NF: @assert_cwd_unchanged() to check if test went somewhere else and chdir back

Side effects from tests, like going to another directory are pain, we should avoid them

Author: yarikoptic

heudiconv/tests/utils.py

@@ -1,9 +1,17 @@
+from functools import wraps
+import os
 import os.path as op
+import sys
+
 import heudiconv.heuristics
 
+
 HEURISTICS_PATH = op.join(heudiconv.heuristics.__path__[0])
 TESTS_DATA_PATH = op.join(op.dirname(__file__), 'data')
 
+import logging
+lgr = logging.getLogger(__name__)
+
 
 def gen_heudiconv_args(datadir, outdir, subject, heuristic_file,
                        anon_cmd=None, template=None, xargs=None):
@@ -58,3 +66,51 @@ def fetch_data(tmpdir, dataset, getpath=None):
     getdir = targetdir + (op.sep + getpath if getpath is not None else '')
     ds.get(getdir)
     return targetdir
+
+
+def assert_cwd_unchanged(ok_to_chdir=False):
+    """Decorator to test whether the current working directory remains unchanged
+
+    Provenance: based on the one in datalad, but simplified.
+
+    Parameters
+    ----------
+    ok_to_chdir: bool, optional
+      If True, allow to chdir, so this decorator would not then raise exception
+      if chdir'ed but only return to original directory
+    """
+
+    def decorator(func=None):  # =None to avoid pytest treating it as a fixture
+        @wraps(func)
+        def newfunc(*args, **kwargs):
+            cwd_before = os.getcwd()
+            exc = None
+            try:
+                return func(*args, **kwargs)
+            except Exception as exc_:
+                exc = exc_
+            finally:
+                try:
+                    cwd_after = os.getcwd()
+                except OSError as e:
+                    lgr.warning("Failed to getcwd: %s" % e)
+                    cwd_after = None
+
+                if cwd_after != cwd_before:
+                    os.chdir(cwd_before)
+                    if not ok_to_chdir:
+                        lgr.warning(
+                            "%s changed cwd to %s. Mitigating and changing back to %s"
+                            % (func, cwd_after, cwd_before))
+                        # If there was already exception raised, we better reraise
+                        # that one since it must be more important, so not masking it
+                        # here with our assertion
+                        if exc is None:
+                            assert cwd_before == cwd_after, \
+                                     "CWD changed from %s to %s" % (cwd_before, cwd_after)
+
+                if exc is not None:
+                    raise exc
+        return newfunc
+
+    return decorator