drop py36 support (#4163)

* drop py36 support

* add a whatsnew entry

* review actions

* warn assert oddness on ci

* update pyproject.toml

* remove py36-linux-64.lock file

.cirrus.yml

@@ -137,8 +137,6 @@ test_minimal_task:
   only_if: ${SKIP_TEST_MINIMAL_TASK} == "" && ${SKIP_ALL_TEST_TASKS} == ""
   << : *CREDITS_TEMPLATE
   matrix:
-    env:
-      PY_VER: 3.6
     env:
       PY_VER: 3.7
     env:
@@ -159,8 +157,6 @@ test_full_task:
   only_if: ${SKIP_TEST_FULL_TASK} == "" && ${SKIP_ALL_TEST_TASKS} == ""
   << : *CREDITS_TEMPLATE
   matrix:
-    env:
-      PY_VER: 3.6
     env:
       PY_VER: 3.7
     env:

.github/workflows/refresh-lockfiles.yml

@@ -69,7 +69,7 @@ jobs:
 
     strategy:
       matrix:
-        python: ['36', '37', '38']
+        python: ['37', '38']
 
     steps:
       - uses: actions/checkout@v2

docs/src/conf.py

@@ -19,9 +19,15 @@
 
 # ----------------------------------------------------------------------------
 
+import datetime
 import ntpath
 import os
+from pathlib import Path
+import re
 import sys
+import warnings
+
+import iris
 
 
 # function to write  useful output to stdout, prefixing the source.
@@ -53,9 +59,6 @@ def autolog(message):
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 
-import datetime
-import warnings
-
 # custom sphinx extensions
 sys.path.append(os.path.abspath("sphinxext"))
 
@@ -80,8 +83,6 @@ def autolog(message):
 # |version| and |release|, also used in various other places throughout the
 # built documents.
 
-import iris
-
 # The short X.Y version.
 if iris.__version__ == "dev":
     version = "dev"
@@ -101,9 +102,35 @@ def autolog(message):
 
 build_python_version = ".".join([str(i) for i in sys.version_info[:3]])
 
+
+def _dotv(version):
+    result = version
+    match = re.match(r"^py(\d+)$", version)
+    if match:
+        digits = match.group(1)
+        if len(digits) > 1:
+            result = f"{digits[0]}.{digits[1:]}"
+    return result
+
+
+# Automate the discovery of the python versions tested with CI.
+python_support = sorted(
+    [fname.stem for fname in Path(".").glob("../../requirements/ci/py*.yml")]
+)
+
+if not python_support:
+    python_support = "unknown Python versions"
+elif len(python_support) == 1:
+    python_support = f"Python {_dotv(python_support[0])}"
+else:
+    rest = ", ".join([_dotv(v) for v in python_support[:-1]])
+    last = _dotv(python_support[-1])
+    python_support = f"Python {rest} and {last}"
+
 rst_epilog = f"""
 .. |copyright_years| replace:: {copyright_years}
 .. |python_version| replace:: {build_python_version}
+.. |python_support| replace:: {python_support}
 .. |iris_version| replace:: v{version}
 .. |build_date| replace:: ({datetime.datetime.now().strftime('%d %b %Y')})
 """

docs/src/installing.rst

@@ -16,8 +16,7 @@ any WSL_ distributions.
 
 .. _WSL: https://docs.microsoft.com/en-us/windows/wsl/install-win10
 
-.. note:: Iris is currently supported and tested against Python ``3.6``,
-          ``3.7``, and ``3.8``.
+.. note:: Iris is currently supported and tested against |python_support|.
 
 .. note:: This documentation was built using Python |python_version|.
 

docs/src/whatsnew/latest.rst

@@ -12,10 +12,11 @@ This document explains the changes made to Iris for this release
    :title: text-primary text-center font-weight-bold
    :body: bg-light
    :animate: fade-in
+   :open:
 
-   The highlights for this major/minor release of Iris include:
+   The highlights for this minor release of Iris include:
 
-   * N/A
+   * We've dropped support for `Python 3.6`_
 
    And finally, get in touch with us on `GitHub`_ if you have any issues or
    feature requests for improving Iris. Enjoy!
@@ -27,8 +28,8 @@ This document explains the changes made to Iris for this release
 #. Congratulations to `@jamesp`_ who recently became an Iris core developer
    after joining the Iris development team at the `Met Office`_. 🎉
 
-#. A special thanks goes to `@akuhnregnier`_, `@gcaria`_, `@jamesp`_ and
-   `@MHBalsmeier`_ all of whom made their first contributions to Iris, which
+#. A special thanks goes to `@akuhnregnier`_, `@gcaria`_, `@jamesp`_, `@MHBalsmeier`_
+   and `@Badboy-16`_ all of whom made their first contributions to Iris, which
    were gratefully received and included in this release. Keep up the awesome
    work! 🍻
 
@@ -168,6 +169,10 @@ This document explains the changes made to Iris for this release
    templates to remove an external URL reference that caused un-posted user issue
    content to be lost in the browser when followed. (:pull:`4147`)
 
+#. `@bjlittle`_ dropped `Python 3.6`_ support, and automated the discovery of
+   supported Python versions tested by `cirrus-ci`_ for documentation.
+   (:pull:`4163`)
+
 .. comment
     Whatsnew author names (@github name) in alphabetical order. Note that,
     core dev names are automatically included by the common_links.inc:
@@ -191,6 +196,7 @@ This document explains the changes made to Iris for this release
 .. |PyPI| image:: https://img.shields.io/pypi/v/scitools-iris?color=orange&label=pypi%7Cscitools-iris
 .. _PyPI: https://pypi.org/project/scitools-iris/
 .. _Python 3.8: https://www.python.org/downloads/release/python-380/
+.. _Python 3.6: https://www.python.org/downloads/release/python-360/
 .. _README.md: https://github.com/SciTools/iris#-----
 .. _xxhash: http://cyan4973.github.io/xxHash/
 .. _conda-lock: https://github.com/conda-incubator/conda-lock

lib/iris/tests/integration/test_pp.py

@@ -414,7 +414,7 @@ def test_hybrid_height_round_trip_no_reference(self):
             "Unable to create instance of HybridHeightFactory. "
             "The source data contains no field(s) for 'orography'."
         )
-        warn.assert_called_once_with(msg)
+        warn.assert_called_with(msg)
 
         # Check the data cube is set up to use hybrid height.
         self._test_coord(

noxfile.py

@@ -20,7 +20,7 @@
 PACKAGE = str("lib" / Path("iris"))
 
 #: Cirrus-CI environment variable hook.
-PY_VER = os.environ.get("PY_VER", ["3.6", "3.7", "3.8"])
+PY_VER = os.environ.get("PY_VER", ["3.7", "3.8"])
 
 #: Default cartopy cache directory.
 CARTOPY_CACHE_DIR = os.environ.get("HOME") / Path(".local/share/cartopy")

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.black]
 line-length = 79
-target-version = ['py36', 'py37', 'py38']
+target-version = ['py37', 'py38']
 include = '\.pyi?$'
 exclude = '''