Added option for debugprint to return a string rather than printing it.

theano/printing.py

 """Pretty-printing (pprint()), the 'Print' Op, debugprint() and pydotprint().
 They all allow different way to print a graph or the result of an Op in a graph(Print Op)
 """
-import gof
+import sys, os, StringIO
 from copy import copy
-import sys,os
+
+import gof
 from theano import config
 from gof import Op, Apply
 from theano.gof.python25 import any
@@ -17,11 +18,10 @@ def debugprint(obj, depth=-1, file=None):
     :param obj: symbolic thing to print
     :type depth: integer
     :param depth: print graph to this depth (-1 for unlimited)
-    :type file: None or file-like object
-    :param file: print to this file (None means sys.stdout)
+    :type file: None, 'str', or file-like object
+    :param file: print to this file ('str' means to return a string)
 
-    :rtype: None or file-like object
-    :returns: `file` argument
+    :returns: str if `file`=='str', else file arg
 
     Each line printed represents a Variable in the graph.
     The indentation of each line corresponds to its depth in the symbolic graph.
@@ -36,7 +36,9 @@ def debugprint(obj, depth=-1, file=None):
     identifier, to indicate which output a line corresponds to.
 
     """
-    if file is None:
+    if file == 'str':
+        _file = StringIO.StringIO()
+    elif file is None:
         _file = sys.stdout
     else:
         _file = file
@@ -50,9 +52,12 @@ def debugprint(obj, depth=-1, file=None):
         results_to_print.extend(obj.maker.env.outputs)
     for r in results_to_print:
         debugmode.debugprint(r, depth=depth, done=done, file=_file)
-    if file is None:
+    if file is _file:
+        return file
+    elif file=='str':
+        return _file.getvalue()
+    else:
         _file.flush()
-    return file
 
 class Print(Op):
     """This identity-like Op has the side effect of printing a message followed by its inputs