Python 2 --> 3: object of type 'zip' has no len()

I'm following a tutorial on neural nets1

It's in Python 2.7. I'm using 3.4. This is the line that troubles me:

if test_data: n_test = len(test_data)

I get: TypeError: object of type 'zip' has no len().

Is there a way to rewrite it so that it works in 3.4?

A bit late now to answer, but in case anyone else stumbles on it: for that same neural net example tutorial, it turned out I had to wrap the 3 zip calls in the mnist_loader with a list(zip(...)) construct:

training_data = list(zip(training_inputs, training_results))
(...)
validation_data = list(zip(validation_inputs, va_d[1]))
(...)
test_data = list(zip(test_inputs, te_d[1]))

And then it worked.

If you know that the iterator is finite:

#NOTE: `sum()` consumes the iterator
n_test = sum(1 for _ in test_data) # find len(iterator)

Or if you know that test_data is always small and a profiler says that the code is the bottleneck in your application then here's code that might be more efficient for small n_test:

test_data = list(test_data)
n_test = len(test_data)

Unfortunately, operator.length_hint() (Python 3.4+) returns zero for a zip() object. See PEP 0424 -- A method for exposing a length hint.