Re: [python-control-discuss] NoneType and circular relative imports

[Scott C. L. <sli...@ca...>]

Howdy Luke,

On 29 Oct 2012, at 13:53, Dale Lukas Peterson wrote:

> Richard,
>  I am running Python 3.2.  In order to run python control, I have to
> use the 2to3 tool to convert on each .py file.  It fixes things like
> print "x" by changing it to print("x").  For some reason, it is

Is anyone ensuring that running the 2to3 tool is sufficient for use of python-control with Python 3, or otherwise working on a script that makes necessary changes?

> 
> I think this may stem from how src/__init__.py and each of the modules
> in src/ are importing each others functionality.  The design patterned
> that I have been following and have used with no issues in the sympy
> project can be seen in these two files:
> https://github.com/hazelnusse/sympy/blob/master/sympy/physics/mechanics/__init__.py
> https://github.com/hazelnusse/sympy/blob/master/sympy/physics/mechanics/particle.py
> 
> Basically, in each module, we set the __all__ list to include the
> names of the publicly available class and functions.  Within each
> module, we never do relative imports, we always use the absolute
> import syntax.  The __init__.py file is heavily commented so you
> should be able to see how we make those symbols available when people
> import the module or sub-package.

While that design pattern is nice, it may not yet be warranted for python-control, which is contained entirely in a single directory with no subpackages.  The simplest change I see to avoid the import error you found is below (given as diff output).

~Scott


Index: src/statesp.py
===================================================================
--- src/statesp.py	(revision 211)
+++ src/statesp.py	(working copy)
@@ -82,7 +82,6 @@
 from scipy.signal import lti
 import warnings
 from lti import Lti, timebaseEqual, isdtime
-import xferfcn
 
 class StateSpace(Lti):
 
@@ -511,7 +510,7 @@
                                                   [1., 1., 1.]].
     
     """
-    
+    from xferfcn import TransferFunction
     if isinstance(sys, StateSpace):
         if len(kw):
             raise TypeError("If sys is a StateSpace, _convertToStateSpace \
@@ -519,7 +518,7 @@
 
         # Already a state space system; just return it
         return sys
-    elif isinstance(sys, xferfcn.TransferFunction):
+    elif isinstance(sys, TransferFunction):
         try:
             from slycot import td04ad
             if len(kw):
Index: src/xferfcn.py
===================================================================
--- src/xferfcn.py	(revision 211)
+++ src/xferfcn.py	(working copy)
@@ -81,7 +81,7 @@
 from copy import deepcopy
 from lti import Lti, timebaseEqual, timebase, isdtime
 from warnings import warn
-import statesp
+from statesp import StateSpace
 
 class TransferFunction(Lti):
 
@@ -308,7 +308,7 @@
         """Add two LTI objects (parallel connection)."""
         
         # Convert the second argument to a transfer function.
-        if (isinstance(other, statesp.StateSpace)):
+        if (isinstance(other, StateSpace)):
             other = _convertToTransferFunction(other)
         elif not isinstance(other, TransferFunction):
             other = _convertToTransferFunction(other, inputs=self.inputs, 
@@ -868,7 +868,7 @@
                     "_convertToTransferFunction cannot take keywords.")
 
         return sys
-    elif isinstance(sys, statesp.StateSpace):
+    elif isinstance(sys, StateSpace):
         try:
             from slycot import tb04ad
             if len(kw):
knoxville:python-control-py3%