Enhance run() placeholder substitutions to honor configuration defa…

…ults

Previously, `run()` would not recognize configuration defaults for
placeholder substitution. This means that any placeholders globally
declared in `datalad.interface.common_cfg`, or via `register_config()`
in DataLad extensions would not be effective.

This changeset makes run's `format_command()` helper include such
defaults explicitly, and thereby enable the global declaration of
substitution defaults.

Moreoever a `{python}` placeholder is now defined via this mechanism,
and points to the value of `sys.executable` by default. This particular
placeholder was found to be valueable for improving the portability of
run-recording across (specific) Python versions, or across different
(virtual) environments. See
datalad/datalad-container#224 for an example
use case.

A corresponding test is included.

The ability to keep run-records paramterizable, in particular, for
interpreters can also aid longevity (think platform-specific choices
to call a system Python executable `python3` for some years, and then
switch back.

Related datalad/datalad-container#250

datalad/core/local/run.py

@@ -38,6 +38,7 @@
     build_doc,
     eval_results,
 )
+from datalad.interface.common_cfg import definitions as cfg_defs
 from datalad.interface.common_opts import (
     jobs_opt,
     save_message_opt,
@@ -623,7 +624,12 @@ def format_command(dset, command, **kwds):
     command = normalize_command(command)
     sfmt = SequenceFormatter()
 
-    for k, v in dset.config.items("datalad.run.substitutions"):
+    for k in set(cfg_defs.keys()).union(dset.config.keys()):
+        v = dset.config.get(
+            k,
+            # pull a default from the config definitions
+            # if we have no value, but a key
+            cfg_defs.get(k, {}).get('default', None))
         sub_key = k.replace("datalad.run.substitutions.", "")
         if sub_key not in kwds:
             kwds[sub_key] = v

datalad/core/local/tests/test_run.py

@@ -21,6 +21,7 @@
     mkdir,
     remove,
 )
+import pytest
 from unittest.mock import patch
 
 from datalad.api import (
@@ -43,6 +44,7 @@
 from datalad.tests.utils_pytest import (
     DEFAULT_BRANCH,
     OBSCURE_FILENAME,
+    SkipTest,
     assert_false,
     assert_in,
     assert_in_results,
@@ -781,3 +783,21 @@ class dset:
         eq_(_format_iospecs(['{dummy}'],
                             **_get_substitutions(dset)),
             ['a', 'b'])
+
+
+def test_substitution_config_default(tmp_path):
+    ds = Dataset(tmp_path).create(result_renderer='disabled')
+
+    if ds.config.get('datalad.run.substitutions.python') is not None:
+        # we want to test default handling when no config is set
+        raise SkipTest(
+            'Test assumptions conflict with effective configuration')
+
+    # the {python} placeholder is not explicitly defined, but it has
+    # a default, which run() should discover and use
+    res = ds.run('{python} -c "True"', result_renderer='disabled')
+    assert_result_count(res, 1, action='run', status='ok')
+
+    # make sure we could actually detect breakage with the check above
+    with pytest.raises(IncompleteResultsError):
+        ds.run('{python} -c "breakage"', result_renderer='disabled')

datalad/interface/common_cfg.py

@@ -17,6 +17,7 @@
 from os import environ
 from os.path import expanduser
 from os.path import join as opj
+import sys
 import time
 
 from platformdirs import AppDirs
@@ -546,6 +547,13 @@ def get_default_ssh():
         'type': EnsureInt(),
         'default': 8,
     },
+    'datalad.run.substitutions.python': {
+        'ui': ('question', {
+               'title': 'Substitution for {python} placeholder',
+               'text': 'Path to a Python interpreter executable'}),
+        'type': EnsureStr(),
+        'default': sys.executable,
+    },
     'datalad.runtime.max-annex-jobs': {
         'ui': ('question', {
                'title': 'Maximum number of git-annex jobs to request when "jobs" option set to "auto" (default)',