Add function_dump() to help user give us reproducable example.

doc/tutorial/debug_faq.txt

@@ -410,3 +410,29 @@ the function itself (a "thunk" is a concept related to closures). Here, to
 get the current node's first input's shape, you'd therefore do "p
 thunk.inputs[0][0].shape", which prints out "(3, 4)".
 
+.. _faq_dump_fct:
+
+Dumping a Function to help debug
+--------------------------------
+
+If you are reading this, there is high chance that you emailed our
+mailing list and we asked you to read this section. This section
+explain how to dump all the parameter passed to
+``theano.function()``. This is useful to help us reproduce a problem
+during compilation and it don't request you to make a self contained
+example.
+
+For this to work, we need to be able to import the code for all Op in
+the graph. So if you create your own Op, we will need this
+code. Otherwise, we won't be able to unpickle it. We already have all
+the Ops from Theano and Pylearn2.
+
+.. code-block:: python
+
+    # Replace this line:
+    theano.function(...)
+    # with
+    theano.function_dump(filename, ...)
+    # Where filename is a string to a file that we will write to.
+
+Then send us filename.

theano/__init__.py

@@ -57,7 +57,7 @@ from theano.compile import \
     SymbolicOutput, Out, \
     Mode, \
     predefined_modes, predefined_linkers, predefined_optimizers, \
-    FunctionMaker, function, OpFromGraph, \
+    FunctionMaker, function, function_dump, OpFromGraph, \
     Component, External, Member, Method, \
     Composite, ComponentList, ComponentDict, Module, \
     ProfileMode, ProfileStats, \

theano/compile/__init__.py

@@ -28,4 +28,4 @@ from theano.compile.pfunc import pfunc, Param, rebuild_collect_shared
 
 from theano.compile.builders import *
 
-from theano.compile.function import function
+from theano.compile.function import function, function_dump

theano/compile/function.py

@@ -2,6 +2,7 @@
 """
 __docformat__ = "restructuredtext en"
 
+import cPickle
 import logging
 _logger = logging.getLogger('theano.compile.function')
 
@@ -12,6 +13,27 @@ from numpy import any  # to work in python 2.4
 import warnings
 from theano import gof
 
+
+def function_dump(filename, inputs, outputs=None, mode=None, updates=None,
+                  givens=None,
+                  no_default_updates=False, accept_inplace=False, name=None,
+                  rebuild_strict=True, allow_input_downcast=None, profile=None,
+                  on_unused_input=None):
+    """This is helpful to make a reproducable case for problem during
+    Theano compilation.
+
+    """
+    assert isinstance(filename, basestring)
+    d = dict(inputs=inputs, outputs=outputs, mode=mode, updates=updates,
+             givens=givens, no_default_updates=no_default_updates,
+             accept_inplace=accept_inplace, name=name,
+             rebuild_strict=rebuild_strict,
+             allow_input_downcast=allow_input_downcast, profile=profile,
+             on_unused_input=on_unused_input)
+    with open(filename, 'wb') as f:
+        cPickle.dump(d, f, -1)
+
+
 def function(inputs, outputs=None, mode=None, updates=None, givens=None,
              no_default_updates=False, accept_inplace=False, name=None,
              rebuild_strict=True, allow_input_downcast=None, profile=None,

theano/compile/tests/test_function.py

+import cPickle
+import os
+import shutil
+import tempfile
+
+import numpy
+
+import theano
+
+
+def test_function_dump():
+    v = theano.tensor.vector()
+    fct1 = theano.function([v], v + 1)
+
+    try:
+        tmpdir = tempfile.mkdtemp()
+        fname = os.path.join(tmpdir, 'test_function_dump.pkl')
+        theano.function_dump(fname, [v], v + 1)
+        f = open(fname, 'rb')
+        l = cPickle.load(f)
+        f.close()
+    finally:
+        if tmpdir is not None:
+            shutil.rmtree(tmpdir)
+
+    fct2 = theano.function(**l)
+    x = [1, 2, 3]
+    assert numpy.allclose(fct1(x), fct2(x))