add option for delayed imports

will make setup checking easier, although it does not work for all cases
(from imports for example)

Change-Id: I2e5cf9b427a6f8e6f603d9662cdb9700d5f9ad9a
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/33893
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>

frappy/client/interactive.py

@@ -30,13 +30,12 @@ import traceback
 import threading
 import logging
 from os.path import expanduser
+from frappy.lib import delayed_import
 from frappy.client import SecopClient, UnregisterCallback
 from frappy.errors import SECoPError
 from frappy.datatypes import get_datatype, StatusType
-try:
-    import readline
-except ImportError:
-    readline = None
+
+readline = delayed_import('readline')
 
 
 USAGE = """

frappy/gui/plotting.py

@@ -25,13 +25,10 @@ import time
 from frappy.gui.qt import QLabel, Qt, QVBoxLayout, QWidget, pyqtSignal
 
 from frappy.gui.util import Colors
+from frappy.lib import delayed_import
 
-try:
-    import numpy as np
-    import pyqtgraph as pg
-except ImportError:
-    pg = None
-    np = None
+np = delayed_import('numpy')
+pg = delayed_import('pyqtgraph')
 
 
 def getPlotWidget(parent):

frappy/lib/__init__.py

@@ -419,3 +419,26 @@ def merge_status(*args):
     # use dict instead of set for preserving order
     merged = {m: True for mm in merged for m in mm.split(', ')}
     return maxcode, ', '.join(merged)
+
+
+class _Raiser:
+    def __init__(self, modname):
+        self.modname = modname
+
+    def __getattr__(self, name):
+        # Just retry the import, it will give the most useful exception.
+        __import__(self.modname)
+
+    def __bool__(self):
+        return False
+
+
+def delayed_import(modname):
+    """Import a module, and return an object that raises a delayed exception
+    on access if it failed.
+    """
+    try:
+        module = __import__(modname, None, None, ['*'])
+    except Exception:
+        return _Raiser(modname)
+    return module