
.. _faq:

==========================
Frequently Asked Questions
==========================

TypeError: object of type 'TensorVariable' has no len()
-------------------------------------------------------

If you receive the following error, it is because the Python function *__len__* cannot 
be implemented on Theano variables:

.. code-block:: python

   TypeError: object of type 'TensorVariable' has no len()

Python requires that *__len__* returns an integer, yet it cannot be done as Theano's variables are symbolic. However, `var.shape[0]` can be used as a workaround.

This error message cannot be made more explicit because the relevant aspects of Python's 
internals cannot be modified.


Faster gcc optimization
-----------------------

You can enable faster gcc optimization with the ``cxxflags`` option.
This list of flags was suggested on the mailing list::

    -O3 -ffast-math -ftree-loop-distribution -funroll-loops -ftracer

Use it at your own risk. Some people warned that the ``-ftree-loop-distribution`` optimization resulted in wrong results in the past.

In the past we said that if the ``compiledir`` was not shared by multiple
computers, you could add the ``-march=native`` flag. Now we recommend
to remove this flag as Theano does it automatically and safely,
even if the ``compiledir`` is shared by multiple computers with different
CPUs. In fact, Theano asks g++ what are the equivalent flags it uses, and re-uses
them directly.


Faster Theano function
----------------------

You can set the Theano flag `allow_gc` to `False` to get a speed-up by using
more memory. By default, Theano frees intermediate results when we don't need
them anymore. Doing so prevents us from reusing this memory. So disabling the
garbage collection will keep all intermediate results' memory space to allow to
reuse them during the next call to the same Theano function, if they are of the
correct shape. The shape could change if the shapes of the inputs change.

Faster Small Theano function
----------------------------

.. note::

   For Theano 0.6 and up.

For Theano functions that don't do much work, like a regular logistic
regression, the overhead of checking the input can be significant. You
can disable it by setting ``f.trust_input`` to True.
Make sure the types of arguments you provide match those defined when
the function was compiled.

Also, for small Theano functions, you can remove more Python overhead by
making a Theano function that does not take any input. You can use shared
variables to achieve this. Then you can call it like this: ``f.fn()`` or
``f.fn(n_calls=N)`` to speed it up. In the last case, only the last
function output (out of N calls) is returned.

Related Projects
----------------

We try to list in this `wiki page <https://github.com/Theano/Theano/wiki/Related-projects>`_ other Theano related projects.


"What are Theano's Limitations?"
--------------------------------

Theano offers a good amount of flexibility, but has some limitations too.
You must answer for yourself the following question: How can my algorithm be cleverly written 
so as to make the most of what Theano can do?

Here is a list of some of the known limitations:

- *While*- or *for*-Loops within an expression graph are supported, but only via
  the :func:`theano.scan` op (which puts restrictions on how the loop body can
  interact with the rest of the graph).

- Neither *goto* nor *recursion* is supported or planned within expression graphs.


"float32 / int{32, 64} gives float64"
-------------------------------------

It should be noted that using float32 and int{32, 64} together
inside a function would provide float64 as output.

Since the GPU can't compute this kind of output, it would be
preferable not to use those dtypes together.
