

Use `tm.assert_produces_warning` as a context manager to check that a block of code raises a warning and specify the warning message using the `match` argument.

    with tm.assert_produces_warning(DeprecationWarning, match="the warning message"):
        pd.deprecated_function()

If a warning should specifically not happen in a block of code, pass `False` into the context manager.

    with tm.assert_produces_warning(False):
        pd.no_warning_function()

If you have a test that would emit a warning, but you aren’t actually testing the warning itself (say because it’s going to be removed in the future, or because we’re matching a 3rd-party library’s behavior), then use `pytest.mark.filterwarnings` to ignore the error.

    @pytest.mark.filterwarnings("ignore:msg:category")
    def test_thing(self):
        pass

#### Testing an exception

Use pytest.raises as a context manager with the specific exception subclass (i.e. never use `Exception`) and the exception message in `match`.

    with pytest.raises(ValueError, match="an error"):
        raise ValueError("an error")

#### Testing involving files

The `temp_file` pytest fixture creates a temporary file `Pathlib` object for testing:

    def test_something(temp_file):
        pd.DataFrame([1]).to_csv(str(temp_file))

Please reference pytest’s documentation for the file retention policy.

#### Testing involving network connectivity

A unit test should not access a public data set over the internet due to flakiness of network connections and lack of ownership of the server that is being connected to. To mock this interaction, use the `httpserver` fixture from the pytest-localserver plugin. with synthetic data.

    @pytest.mark.network
    @pytest.mark.single_cpu
    def test_network(httpserver):
        httpserver.serve_content(content="content")
        result = pd.read_html(httpserver.url)

#### Example

Here is an example of a self-contained set of tests in a file `pandas/tests/test_cool_feature.py` that illustrate multiple features that we like to use. Please remember to add the GitHub Issue Number as a comment to a new test.

    import pytest
    import numpy as np
    import pandas as pd

    @pytest.mark.parametrize('dtype', ['int8', 'int16', 'int32', 'int64'])
    def test_dtypes(dtype):
        assert str(np.dtype(dtype)) == dtype

    @pytest.mark.parametrize(
        'dtype', ['float32', pytest.param('int16', marks=pytest.mark.skip),
                  pytest.param('int32', marks=pytest.mark.xfail(
                      reason='to show how it works'))])
    def test_mark(dtype):
        assert str(np.dtype(dtype)) == 'float32'

    @pytest.fixture
    def series():
        return pd.Series([1, 2, 3])

    @pytest.fixture(params=['int8', 'int16', 'int32', 'int64'])
    def dtype(request):
        return request.param

    def test_series(series, dtype):
        # GH <issue_number>
        result = series.astype(dtype)
        assert result.dtype == dtype

        expected = pd.Series([1, 2, 3], dtype=dtype)
        tm.assert_series_equal(result, expected)

A test run of this yields

    ((pandas) bash-3.2$ pytest  test_cool_feature.py  -v
    =========================== test session starts ===========================
    platform darwin -- Python 3.6.2, pytest-3.6.0, py-1.4.31, pluggy-0.4.0
    collected 11 items

    tester.py::test_dtypes[int8] PASSED
    tester.py::test_dtypes[int16] PASSED
    tester.py::test_dtypes[int32] PASSED
    tester.py::test_dtypes[int64] PASSED
    tester.py::test_mark[float32] PASSED
    tester.py::test_mark[int16] SKIPPED
    tester.py::test_mark[int32] xfail
    tester.py::test_series[int8] PASSED
    tester.py::test_series[int16] PASSED
    tester.py::test_series[int32] PASSED
    tester.py::test_series[int64] PASSED

Tests that we have `parametrized` are now accessible via the test name, for example we could run these with `-k int8` to sub-select only those tests which match `int8`.

    ((pandas) bash-3.2$ pytest  test_cool_feature.py  -v -k int8
    =========================== test session starts ===========================
    platform darwin -- Python 3.6.2, pytest-3.6.0, py-1.4.31, pluggy-0.4.0
    collected 11 items

    test_cool_feature.py::test_dtypes[int8] PASSED
    test_cool_feature.py::test_series[int8] PASSED

### Using `hypothesis`

Hypothesis is a library for property-based testing. Instead of explicitly parametrizing a test, you can describe all valid inputs and let Hypothesis try to find a failing input. Even better, no matter how many random examples it tries, Hypothesis always reports a single minimal counterexample to your assertions - often an example that you would never have thought to test.

See Getting Started with Hypothesis for more of an introduction, then refer to the Hypothesis documentation for details.

    import json
    from hypothesis import given, strategies as st

    any_json_value = st.deferred(lambda: st.one_of(
        st.none(), st.booleans(), st.floats(allow_nan=False), st.text(),
        st.lists(any_json_value), st.dictionaries(st.text(), any_json_value)
    ))

    @given(value=any_json_value)
    def test_json_roundtrip(value):
        result = json.loads(json.dumps(value))
        assert value == result

This test shows off several useful features of Hypothesis, as well as demonstrating a good use-case: checking properties that should hold over a large or complicated domain of inputs.

To keep the pandas test suite running quickly, parametrized tests are preferred if the inputs or logic are simple, with Hypothesis tests reserved for cases with complex logic or where there are too many combinations of options or subtle interactions to test (or think of!) all of them.

## Running the test suite

The tests can then be run directly inside your Git clone (without having to install pandas) by typing:

    pytest pandas

Note

If a handful of tests don’t pass, it may not be an issue with your pandas installation. Some tests (e.g. some SQLAlchemy ones) require additional setup, others might start failing because a non-pinned library released a new version, and others might be flaky if run in parallel. As long as you can import pandas from your locally built version, your installation is probably fine and you can start contributing!

Often it is worth running only a subset of tests first around your changes before running the entire suite.

The easiest way to do this is with:

    pytest pandas/path/to/test.py -k regex_matching_test_name

Or with one of the following constructs:

    pytest pandas/tests/[test-module].py
    pytest pandas/tests/[test-module].py::[TestClass]
    pytest pandas/tests/[test-module].py::[TestClass]::[test_method]

Using pytest-xdist, which is included in our ‘pandas-dev’ environment, one can speed up local testing on multicore machines. The `-n` number flag then can be specified when running pytest to parallelize a test run across the number of specified cores or auto to utilize all the available cores on your machine.

    # Utilize 4 cores
    pytest -n 4 pandas

    # Utilizes all available cores
    pytest -n auto pandas

If you’d like to speed things along further a more advanced use of this command would look like this

    pytest pandas -n 4 -m "not slow and not network and not db and not single_cpu" -r sxX

In addition to the multithreaded performance increase this improves test speed by skipping some tests using the `-m` mark flag:

- slow: any test taking long (think seconds rather than milliseconds)

- network: tests requiring network connectivity

- db: tests requiring a database (mysql or postgres)

- single_cpu: tests that should run on a single cpu only

You might want to enable the following option if it’s relevant for you:

- arm_slow: any test taking long on arm64 architecture

These markers are defined in this toml file , under `[tool.pytest.ini_options]` in a list called `markers`, in case you want to check if new ones have been created which are of interest to you.

The `-r` report flag will display a short summary info (see pytest documentation) . Here we are displaying the number of:

- s: skipped tests

- x: xfailed tests

- X: xpassed tests

The summary is optional and can be removed if you don’t need the added information. Using the parallelization option can significantly reduce the time it takes to locally run tests before submitting a pull request.

If you require assistance with the results, which has happened in the past, please set a seed before running the command and opening a bug report, that way we can reproduce it. Here’s an example for setting a seed on windows

    set PYTHONHASHSEED=314159265
    pytest pandas -n 4 -m "not slow and not network and not db and not single_cpu" -r sxX

On Unix use

    export PYTHONHASHSEED=314159265
    pytest pandas -n 4 -m "not slow and not network and not db and not single_cpu" -r sxX

For more, see the pytest documentation.

Furthermore one can run

    pd.test()

with an imported pandas to run tests similarly.

## Running the performance test suite

Performance matters and it is worth considering whether your code has introduced performance regressions. pandas is in the process of migrating to asv benchmarks to enable easy monitoring of the performance of critical pandas operations. These benchmarks are all found in the `pandas/asv_bench` directory, and the test results can be found here.

To use all features of asv, you will need either `conda` or `virtualenv`. For more details please check the asv installation webpage.

To install asv:

    pip install git+https://github.com/airspeed-velocity/asv

If you need to run a benchmark, change your directory to `asv_bench/` and run:

    asv continuous -f 1.1 upstream/main HEAD

You can replace `HEAD` with the name of the branch you are working on, and report benchmarks that changed by more than 10%. The command uses `conda` by default for creating the benchmark environments. If you want to use virtualenv instead, write:

    asv continuous -f 1.1 -E virtualenv upstream/main HEAD

The `-E virtualenv` option should be added to all `asv` commands that run benchmarks. The default value is defined in `asv.conf.json`.

Running the full benchmark suite can be an all-day process, depending on your hardware and its resource utilization. However, usually it is sufficient to paste only a subset of the results into the pull request to show that the committed changes do not cause unexpected performance regressions. You can run specific benchmarks using the `-b` flag, which takes a regular expression. For example, this will only run benchmarks from a `pandas/asv_bench/benchmarks/groupby.py` file:

    asv continuous -f 1.1 upstream/main HEAD -b ^groupby

If you want to only run a specific group of benchmarks from a file, you can do it using `.` as a separator. For example:

    asv continuous -f 1.1 upstream/main HEAD -b groupby.GroupByMethods

will only run the `GroupByMethods` benchmark defined in `groupby.py`.

You can also run the benchmark suite using the version of `pandas` already installed in your current Python environment. This can be useful if you do not have virtualenv or conda; for the in-place build you need to set `PYTHONPATH`, e.g. `PYTHONPATH="$PWD/.." asv [remaining arguments]`. You can run benchmarks using an existing Python environment by:

    asv run -e -E existing

or, to use a specific Python interpreter,:

    asv run -e -E existing:python3.6

This will display stderr from the benchmarks, and use your local `python` that comes from your `$PATH`.

Information on how to write a benchmark and how to use asv can be found in the asv documentation.

## Documenting your code

Changes should be reflected in the release notes located in `doc/source/whatsnew/vx.y.z.rst`. This file contains an ongoing change log for each release. Add an entry to this file to document your fix, enhancement or (unavoidable) breaking change. Make sure to include the GitHub issue number when adding your entry (using `:issue:`1234`` where `1234` is the issue/pull request number). Your entry should be written using full sentences and proper grammar.

When mentioning parts of the API, use a Sphinx `:func:`, `:meth:`, or `:class:` directive as appropriate. Not all public API functions and methods have a documentation page; ideally links would only be added if they resolve. You can usually find similar examples by checking the release notes for one of the previous versions.

If your code is a bugfix, add your entry to the relevant bugfix section. Avoid adding to the `Other` section; only in rare cases should entries go there. Being as concise as possible, the description of the bug should include how the user may encounter it and an indication of the bug itself, e.g. “produces incorrect results” or “incorrectly raises”. It may be necessary to also indicate the new behavior.

If your code is an enhancement, it is most likely necessary to add usage examples to the existing documentation. This can be done following the section regarding documentation. Further, to let users know when this feature was added, the `versionadded` directive is used. The sphinx syntax for that is:

    .. versionadded:: 2.1.0

This will put the text New in version 2.1.0 wherever you put the sphinx directive. This should also be put in the docstring when adding a new function or method (example) or a new keyword argument (example).