API: Add entrypoint for plotting (#27488)

Author: TomAugspurger

Diff for: Makefile

@@ -15,7 +15,7 @@ lint-diff:
 	git diff upstream/master --name-only -- "*.py" | xargs flake8
 
 black:
-	black . --exclude '(asv_bench/env|\.egg|\.git|\.hg|\.mypy_cache|\.nox|\.tox|\.venv|_build|buck-out|build|dist)'
+	black . --exclude '(asv_bench/env|\.egg|\.git|\.hg|\.mypy_cache|\.nox|\.tox|\.venv|_build|buck-out|build|dist|setup.py)'
 
 develop: build
 	python setup.py develop

Diff for: ci/code_checks.sh

@@ -56,7 +56,7 @@ if [[ -z "$CHECK" || "$CHECK" == "lint" ]]; then
     black --version
 
     MSG='Checking black formatting' ; echo $MSG
-	black . --check --exclude '(asv_bench/env|\.egg|\.git|\.hg|\.mypy_cache|\.nox|\.tox|\.venv|_build|buck-out|build|dist)'
+	black . --check --exclude '(asv_bench/env|\.egg|\.git|\.hg|\.mypy_cache|\.nox|\.tox|\.venv|_build|buck-out|build|dist|setup.py)'
     RET=$(($RET + $?)) ; echo $MSG "DONE"
 
     # `setup.cfg` contains the list of error codes that are being ignored in flake8

Diff for: doc/source/development/extending.rst

@@ -441,5 +441,22 @@ This would be more or less equivalent to:
 The backend module can then use other visualization tools (Bokeh, Altair,...)
 to generate the plots.
 
+Libraries implementing the plotting backend should use `entry points <https://setuptools.readthedocs.io/en/latest/setuptools.html#dynamic-discovery-of-services-and-plugins>`__
+to make their backend discoverable to pandas. The key is ``"pandas_plotting_backends"``. For example, pandas
+registers the default "matplotlib" backend as follows.
+
+.. code-block:: python
+
+   # in setup.py
+   setup(  # noqa: F821
+       ...,
+       entry_points={
+           "pandas_plotting_backends": [
+               "matplotlib = pandas:plotting._matplotlib",
+           ],
+       },
+   )
+
+
 More information on how to implement a third-party plotting backend can be found at
 https://github.com/pandas-dev/pandas/blob/master/pandas/plotting/__init__.py#L1.

Diff for: doc/source/whatsnew/v0.25.1.rst

@@ -114,7 +114,7 @@ I/O
 Plotting
 ^^^^^^^^
 
--
+- Added a pandas_plotting_backends entrypoint group for registering plot backends. See :ref:`extending.plotting-backends` for more (:issue:`26747`).
 -
 -
 

Diff for: pandas/plotting/_core.py

@@ -1533,6 +1533,53 @@ def hexbin(self, x, y, C=None, reduce_C_function=None, gridsize=None, **kwargs):
         return self(kind="hexbin", x=x, y=y, C=C, **kwargs)
 
 
+_backends = {}
+
+
+def _find_backend(backend: str):
+    """
+    Find a pandas plotting backend>
+
+    Parameters
+    ----------
+    backend : str
+        The identifier for the backend. Either an entrypoint item registered
+        with pkg_resources, or a module name.
+
+    Notes
+    -----
+    Modifies _backends with imported backends as a side effect.
+
+    Returns
+    -------
+    types.ModuleType
+        The imported backend.
+    """
+    import pkg_resources  # Delay import for performance.
+
+    for entry_point in pkg_resources.iter_entry_points("pandas_plotting_backends"):
+        if entry_point.name == "matplotlib":
+            # matplotlib is an optional dependency. When
+            # missing, this would raise.
+            continue
+        _backends[entry_point.name] = entry_point.load()
+
+    try:
+        return _backends[backend]
+    except KeyError:
+        # Fall back to unregisted, module name approach.
+        try:
+            module = importlib.import_module(backend)
+        except ImportError:
+            # We re-raise later on.
+            pass
+        else:
+            _backends[backend] = module
+            return module
+
+    raise ValueError("No backend {}".format(backend))
+
+
 def _get_plot_backend(backend=None):
     """
     Return the plotting backend to use (e.g. `pandas.plotting._matplotlib`).
@@ -1546,7 +1593,18 @@ def _get_plot_backend(backend=None):
     The backend is imported lazily, as matplotlib is a soft dependency, and
     pandas can be used without it being installed.
     """
-    backend_str = backend or pandas.get_option("plotting.backend")
-    if backend_str == "matplotlib":
-        backend_str = "pandas.plotting._matplotlib"
-    return importlib.import_module(backend_str)
+    backend = backend or pandas.get_option("plotting.backend")
+
+    if backend == "matplotlib":
+        # Because matplotlib is an optional dependency and first-party backend,
+        # we need to attempt an import here to raise an ImportError if needed.
+        import pandas.plotting._matplotlib as module
+
+        _backends["matplotlib"] = module
+
+    if backend in _backends:
+        return _backends[backend]
+
+    module = _find_backend(backend)
+    _backends[backend] = module
+    return module

Diff for: pandas/tests/plotting/test_backend.py

@@ -1,5 +1,11 @@
+import sys
+import types
+
+import pkg_resources
 import pytest
 
+import pandas.util._test_decorators as td
+
 import pandas
 
 
@@ -36,3 +42,44 @@ def test_backend_is_correct(monkeypatch):
         pandas.set_option("plotting.backend", "matplotlib")
     except ImportError:
         pass
+
+
+@td.skip_if_no_mpl
+def test_register_entrypoint():
+    mod = types.ModuleType("my_backend")
+    mod.plot = lambda *args, **kwargs: 1
+
+    backends = pkg_resources.get_entry_map("pandas")
+    my_entrypoint = pkg_resources.EntryPoint(
+        "pandas_plotting_backend",
+        mod.__name__,
+        dist=pkg_resources.get_distribution("pandas"),
+    )
+    backends["pandas_plotting_backends"]["my_backend"] = my_entrypoint
+    # TODO: the docs recommend importlib.util.module_from_spec. But this works for now.
+    sys.modules["my_backend"] = mod
+
+    result = pandas.plotting._core._get_plot_backend("my_backend")
+    assert result is mod
+
+    # TODO: https://github.com/pandas-dev/pandas/issues/27517
+    # Remove the td.skip_if_no_mpl
+    with pandas.option_context("plotting.backend", "my_backend"):
+        result = pandas.plotting._core._get_plot_backend()
+
+    assert result is mod
+
+
+def test_register_import():
+    mod = types.ModuleType("my_backend2")
+    mod.plot = lambda *args, **kwargs: 1
+    sys.modules["my_backend2"] = mod
+
+    result = pandas.plotting._core._get_plot_backend("my_backend2")
+    assert result is mod
+
+
+@td.skip_if_mpl
+def test_no_matplotlib_ok():
+    with pytest.raises(ImportError):
+        pandas.plotting._core._get_plot_backend("matplotlib")

Diff for: setup.py

@@ -830,5 +830,10 @@ def srcpath(name=None, suffix=".pyx", subdir="src"):
             "hypothesis>=3.58",
         ]
     },
+    entry_points={
+        "pandas_plotting_backends": [
+            "matplotlib = pandas:plotting._matplotlib",
+        ],
+    },
     **setuptools_kwargs
 )