Sanitize cal_tools.tools.get_creation_date

This is just a little bit of maintenance, as I'm wading through the code.

This MR cleans

It was tested so:

In [1]: from cal_tools.tools import get_dir_creation_date

In [2]: folder = "/gpfs/exfel/exp/DETLAB/202031/p900172/raw"

In [3]: get_dir_creation_date(folder, 10)
Out[3]: datetime.datetime(2020, 7, 20, 12, 39, 20, 631633)

In [4]: get_dir_creation_date(folder, 4)  # no such run, None is returned

In [5]: 

One question maybe is, should this function return None when the date could not be extracted, as it is now, or to raise?
I'd prefer to raise an error, so that it's clear that this step failed, rather than failing later.

@kamile @ahmedk

cal_tools/cal_tools/tools.py

@@ -2,8 +2,9 @@ from collections import OrderedDict
 import datetime
 from glob import glob
 import json
-from os import environ, listdir, path, stat
-from os.path import isfile, splitext
+from os import environ, listdir, path
+from os.path import isfile
+from pathlib import Path
 from queue import Queue
 import re
 from time import sleep
@@ -230,7 +231,7 @@ def get_run_info(proposal, run):
 
 def get_dir_creation_date(directory: str, run: int,
                           tsdir: Optional[bool] = False,
-                          verbosity: Optional[int] = 0):
+                          verbosity: Optional[int] = 0) -> Optional[datetime.datetime]:
     """
     Return run starting time from the MDC.
     If not succeeded, return modification time of oldest file.h5
@@ -241,35 +242,35 @@ def get_dir_creation_date(directory: str, run: int,
     :param tsdir: to get modification time of [directory]/[run]04.
     :param verbosity: Level of verbosity (0 - silent)
     :return: (datetime) modification time
+
     """
+    directory = Path(directory)
+    proposal = int(directory.parent.name[1:])
 
     try:
-        path_list = list(filter(None, directory.strip('/').split('/')))
-        proposal = int(path_list[-2][1:])
         run_info = get_run_info(proposal, run)
         return dateutil.parser.parse(run_info['begin_at'])
     except Exception as e:
         if verbosity > 0:
             print(e)
 
+    directory = directory / 'r{:04d}'.format(run)
+
     ntries = 100
     while ntries > 0:
         try:
             if tsdir:
-                creation_time = stat("{}/r{:04d}".format(directory, run)).st_mtime
+                creation_time = directory.stat().st_mtime
             else:
-                rfiles = glob("{}/r{:04d}/*.h5".format(directory, run))
-                rfiles.sort(key=path.getmtime)
-                creation_time = stat(rfiles[0]).st_mtime
-
-            creation_time = datetime.datetime.fromtimestamp(creation_time)
-            return creation_time
-        except:  # catch stale file handle errors etc and try again
+                creation_time = min(f.stat().st_mtime
+                                    for f in directory.glob('*.h5'))
+            return datetime.datetime.fromtimestamp(creation_time)
+        except IOError:  # catch stale file handle errors etc and try again
             ntries -= 1
 
 
 def save_const_to_h5(device, constant, condition, data,
-                    file_loc, creation_time, out_folder):
+                     file_loc, creation_time, out_folder):
     """
     Save constant in h5 file with its metadata
     (e.g. db_module, condition, creation_time)
@@ -280,7 +281,7 @@ def save_const_to_h5(device, constant, condition, data,
     :type constant: iCalibrationDB.know_constants object
     :param condition: Calibration condition
     :type condition: iCalibrationDB.know_detector_conditions object
-    :param data: Constant data to save 
+    :param data: Constant data to save
     :type data: ndarray
     :param file_loc: Location of raw data "proposal:{} runs:{} {} {}"
     :type file_loc: str